13.1. Definitions
The most important classes of inorganic substances traditionally include simple substances (metals and non-metals), oxides (acidic, basic and amphoteric), hydroxides (some acids, bases, amphoteric hydroxides) and salts. Substances belonging to the same class have similar chemical properties. But you already know that when identifying these classes, different classification criteria are used.
In this section we will finally formulate the definitions of all the most important classes of chemical substances and understand by what criteria these classes are distinguished.
Let's start with simple substances
(classification according to the number of elements that make up the substance). They are usually divided into metals And nonmetals(Fig. 13.1- A).
You already know the definition of “metal”.
From this definition it is clear that the main feature that allows us to divide simple substances into metals and non-metals is the type of chemical bond.
Most nonmetals have covalent bonds. But there are also noble gases (simple substances of group VIIIA elements), the atoms of which in the solid and liquid states are connected only by intermolecular bonds. Hence the definition.
According to their chemical properties, metals are divided into a group of so-called amphoteric metals. This name reflects the ability of these metals to react with both acids and alkalis (as amphoteric oxides or hydroxides) (Fig. 13.1- b).
In addition, due to chemical inertness among metals there are noble metals. These include gold, ruthenium, rhodium, palladium, osmium, iridium, and platinum. According to tradition, the slightly more reactive silver is also classified as noble metals, but inert metals such as tantalum, niobium and some others are not included. There are other classifications of metals, for example, in metallurgy, all metals are divided into black and colored, referring to ferrous metals iron and its alloys.
From complex substances
are most important, first of all, oxides(see §2.5), but since their classification takes into account the acid-base properties of these compounds, we first recall what acids And grounds.
Thus, we distinguish acids and bases from the total mass of compounds using two characteristics: composition and chemical properties.
According to their composition, acids are divided into oxygen-containing
(oxoacids) And oxygen-free(Fig. 13.2).
It should be remembered that oxygen-containing acids, by their structure, are hydroxides.
Note. Traditionally, for oxygen-free acids, the word “acid” is used in cases where we are talking about a solution of the corresponding individual substance, for example: the substance HCl is called hydrogen chloride, and its aqueous solution is called hydrochloric or hydrochloric acid.
Now let's return to oxides. We assigned oxides to the group acidic or main by how they react with water (or by whether they are made from acids or bases). But not all oxides react with water, but most of them react with acids or alkalis, so it is better to classify oxides according to this property.
There are several oxides that under normal conditions do not react with either acids or alkalis. Such oxides are called non-salt-forming. These are, for example, CO, SiO, N 2 O, NO, MnO 2. In contrast, the remaining oxides are called salt-forming(Fig. 13.3).
As you know, most acids and bases are hydroxides. Based on the ability of hydroxides to react with both acids and alkalis, they (as well as among oxides) are divided into amphoteric hydroxides(Fig. 13.4).
Now we just need to define salts. The term salt has been used for a long time. As science developed, its meaning was repeatedly changed, expanded and clarified. In the modern understanding, salt is an ionic compound, but traditionally salts do not include ionic oxides (as they are called basic oxides), ionic hydroxides (bases), as well as ionic hydrides, carbides, nitrides, etc. Therefore, in a simplified way, we can say, What
Another, more precise definition of salts can be given.
When given this definition, oxonium salts are usually classified as both salts and acids.
Salts are usually divided according to their composition into sour,
average And basic(Fig. 13.5).
That is, the anions of acid salts include hydrogen atoms linked by covalent bonds to other atoms of the anions and capable of being torn off under the action of bases.
Basic salts usually have a very complex composition and are often insoluble in water. A typical example of a basic salt is the mineral malachite Cu 2 (OH) 2 CO 3 .
As you can see, the most important classes of chemical substances are distinguished according to different classification criteria. But no matter how we distinguish a class of substances, all substances of this class have common chemical properties.
In this chapter you will become acquainted with the most characteristic chemical properties of substances representing these classes and with the most important methods for their preparation.
METALS, NON-METALS, AMPHOTERIC METALS, ACIDS, BASES, OXO ACIDS, OXYGEN-FREE ACIDS, BASIC OXIDES, ACID OXIDES, AMPHOTERIC OXIDES, AMPHOTERIC HYDROXIDES, SALTS, ACID SALTS, MEDIUM SALTS, BASIC SALT
1.Where in the natural system of elements are the elements that form metals located, and where are the elements that form non-metals?
2.Write the formulas of five metals and five non-metals.
3. Make up the structural formulas of the following compounds:
(H 3 O)Cl, (H 3 O) 2 SO 4, HCl, H 2 S, H 2 SO 4, H 3 PO 4, H 2 CO 3, Ba(OH) 2, RbOH.
4.Which oxides correspond to the following hydroxides:
H2SO4, Ca(OH)2, H3PO4, Al(OH)3, HNO3, LiOH?
What is the nature (acidic or basic) of each of these oxides?
5. Find salts among the following substances. Make up their structural formulas.
KNO 2, Al 2 O 3, Al 2 S 3, HCN, CS 2, H 2 S, K 2, SiCl 4, CaSO 4, AlPO 4
6. Make up the structural formulas of the following acid salts:
NaHSO 4, KHSO 3, NaHCO 3, Ca(H 2 PO 4) 2, CaHPO 4.
13.2. Metals
In metal crystals and their melts, the atomic cores are connected by a single electron cloud of metallic bonding. Like an individual atom of the element that forms a metal, a metal crystal has the ability to donate electrons. The tendency of a metal to give up electrons depends on its structure and, above all, on the size of the atoms: the larger the atomic cores (that is, the larger the ionic radii), the more easily the metal gives up electrons.
Metals are simple substances, therefore the oxidation state of the atoms in them is 0. When entering into reactions, metals almost always change the oxidation state of their atoms. Metal atoms, not having the tendency to accept electrons, can only donate or share them. The electronegativity of these atoms is low, therefore, even when they form covalent bonds, the metal atoms acquire a positive oxidation state. Consequently, all metals exhibit, to one degree or another, restorative properties. They react:
1) C non-metals(but not all and not with everyone):
4Li + O 2 = 2Li 2 O,
3Mg + N 2 = Mg 3 N 2 (when heated),
Fe + S = FeS (when heated).
The most active metals easily react with halogens and oxygen, and only lithium and magnesium react with very strong nitrogen molecules.
When reacting with oxygen, most metals form oxides, and the most active ones form peroxides (Na 2 O 2, BaO 2) and other more complex compounds.
2) C oxides less active metals:
2Ca + MnO 2 = 2CaO + Mn (when heated),
2Al + Fe 2 O 3 = Al 2 O 3 + 2Fe (with preheating).
The possibility of these reactions occurring is determined by the general rule (redox reactions proceed in the direction of the formation of weaker oxidizing and reducing agents) and depends not only on the activity of the metal (a more active metal, that is, a metal that more easily gives up its electrons, reduces a less active one), but also on the energy of the oxide crystal lattice ( the reaction proceeds in the direction of the formation of a more “strong” oxide).
3) C acid solutions(§ 12.2):
Mg + 2H 3 O = Mg 2B + H 2 + 2H 2 O, Fe + 2H 3 O = Fe 2 + H 2 + 2H 2 O,
Mg + H 2 SO 4p = MgSO 4p + H 2, Fe + 2HCl p = FeCl 2p + H 2.
In this case, the possibility of a reaction is easily determined by a series of voltages (the reaction occurs if the metal in the voltage series is to the left of hydrogen).
4) C salt solutions(§ 12.2):
Fe + Cu 2 = Fe 2 + Cu, Cu + 2Ag = Cu 2 +2Ag,
Fe + CuSO 4p = Cu + FeSO 4p, Cu + 2AgNO 3p = 2Ag + Cu(NO 3) 2p.
A number of voltages are also used here to determine whether a reaction can occur.
5) In addition, the most active metals (alkali and alkaline earth) react with water (§ 11.4):
2Na + 2H 2 O = 2Na + H 2 + 2OH, Ca + 2H 2 O = Ca 2 + H 2 + 2OH,
2Na + 2H 2 O = 2NaOH p + H 2, Ca + 2H 2 O = Ca(OH) 2p + H 2.
In the second reaction, the formation of a Ca(OH) 2 precipitate is possible.
Most metals in industry get, reducing their oxides:
Fe 2 O 3 + 3CO = 2Fe + 3CO 2 (at high temperature),
MnO 2 + 2C = Mn + 2CO (at high temperature).
Hydrogen is often used for this in the laboratory:
The most active metals, both in industry and in the laboratory, are obtained by electrolysis (§ 9.9).
In the laboratory, less active metals can be reduced from solutions of their salts by more active metals (for restrictions, see § 12.2).
1.Why do metals not tend to exhibit oxidizing properties?
2.What primarily determines the chemical activity of metals?
3. Carry out transformations
a) Li Li 2 O LiOH LiCl; b) NaCl Na Na 2 O 2;
c) FeO Fe FeS Fe 2 O 3; d) CuCl 2 Cu(OH) 2 CuO Cu CuBr 2.
4.Restore the left sides of the equations:
a) ... = H 2 O + Cu;
b) ... = 3CO + 2Fe;
c) ... = 2Cr + Al 2 O 3
. Chemical properties of metals.
13.3. Nonmetals
Unlike metals, non-metals differ very much from each other in their properties - both physical and chemical, and even in type of structure. But, not counting the noble gases, in all nonmetals the bond between atoms is covalent.
The atoms that make up nonmetals have a tendency to gain electrons, but when forming simple substances, they cannot “satisfy” this tendency. Therefore, non-metals (to one degree or another) have a tendency to add electrons, that is, they can exhibit oxidizing properties. The oxidative activity of nonmetals depends, on the one hand, on the size of the atoms (the smaller the atoms, the more active the substance), and on the other, on the strength of covalent bonds in a simple substance (the stronger the bonds, the less active the substance). When forming ionic compounds, nonmetal atoms actually add “extra” electrons, and when forming compounds with covalent bonds, they only shift common electron pairs in their direction. In both cases, the oxidation state decreases.
Nonmetals can oxidize:
1) metals(substances more or less inclined to donate electrons):
3F 2 + 2Al = 2AlF 3,
O 2 + 2Mg = 2MgO (with preheating),
S + Fe = FeS (when heated),
2C + Ca = CaC 2 (when heated).
2) other non-metals(less prone to accept electrons):
2F 2 + C = CF 4 (when heated),
O 2 + S = SO 2 (with preheating),
S + H 2 = H 2 S (when heated),
3) many complex
substances:
4F 2 + CH 4 = CF 4 + 4HF,
3O 2 + 4NH 3 = 2N 2 + 6H 2 O (when heated),
Cl 2 + 2HBr = Br 2 + 2HCl.
Here, the possibility of a reaction occurring is determined primarily by the strength of the bonds in the reagents and reaction products and can be determined by calculation G.
The strongest oxidizing agent is fluorine. Oxygen and chlorine are not much inferior to it (pay attention to their position in the system of elements).
To a much lesser extent, boron, graphite (and diamond), silicon and other simple substances formed by elements adjacent to the boundary between metals and non-metals exhibit oxidizing properties. Atoms of these elements are less likely to gain electrons. It is these substances (especially graphite and hydrogen) that are capable of exhibiting restorative properties:
2C + MnO 2 = Mn + 2CO,
4H 2 + Fe 3 O 4 = 3Fe + 4H 2 O.
You will study the remaining chemical properties of nonmetals in the following sections as you become familiar with the chemistry of individual elements (as was the case with oxygen and hydrogen). There you will also learn how to obtain these substances.
1. Which of the following substances are non-metals: Be, C, Ne, Pt, Si, Sn, Se, Cs, Sc, Ar, Ra?
2. Give examples of non-metals that, under normal conditions, are a) gases, b) liquids, c) solids.
3. Give examples of a) molecular and b) non-molecular simple substances.
4. Give three examples of chemical reactions in which a) chlorine and b) hydrogen exhibit oxidizing properties.
5.Give three examples of chemical reactions that are not in the text of the paragraph, in which hydrogen exhibits reducing properties.
6. Carry out transformations:
a) P 4 P 4 O 10 H 3 PO 4 ; b) H 2 NaH H 2 ; c) Cl 2 NaCl Cl 2 .
Chemical properties of nonmetals.
13.4. Basic oxides
You already know that all basic oxides are non-molecular solids with ionic bonds.
The main oxides include:
a) oxides of alkali and alkaline earth elements,
b) oxides of some other elements that form metals in lower oxidation states, for example: CrO, MnO, FeO, Ag 2 O, etc.
They contain singly charged, doubly charged (very rarely triply charged cations) and oxide ions. The most characteristic Chemical properties basic oxides are precisely due to the presence in them of doubly charged oxide ions (very strong base particles). The chemical activity of basic oxides depends primarily on the strength of the ionic bonds in their crystals.
1) All basic oxides react with solutions of strong acids (§ 12.5):
Li 2 O + 2H 3 O = 2Li + 3H 2 O, NiO + 2H 3 O = Ni 2 + 3H 2 O,
Li 2 O + 2HCl p = 2LiCl p + H 2 O, NiO + H 2 SO 4p = NiSO 4p + H 2 O.
In the first case, in addition to the reaction with oxonium ions, a reaction with water also occurs, but since its rate is much lower, it can be neglected, especially since in the end the same products are still obtained.
The possibility of reaction with a solution of a weak acid is determined both by the strength of the acid (the stronger the acid, the more active it is) and the strength of the bond in the oxide (the weaker the bond, the more active the oxide).
2) Oxides of alkali and alkaline earth metals react with water (§ 11.4):
Li 2 O + H 2 O = 2Li + 2OH BaO + H 2 O = Ba 2 + 2OH
Li 2 O + H 2 O = 2LiOH p, BaO + H 2 O = Ba(OH) 2p.
3) In addition, basic oxides react with acidic oxides:
BaO + CO 2 = BaCO 3,
FeO + SO 3 = FeSO 4,
Na 2 O + N 2 O 5 = 2NaNO 3.
Depending on the chemical activity of these and other oxides, reactions can occur at ordinary temperatures or when heated.
What is the reason for such reactions? Let's consider the reaction of the formation of BaCO 3 from BaO and CO 2. The reaction proceeds spontaneously, and the entropy in this reaction decreases (from two substances, solid and gaseous, one crystalline substance is formed), therefore, the reaction is exothermic. In exothermic reactions, the energy of bonds formed is greater than the energy of bonds broken; therefore, the energy of bonds in BaCO 3 is greater than in the original BaO and CO 2. There are two types of chemical bonds in both the starting materials and the reaction products: ionic and covalent. The ionic bond energy (lattice energy) in BaO is slightly greater than in BaCO 3 (the size of the carbonate ion is larger than the oxide ion), therefore, the energy of the O 2 + CO 2 system is greater than the energy of CO 3 2.
+ Q
In other words, the CO 3 2 ion is more stable than the O 2 ion and CO 2 molecule taken separately. And the greater stability of the carbonate ion (its lower internal energy) is associated with the charge distribution of this ion (– 2 e) by three oxygen atoms of the carbonate ion instead of one in the oxide ion (see also § 13.11).
4) Many basic oxides can be reduced to the metal by a more active metal or nonmetal reducing agent:
MnO + Ca = Mn + CaO (when heated),
FeO + H 2 = Fe + H 2 O (when heated).
The possibility of such reactions occurring depends not only on the activity of the reducing agent, but also on the strength of the bonds in the initial and resulting oxide.
General method of obtaining Almost all basic oxides involve oxidation of the corresponding metal with oxygen. In this way, oxides of sodium, potassium and some other very active metals (under these conditions they form peroxides and more complex compounds), as well as gold, silver, platinum and other very low-active metals (these metals do not react with oxygen) cannot be obtained. Basic oxides can be obtained by thermal decomposition of the corresponding hydroxides, as well as some salts (for example, carbonates). Thus, magnesium oxide can be obtained in all three ways:
2Mg + O 2 = 2MgO,
Mg(OH) 2 = MgO + H 2 O,
MgCO 3 = MgO + CO 2.
1. Make up reaction equations:
a) Li 2 O + CO 2 b) Na 2 O + N 2 O 5 c) CaO + SO 3
d) Ag 2 O + HNO 3 e) MnO + HCl f) MgO + H 2 SO 4
2. Make up equations for the reactions that occur during the following transformations:
a) Mg MgO MgSO 4 b) Na 2 O Na 2 SO 3 NaCl
c) CoO Co CoCl 2 d) Fe Fe 3 O 4 FeO
3. A portion of nickel weighing 8.85 g was calcined in a stream of oxygen to obtain nickel(II) oxide, then treated with an excess of hydrochloric acid. A sodium sulfide solution was added to the resulting solution until the precipitation ceased. Determine the mass of this sediment.
Chemical properties of basic oxides.
13.5. Acidic oxides
All acid oxides are substances with
covalent bond.
Acid oxides include:
a) oxides of elements forming non-metals,
b) some oxides of elements that form metals, if the metals in these oxides are in higher oxidation states, for example, CrO 3, Mn 2 O 7.
Among the acid oxides there are substances that are gases at room temperature (for example: CO 2, N 2 O 3, SO 2, SeO 2), liquids (for example, Mn 2 O 7) and solids (for example: B 2 O 3, SiO 2, N 2 O 5, P 4 O 6, P 4 O 10, SO 3, I 2 O 5, CrO 3). Most acid oxides are molecular substances (exceptions are B 2 O 3, SiO 2, solid SO 3, CrO 3 and some others; there are also non-molecular modifications of P 2 O 5). But non-molecular acid oxides also become molecular upon transition to a gaseous state.
The following are characteristic of acid oxides: Chemical properties.
1) All acidic oxides react with strong bases as with solids:
CO 2 + Ca(OH) 2 = CaCO 3 + H 2 O
SiO 2 + 2KOH = K 2 SiO 3 + H 2 O (when heated),
and with alkali solutions (§ 12.8):
SO 3 + 2OH = SO 4 2 + H 2 O, N 2 O 5 + 2OH = 2NO 3 + H 2 O,
SO 3 + 2NaOH р = Na 2 SO 4р + H 2 O, N 2 O 5 + 2KOH р = 2KNO 3р + H 2 O.
The reason for reactions with solid hydroxides is the same as with oxides (see § 13.4).
The most active acidic oxides (SO 3, CrO 3, N 2 O 5, Cl 2 O 7) can also react with insoluble (weak) bases.
2) Acidic oxides react with basic oxides (§ 13.4):
CO 2 + CaO = CaCO 3
P 4 O 10 + 6FeO = 2Fe 3 (PO 4) 2 (when heated)
3) Many acidic oxides react with water (§11.4).
N 2 O 3 + H 2 O = 2HNO 2 SO 2 + H 2 O = H 2 SO 3 (more correct writing of the formula of sulfurous acid -SO 2. H 2 O
N 2 O 5 + H 2 O = 2HNO 3 SO 3 + H 2 O = H 2 SO 4
Many acid oxides can be received by oxidation with oxygen (combustion in oxygen or in air) of the corresponding simple substances (C gr, S 8, P 4, P cr, B, Se, but not N 2 and not halogens):
C + O 2 = CO 2,
S 8 + 8O 2 = 8SO 2,
or upon decomposition of the corresponding acids:
H 2 SO 4 = SO 3 + H 2 O (with strong heating),
H 2 SiO 3 = SiO 2 + H 2 O (when dried in air),
H 2 CO 3 = CO 2 + H 2 O (at room temperature in solution),
H 2 SO 3 = SO 2 + H 2 O (at room temperature in solution).
The instability of carbonic and sulfurous acids makes it possible to obtain CO 2 and SO 2 by the action of strong acids on carbonates Na 2 CO 3 + 2HCl p = 2NaCl p + CO 2 +H 2 O
(the reaction occurs both in solution and with solid Na 2 CO 3), and sulfites
K 2 SO 3tv + H 2 SO 4conc = K 2 SO 4 + SO 2 + H 2 O (if there is a lot of water, sulfur dioxide is not released as a gas).
8th grade
Lesson type. Acquiring new knowledge.
Goals. Educational – explain the essence of metabolic reactions; teach students to write equations of exchange reactions.
Developmental– develop the ability to pose simple problems, formulate hypotheses and test them experimentally, based on knowledge of chemistry; improve skills in working with laboratory equipment and reagents, documenting the results of educational experiments; to form abilities for adequate self- and mutual control.
Educational– continue the formation of students’ scientific worldview; cultivate a culture of communication through work in pairs student-student, teacher-student; to cultivate such personality qualities as observation, inquisitiveness, initiative, and the desire for independent search.
Methods and methodological techniques. Frontal survey; independent work with cards, mutual checking of the results of independent work in pairs, marking; performing laboratory work in pairs, independently filling out a report on laboratory work; work with visual aids (D.I. Mendeleev’s periodic table of chemical elements, table of solubility of substances, cards).
Equipment and reagents. An overhead projector, a table for compiling a report for the laboratory work “Exchange Reactions,” cards with tasks for independent work on the topic “Types of Chemical Reactions,” a laboratory stand with test tubes, a crystallizer, an alcohol lamp, a test tube holder, matches; copper(II) oxide, solutions of sodium and potassium hydroxides, hydrochloric and sulfuric acids, iron(III) chloride, phenolphthalein.
DURING THE CLASSES
Updating knowledge
The lesson begins with a frontal conversation on the material studied*. During the conversation, the teacher asks questions. For each correct answer, a chip is awarded. At the end of the lesson, marks are given based on the number of chips collected. Criteria for converting the number of chips to a mark: on “5” you need to score 5 chips, on “4” – 4 chips.
Teacher. We are studying the chapter “Changes that occur in substances.” Such changes can be physical or chemical. What is the difference between a chemical phenomenon and a physical one?
Student. As a result of a chemical phenomenon, the composition of a substance changes, but as a result of a physical phenomenon, it does not.
Teacher. What signs can be used to determine that a chemical reaction has occurred?(Each answerer must name only one sign of a chemical reaction.)
Students. Change in color, release of gas, precipitation or dissolution of sediment, appearance of odor, release of light, release of heat.
Teacher. What is a chemical equation called?
Student. A chemical equation is a conventional representation of a chemical reaction using chemical formulas and mathematical symbols.
Teacher. What types of chemical reactions do you know?
Student. We know three types of chemical reactions: combination, decomposition, substitution.
Teacher. Define a compound reaction and give an example of such a chemical reaction.
Student. A compound reaction is a reaction in which two or more simple or complex substances combine to form one complex substance. For example, when two simple substances oxygen and hydrogen combine, the complex substance water is formed:
2H 2 + O 2 = 2H 2 O.
Teacher. What reaction is called a decomposition reaction? Give an example of a decomposition reaction.
Student. A decomposition reaction is a reaction in which several simple or complex substances are obtained from one complex substance. For example, when the complex substance malachite decomposes, three new complex substances are formed: copper(II) oxide, water and carbon dioxide:
(CuOH) 2 CO 3 2CuO + H 2 O + CO 2 .
Teacher. What reaction is called a substitution reaction? Give an example of such a reaction.
Student. A substitution reaction is a reaction in which a simple substance replaces one type of atom in a complex substance. For example, if you dip an iron nail into a solution of copper(II) sulfate, the iron will displace copper from the salt solution:
Fe + CuSO 4 = FeSO 4 + Cu.
Teacher. You have learned the material well about the types of chemical reactions. Try to apply your theoretical knowledge in practice. Determine the types of chemical reactions, the schemes of which are given in the cards for independent work. In addition, you need to arrange the coefficients in the reaction equations.
Independent work (7–8 min)
Exercise. Arrange the coefficients in the reaction equations and indicate the type of each reaction.
Option 1
CO + O 2 CO 2, NaNO 3 NaNO 2 + O 2,
CuO + Al Al 2 O 3 + Cu,
AgNO 3 + Cu Cu(NO 3) 2 + Ag,
HBr H 2 + Br 2, Ca + O 2 CaO.
Option 2
Fe + O 2 Fe 3 O 4, KClO 3 KCl + O 2,
Al + HCl AlCl 3 + H 2, Al + O 2 Al 2 O 3,
Fe + HCl FeCl 2 + H 2, KNO 3 KNO 2 + O 2.
Criteria for evaluation
You can score a maximum of 6 points (0.5 points for correctly placed coefficients in each equation and 0.5 points for the correctly indicated type of reaction).
At “5” – 6–5.5 points,
for “4” – 5–4.5 points,
for “3” – 4–3 points.
After completing the assignments, students sitting at the same desk exchange their work. The work is mutually checked using an overhead projector and marks are given according to the above criteria.
Teacher. Guys, raise your hands who did the job with an “A”. Who did it with a 4? So, to summarize today's independent work, I can say that you are well aware of three types of chemical reactions: combination, decomposition and substitution reactions. We are faced with the task of studying another type of chemical reactions - exchange reactions.
Learning new material
(using chips)
Teacher. Based on the name of the reaction type, guess what the essence of the exchange reaction is.
Student. The essence of such a reaction is that substances exchange their components.
Teacher. What substances - simple or complex - can exchange their constituent parts?
Student. Both substances must be complex.
Teacher. What does the general scheme of an exchange reaction look like?
The student writes on the board the general scheme of the exchange reaction:
AB + CD = AD + CB.
Students return to the summary table (Table 1) for the types of chemical reactions made in the two previous lessons, and, under the guidance of the teacher, fill out the last line in this table.
Table 1
Classification of reactions based on
quantity and composition of reacting substances
| Reaction type | Reaction equations in general form |
|---|---|
| Compound reaction | The combination of two (several) simple substances into one complex substance: A + B = AB. The combination of two binary substances into one three-element complex substance: AB + CB = DIA 2 |
| Decomposition reaction | Decomposition of a complex substance into two (several) simple substances: Decomposition of a three-element complex substance into two binary substances: DIA 2 = AB + BC |
| Substitution reaction | The interaction of a simple substance with a complex one, as a result of which other - simple and complex - substances are formed: AB + C = A + CB |
| Exchange reaction | The interaction of two complex substances to form two other complex substances: AB + CD = AD + CB |
Teacher. An exchange reaction is a reaction between two complex substances that exchange their constituent parts.
We examined the essence of the exchange reaction from a theoretical point of view. To practically check whether exchange reactions really occur between complex substances, we will carry out laboratory work. (Students receive cards with a table (Table 2) to compile a report on the laboratory work “Exchange Reactions.”) The table contains a column that gives an idea of what needs to be done. You will fill in the other two columns after completing the experiments.
table 2
Laboratory work “Exchange reactions”
| Experience no. | Work progress (what needs to be done) | Observations (what we saw) | Chemical reaction equations, conclusions |
|---|---|---|---|
| 1 | Pour sodium hydroxide solution into a test tube, add a drop of phenolphthalein solution, then add hydrochloric acid solution | A chemical reaction has occurred: NaOH + HCl = NaCl + H 2 O. |
|
| 2 | Pour potassium hydroxide solution into a test tube, add a drop of phenolphthalein solution, then add sulfuric acid solution | The indicator in the alkali solution turned crimson, and when acid was added it became discolored | A chemical reaction has occurred: 2KOH + H 2 SO 4 = This is an exchange reaction, because alkali and acid exchange their constituent parts |
| 3 | a) Add sodium hydroxide solution drop by drop to a solution of iron(III) chloride | A brown precipitate fell | A chemical reaction has occurred: FeCl 3 + 3NaOH = This is an exchange reaction, because salt and alkali exchanged their constituents |
| b) Add a solution of sulfuric acid to the resulting precipitate | The brown precipitate dissolved | A chemical reaction has occurred: 2Fe(OH) 3 + 3H 2 SO 4 = This is an exchange reaction, because insoluble base and acid exchange their constituents |
|
| 4 | Pour copper(II) oxide powder into a test tube, add sulfuric acid and heat in the upper flame of an alcohol lamp. | The black powder dissolved to form a blue solution | A chemical reaction has occurred: CuO + H 2 SO 4 = CuSO 4 + H 2 O. This is an exchange reaction, because oxide and acid exchange their constituent parts |
Before you begin performing experiments, remember that you need to work with solutions of acids and alkalis carefully, because they are dangerous. Work with solutions according to the “do not spill” principle, and with solids - according to the “do not spill” principle. Heat the test tube with substances in the upper part of the flame of an alcohol lamp, first heating the entire test tube, and then its bottom.
Who can say what the rules are for using an alcohol lamp?
Student. First you need to check the tank of the alcohol lamp, adjust the wick, then light it. After heating, extinguish the flame of the alcohol lamp with the cap.
Experiments No. 1 and 2 are being carried out.
FRONTAL CONVERSATION
Teacher. Why did we use phenolphthalein during the experiments?
Student. Phenolphthalein is used so that one can see how the solution environment changes from alkaline to neutral. Since the starting materials and reaction products are colorless, a change in the color of the indicator will be a sign of a chemical reaction.
Teacher. Check the correctness of writing the reaction equations for the first and second experiments(it is proposed to write the reaction equations on code tape). Are these reactions exchange reactions?
Student. The reaction between an alkali and an acid is an exchange reaction in which two complex substances exchange their constituents.
Teacher. Why is the reaction between an alkali and an acid called a neutralization reaction?
Student. In a neutralization reaction, an acid neutralizes an alkali to produce salt and water.
Teacher. We investigated the interaction between alkali and acid. However, bases are not only soluble, but also insoluble. Will a reaction occur between an insoluble base and an acid? Will this reaction be an exchange reaction, and also a neutralization reaction? Can anyone solve this problem?
Student. It is necessary to conduct an experiment between an insoluble base and an acid..
Teacher. First, by reacting an iron(III) salt with sodium alkali, we obtain an insoluble base. To do this, we will conduct experiment 3a. Then let's see if an insoluble base can interact with an acid - experiment 3b.
(discussion of experimental results)
Teacher. By what signs can you determine that the reactions have passed?
Student. In the first case, a precipitate formed; in the second case, the precipitate dissolved and a brown solution was obtained..
Teacher. Check the correctness of the written reaction equations(it is proposed to write the reaction equations on code tape). Do these reactions relate to exchange reactions?
Student. These reactions belong to exchange reactions, because they involve complex substances that exchange components.
Teacher. Please note that in experiment 3a, a salt and an alkali enter into the exchange reaction, and in the case of experiment 3b, an insoluble base and an acid. Is the reaction between an insoluble base and an acid a neutralization reaction?
Student. Yes, because As a result of this reaction, salt and water are formed.
Teacher. Between what substances does a neutralization reaction occur?
Student. The neutralization reaction occurs between acids and bases, both soluble and insoluble.
Teacher. The neutralization reaction is a special case of an exchange reaction. Substances of what other classes of compounds can enter into exchange reactions?
Student. Basic oxides also undergo exchange reactions.
Teacher. In order to solve this problem, let's conduct experiment 4. During the experiment, do not forget about the rules for heating substances.
FRONTAL CONVERSATION
(discussion of experimental results)
Teacher. What signs indicate that the reaction has passed?
Student. The precipitate dissolved, a blue solution was formed.
Teacher. How did you write the reaction equation?(The student writes down the reaction equation at the blackboard.) So, the metal oxide and acid enter into an exchange reaction.
Closing conversation
Teacher. How many types of chemical reactions do you now know?
Student. We know four types of chemical reactions: combination, decomposition, substitution and exchange reactions.
Teacher. Between which classes of substances can exchange reactions occur?
Student. Exchange reactions can occur between bases and acids, acids and basic oxides, salts and alkalis.
Teacher. What reaction is called the neutralization reaction?
Student. A neutralization reaction is an exchange reaction between a base and an acid, resulting in the formation of salt and water.
Teacher. Two soluble salts also enter into an exchange reaction if an insoluble salt is formed as a result. For example:
AgNO 3 + NaCl = AgCl + NaNO 3,
BaCl 2 + MgSO 4 = BaSO 4 + MgCl 2.
The teacher gives marks based on the number of chips collected.
Homework. According to the textbook by O.S. Gabrielyan “Chemistry-8” § 27, ex. 2b, 3a, p. 100.
* See No. 7, 10/2006
Literature
Gabrielyan O.S.. Chemistry-8. M.: Bustard, 2002, 208 pp.; Gabrielyan O.S., Voskoboynikova N.P., Yashukova A.V. Teacher's handbook. 8th grade. M.: Bustard, 2002, 416 pp.; Gabrielyan O.S., Smirnova T.V.. We study chemistry in 8th grade. Methodological guide to the textbook by O.S. Gabrielyan “Chemistry-8” for students and teachers. M.: Blik and Co., 2001, 224 pp.; Kuznetsova N.E., Titova I.M., Gara N.N., Zhegin A.Yu.. Chemistry. 8th grade. M.: Ventana-Graf, 2003, 224 p.
The content of the article
MALACHITE– is a copper compound, the composition of natural malachite is simple: it is basic copper carbonate (CuOH) 2 CO 3, or CuCO 3 ·Cu(OH) 2. This compound is thermally unstable and easily decomposes when heated, even not very strongly. If you heat malachite above 200 o C, it will turn black and turn into black powder of copper oxide, and at the same time water vapor and carbon dioxide will be released: (CuOH) 2 CO 3 ® 2CuO + CO 2 + H 2 O. However, getting malachite again is a very difficult task : This could not be done for many decades, even after the successful synthesis of diamond.
It is not easy to obtain even a compound of the same composition as malachite. If you merge solutions of copper sulfate and sodium carbonate, you will get a loose, voluminous blue precipitate, very similar to copper hydroxide Cu(OH) 2; At the same time, carbon dioxide will be released. But after about a week, the loose blue sediment will become very dense and take on a green color. Repeating the experiment with hot solutions of reagents will lead to the fact that the same changes in the sediment will occur within an hour.
The reaction of copper salts with alkali metal carbonates was studied by many chemists from different countries, but the results of the analysis of the resulting precipitates varied among different researchers, sometimes significantly. If you take too much carbonate, no precipitate will form at all, but you will get a beautiful blue solution containing copper in the form of complex anions, for example, 2–. If you take less carbonate, a voluminous jelly-like precipitate of light blue color falls out, foamed with bubbles of carbon dioxide. Further transformations depend on the ratio of reagents. With an excess of CuSO 4, even a small one, the precipitate does not change over time. With an excess of sodium carbonate, after 4 days the blue precipitate sharply (6 times) decreases in volume and turns into green crystals, which can be filtered, dried and ground into a fine powder, which is close in composition to malachite. If you increase the concentration of CuSO 4 from 0.067 to 1.073 mol/l (with a slight excess of Na 2 CO 3), then the time for the transition of the blue precipitate to green crystals decreases from 6 days to 18 hours. Obviously, in the blue jelly, over time, nuclei of the crystalline phase are formed, which gradually grow. And green crystals are much closer to malachite than shapeless jelly.
Thus, in order to obtain a precipitate of a certain composition corresponding to malachite, you need to take a 10% excess of Na 2 CO 3, a high concentration of reagents (about 1 mol/l) and keep the blue precipitate under the solution until it turns into green crystals. By the way, the mixture obtained by adding soda to copper sulfate has long been used against harmful insects in agriculture under the name “Burgundy mixture.”
Soluble copper compounds are known to be poisonous. Basic copper carbonate is insoluble, but in the stomach under the influence of hydrochloric acid it easily turns into soluble chloride: (CuOH) 2 CO 3 + 2HCl ® 2CuCl 2 + CO 2 + H 2 O. Is malachite dangerous in this case? It was once considered very dangerous to prick yourself with a copper pin or hairpin, the tip of which turned green, indicating the formation of copper salts - mainly basic carbonate under the influence of carbon dioxide, oxygen and moisture in the air. In fact, the toxicity of basic copper carbonate, including that which forms in the form of a green patina on the surface of copper and bronze products, is somewhat exaggerated. As special studies have shown, the dose of basic copper carbonate that is lethal for half of the tested rats is 1.35 g per 1 kg of weight for males and 1.5 g for females. The maximum safe single dose is 0.67 g per 1 kg. Of course, a person is not a rat, but malachite is clearly not potassium cyanide. And it’s hard to imagine that anyone would eat half a glass of powdered malachite. The same can be said about basic copper acetate (historical name is verdigris), which is obtained by treating the basic carbonate with acetic acid and is used, in particular, as a pesticide. Much more dangerous is another pesticide known as “Paris green”, which is a mixture of basic copper acetate with its arsenate Cu(AsO 2) 2.
Chemists have long been interested in the question of whether there is not basic, but simple copper carbonate CuCO 3. In the table of salt solubility there is a dash in place of CuCO 3, which means one of two things: either this substance is completely decomposed by water, or it does not exist at all. Indeed, for a whole century no one managed to obtain this substance, and all textbooks wrote that copper carbonate does not exist. However, in 1959 this substance was obtained, albeit under special conditions: at 150 ° C in an atmosphere of carbon dioxide under a pressure of 60–80 atm.
Malachite as a mineral.
Natural malachite is always formed where there are deposits of copper ores, if these ores occur in carbonate rocks - limestones, dolomites, etc. Often these are sulfide ores, of which the most common are chalcocite (another name is chalcokite) Cu 2 S, chalcopyrite CuFeS 2, bornite Cu 5 FeS 4 or 2Cu 2 S·CuS·FeS, covellite CuS. When copper ore weathers under the influence of groundwater, in which oxygen and carbon dioxide are dissolved, copper goes into solution. This solution, containing copper ions, slowly seeps through the porous limestone and reacts with it to form the basic copper carbonate, malachite. Sometimes droplets of solution, evaporating in the voids, form deposits, something like stalactites and stalagmites, only not calcite, but malachite. All stages of the formation of this mineral are clearly visible on the walls of a huge copper ore quarry up to 300–400 m deep in the province of Katanga (Zaire). The copper ore at the bottom of the quarry is very rich - it contains up to 60% copper (mainly in the form of chalcocite). Chalcocite is a dark silver mineral, but in the upper part of the ore layer all its crystals turned green, and the voids between them were filled with a solid green mass - malachite. This was precisely in those places where surface water penetrated through rock containing a lot of carbonates. When they met chalcocite, they oxidized sulfur, and copper in the form of basic carbonate settled right there, next to the destroyed chalcocite crystal. If there was a void in the rock nearby, malachite stood out there in the form of beautiful deposits.
So, for the formation of malachite, the proximity of limestone and copper ore is necessary. Is it possible to use this process to artificially obtain malachite under natural conditions? Theoretically, this is not impossible. For example, it was proposed to use this technique: pour cheap limestone into old underground workings of copper ore. There will also be no shortage of copper, since even with the most advanced mining technology it is impossible to avoid losses. To speed up the process, water must be supplied to the production. How long can such a process last? Typically, the natural formation of minerals is an extremely slow process and takes thousands of years. But sometimes mineral crystals grow quickly. For example, gypsum crystals under natural conditions can grow at a rate of up to 8 microns per day, quartz - up to 300 microns (0.3 mm), and the iron mineral hematite (bloodstone) can grow by 5 cm in one day. Laboratory studies have shown that and malachite can grow at a rate of up to 10 microns per day. At this speed, in favorable conditions, a ten-centimeter crust of a magnificent gem will grow in about thirty years - this is not such a long time: even forest plantations are designed for 50, or even 100 years or even more.
However, there are cases when discoveries of malachite in nature do not please anyone. For example, as a result of many years of treatment of vineyard soils with Bordeaux mixture, real malachite grains are sometimes formed under the arable layer. This man-made malachite is obtained in the same way as natural one: Bordeaux mixture (a mixture of copper sulfate and milk of lime) seeps into the soil and meets with lime deposits underneath it. As a result, the copper content in the soil can reach 0.05%, and in the ash of grape leaves - more than 1%!
Malachite also forms on products made of copper and its alloys - brass, bronze. This process occurs especially quickly in large cities, where the air contains oxides of sulfur and nitrogen. These acidic agents, together with oxygen, carbon dioxide and moisture, promote corrosion of copper and its alloys. In this case, the color of the main copper carbonate formed on the surface has an earthy tint.
Malachite in nature is often accompanied by the blue mineral azurite - copper azure. This is also basic copper carbonate, but of a different composition - 2CuCO 3 ·Cu(OH) 2. Azurite and malachite are often found together; their banded intergrowths are called azuromalachite. Azurite is less stable and gradually turns green in humid air, turning into malachite. Thus, malachite is not at all rare in nature. It even covers ancient bronze things that are found during archaeological excavations. Moreover, malachite is often used as copper ore: it contains almost 56% copper. However, these tiny malachite grains are of no interest to stone seekers. More or less large crystals of this mineral are found very rarely. Typically, malachite crystals are very thin - from hundredths to tenths of a millimeter, and up to 10 mm in length, and only occasionally, under favorable conditions, can huge multi-ton deposits of a dense substance consisting of a mass of seemingly stuck together crystals form. It is these deposits that form jewelry malachite, which is very rare. Thus, in Katanga, to obtain 1 kg of jewelry malachite, about 100 tons of ore must be processed.
There were once very rich deposits of malachite in the Urals; Unfortunately, at present they are practically depleted. Ural malachite was discovered back in 1635, and in the 19th century. Up to 80 tons of malachite of unsurpassed quality were mined there per year, and malachite was often found in the form of rather weighty blocks. The largest of them, weighing 250 tons, was discovered in 1835, and in 1913 a block weighing more than 100 tons was found. Solid masses of dense malachite were used for decoration, and individual grains distributed in the rock - the so-called earthy malachite, and small accumulations of pure malachite were used to produce high-quality green paint, “malachite green” (this paint should not be confused with “malachite green”, which is an organic dye, and the only thing it has in common with malachite is its color). Before the revolution in Yekaterinburg and Nizhny Tagil, the roofs of many mansions were painted with malachite in a beautiful bluish-green color. Malachite also attracted Ural copper smelters. But copper was mined only from a mineral that was of no interest to jewelers and artists. Solid pieces of dense malachite were used only for decoration.
Malachite as decoration.
Anyone who has seen products made from malachite will agree that this is one of the most beautiful stones. Shimmers of various shades from blue to deep green, combined with a bizarre pattern, give the mineral a unique identity. Depending on the angle of incidence of light, some areas may appear lighter than others, and when the sample is rotated, a “crossing” of light is observed - the so-called moiré or silky tint. According to the classification of academician A.E. Fersman and German mineralogist M. Bauer, malachite occupies the highest first category among semi-precious stones, along with rock crystal, lapis lazuli, jasper, and agate.
The mineral derives its name from the Greek malache - mallow; The leaves of this plant, like malachite, are bright green. The term “malachite” was introduced in 1747 by the Swedish mineralogist J.G. Vallerius.
Malachite has been known since prehistoric times. The oldest known malachite product is a pendant from a Neolithic burial ground in Iraq, which is more than 10.5 thousand years old. Malachite beads found in the vicinity of ancient Jericho are 9 thousand years old. In Ancient Egypt, malachite mixed with fat was used in cosmetics and hygienic purposes. They used it to paint the eyelids green: copper is known to have bactericidal properties. Powdered malachite was used to make colored glass and glaze. Malachite was also used for decorative purposes in Ancient China.
In Russia, malachite has been known since the 17th century, but its widespread use as a jewelry stone began only at the end of the 18th century, when huge malachite monoliths were found at the Gumeshevsky mine. Since then, malachite has become a ceremonial facing stone decorating palace interiors. From the middle of the 19th century. For these purposes, tens of tons of malachite were brought annually from the Urals. Visitors to the State Hermitage can admire the Malachite Hall, the decoration of which took two tons of malachite; There is also a huge malachite vase there. Products made from malachite can also be seen in the Catherine Hall of the Grand Kremlin Palace in Moscow. But the columns at the altar of St. Isaac's Cathedral in St. Petersburg, about 10 m high, can be considered the most remarkable product in terms of beauty and size. It seems to the uninitiated that both the vase and the columns are made of huge solid pieces of malachite. Actually this is not true. The products themselves are made of metal, gypsum, and other materials, and only the outside is lined with malachite tiles, cut from a suitable piece - a kind of “malachite plywood”. The larger the original piece of malachite, the larger the size of the tiles that could be cut from it. And to save valuable stone, the tiles were made very thin: their thickness sometimes reached 1 mm! But that wasn’t even the main trick. If you simply lay out any surface with such tiles, then nothing good will come of it: after all, the beauty of malachite is determined largely by its pattern. It was necessary that the pattern of each tile be a continuation of the pattern of the previous one.
A special method of cutting malachite was brought to perfection by the malachite masters of the Urals and Peterhof, and therefore it is known throughout the world as “Russian mosaic”. In accordance with this method, a piece of malachite is sawn perpendicular to the layered structure of the mineral, and the resulting tiles seem to “unfold” in the form of an accordion. In this case, the pattern of each subsequent tile is a continuation of the pattern of the previous one. With such sawing, a relatively small piece of mineral can be used to cover a large area with a single, continuous pattern. Then, using a special mastic, the resulting tiles were pasted over the product, and this work also required the greatest skill and art. Craftsmen sometimes managed to “stretch” the malachite pattern through a rather large product.
In 1851 Russia took part in the World Exhibition in London. Among other exhibits there was, of course, “Russian mosaic”. Londoners were especially struck by the doors in the Russian pavilion. One of the local newspapers wrote about this: “The transition from a brooch, which is decorated with malachite like a precious stone, to colossal doors seemed incomprehensible: people refused to believe that these doors were made of the same material that everyone was accustomed to consider a jewel.” A lot of jewelry is also made from Ural malachite ( Malachite Box Bazhova).
Artificial malachite.
The fate of any large malachite deposit (and you can count them on one hand in the world) is the same: first, large pieces are mined there, from which vases, writing instruments, and boxes are made; then the sizes of these pieces are gradually reduced, and they are used mainly to make inserts into pendants, brooches, rings, earrings and other small jewelry. In the end, the deposit of ornamental malachite is completely depleted, as happened with the Ural deposits. And although malachite deposits are currently known in Africa (Zaire, Zambia), Australia (Queensland), and the USA (Tennessee, Arizona), the malachite mined there is inferior in color and design beauty to that of the Urals. It is not surprising that considerable effort was devoted to obtaining artificial malachite. But while it is relatively easy to synthesize basic copper carbonate, it is very difficult to obtain real malachite - after all, the precipitate obtained in a test tube or reactor, corresponding in composition to malachite, and a beautiful gem differ from each other no less than a nondescript piece of chalk from a piece of snow-white marble
It seemed that there would be no big problems here: the researchers already had such achievements as the synthesis of diamond, emerald, amethyst, and many other precious stones and minerals. However, numerous attempts to obtain a beautiful mineral, and not just a green powder, did not lead to anything, and jewelry and ornamental malachite for a long time remained one of the few natural gems, the production of which was considered almost impossible.
In principle, there are several ways to obtain artificial minerals. One of them is the creation of composite materials by sintering natural mineral powder in the presence of an inert binder at high pressure. In this case, many processes occur, the main ones being compaction and recrystallization of the substance. This method has become widespread in the USA for obtaining artificial turquoise. Jadeite, lapis lazuli, and other semi-precious stones were also obtained. In our country, composites were obtained by cementing small fragments of natural malachite ranging in size from 2 to 5 mm using organic hardeners (like epoxy resins) with the addition of dyes of the appropriate color and fine powder of the same mineral as a filler. The working mass, composed of the indicated components in a certain percentage, was subjected to compression at pressures up to 1 GPa (10,000 atm) while simultaneously heating above 100 ° C. As a result of various physical and chemical processes, all components were firmly cemented into a solid mass that is well polished . In one working cycle, four plates with a side of 50 mm and a thickness of 7 mm are thus obtained. True, they are quite easy to distinguish from natural malachite.
Another possible method is hydrothermal synthesis, i.e. obtaining crystalline inorganic compounds under conditions simulating the processes of formation of minerals in the bowels of the earth. It is based on the ability of water to dissolve at high temperatures (up to 500 ° C) and pressures up to 3000 atm substances that are practically insoluble under normal conditions - oxides, silicates, sulfides. Every year, hundreds of tons of rubies and sapphires are obtained using this method, and quartz and its varieties, for example, amethyst, are successfully synthesized. It was in this way that malachite was obtained, almost no different from natural one. In this case, crystallization is carried out under milder conditions - from slightly alkaline solutions at a temperature of about 180 ° C and atmospheric pressure.
The difficulty in obtaining malachite was that for this mineral the main thing is not chemical purity and transparency, which is important for stones such as diamond or emerald, but its color shades and texture - a unique pattern on the surface of a polished sample. These properties of the stone are determined by the size, shape, and mutual orientation of the individual crystals of which it consists. One malachite “bud” is formed by a series of concentric layers of different thicknesses - from fractions of a millimeter to 1.5 cm in different shades of green. Each layer consists of many radial fibers (“needles”), tightly adjacent to each other and sometimes indistinguishable to the naked eye. The intensity of the color depends on the thickness of the fibers. For example, fine-crystalline malachite is noticeably lighter than coarse-crystalline malachite, therefore the appearance of malachite, both natural and artificial, depends on the rate of nucleation of new crystallization centers during its formation. It is very difficult to regulate such processes; That is why this mineral was not amenable to synthesis for a long time.
Three groups of Russian researchers succeeded in obtaining artificial malachite, which is not inferior to natural malachite - at the Research Institute for the Synthesis of Mineral Raw Materials (city of Aleksandrov, Vladimir Region), at the Institute of Experimental Mineralogy of the Russian Academy of Sciences (Chernogolovka, Moscow Region) and at St. Petersburg State University. Accordingly, several methods for the synthesis of malachite have been developed, making it possible to obtain under artificial conditions almost all the textural varieties characteristic of natural stone - banded, pleated, kidney-shaped. It was possible to distinguish artificial malachite from natural one only by methods of chemical analysis: artificial malachite did not contain impurities of zinc, iron, calcium, phosphorus, characteristic of natural stone. The development of methods for the artificial production of malachite is considered one of the most significant achievements in the field of synthesis of natural analogues of precious and ornamental stones. Thus, in the museum of the mentioned institute in Aleksandrov there is a large vase made from malachite synthesized here. The institute learned not only to synthesize malachite, but even to program its pattern: satin, turquoise, star-shaped, plush... In all its properties, synthetic malachite can replace natural stone in jewelry and stone cutting. It can be used for cladding architectural details both inside and outside buildings.
Artificial malachite with a beautiful thin-layered pattern is also produced in Canada and in a number of other countries.
Ilya Leenson
The purpose of the lesson: continue the formation of the concept of substance, introduce students to complex substances, methods of proving their complexity - analysis and synthesis.
During the classes
1. Frontal survey.
Which substances are classified as simple: a) Diamond, b) Water, c) Table salt?
What two groups are simple substances divided into if there is a clear boundary between them?
What properties and structures do metals and non-metals have?
How to express the composition of a simple substance (molecular and non-molecular)?
Paperwork.
Make up chemical formulas of molecular simple substances, the models of which are shown in the textbook.
Write the formulas of simple substances formed by elements of the third period.
These exercises are of particular importance, as they help them connect the internal structure of a substance with its iconic model (formula).
2. Discussion of new material.
Questions:
- Discussion of the elemental composition of substances using known examples;
- Experimental proof of the complexity of matter - synthesis of a complex substance;
- Substance analysis;
- Discussion of the structures of complex substances.
We demonstrate a number of simple and complex substances: copper oxide, graphite, quartz (or river sand), basic copper carbonate (malachite), sulfur, hydrogen, carbon dioxide, water. Which of these substances consist of one element, and which of two or more? Students can name sulfur and hydrogen as consisting of one element, and water, based on previous experience, as consisting of two elements. At the same time, they can tell how to prove that water consists of two elements. We conclude that it is impossible to recognize simple and complex substances by their appearance. We need to explore them.
What do we call those substances that consist of one element?
What do we call substances that consist of two or more elements?
As a rule, children answer accurately – complex substances. Let's formulate the definition. Students need to be involved in this.
How to conduct an experiment to prove whether a substance is complex or simple? The substance needs to be decomposed.
By what signs do we know that a substance is complex? If new substances are obtained from it, then it is complex.
Here it is necessary to explain that determining the composition of a substance using decomposition is called analysis, and that decomposition is often carried out using heating. It is very helpful to have students carry out the experiments themselves. Decomposition devices (a test tube with a gas outlet tube mounted in a stand) should be prepared on the student tables. We pour malachite (on some tables) and potassium permanganate (on others) into a test tube. I tell students the names of substances not for memorization, although they remember them already in the first lessons. Students are tasked with proving that these substances are complex.
Before the experiments, I introduce the guys to the rules of working with an alcohol lamp. Students in the group studying malachite need to place a glass of lime water under the gas outlet tube. Another group studying potassium permanganate is a glass of clean water.
How many new substances did the students receive?
When malachite decomposes, three substances are clearly visible: gas, water droplets (on the walls of the test tube), and a black substance remaining in the test tube. Carbon dioxide is tested by the turbidity of lime water. The teacher reports that the black substance remaining in the test tube is copper oxide.
During the decomposition of potassium permanganate, observations are complicated by the masking of the resulting black oxide and almost the same color of manganate, which outwardly differ little from the potassium permanganate taken. Students name two substances as a result of the experiment - a gas and a black solid.
Students test the released gas in an empty glass by bringing a smoldering splinter, which lights up brightly.
I examine the isolated second substance myself. To do this, I dissolve the resulting substance as a result of decomposition and the starting substance – potassium permanganate – in two glasses of water. Potassium permanganate gives a crimson color, and the substance, as a result of decomposition, gives a green color.
Students see the difference between the two substances and conclude that the decomposition of potassium permanganate produces two different substances. Based on the research in groups, students fill out the table.
I bring students to a general conclusion: those substances that decompose into two or more new ones consist of several elements and belong to complex substances, and those that cannot be decomposed consist of one element and belong to simple ones.
Next I move on to the concept of synthesis. I demonstrate an experiment: I heat iron filings with sulfur powder. What substance is formed as a result - stable or complex? What elements does it consist of? The students answer - made of sulfur and iron. This means that we conclude that with the help of synthesis, complex substances can be obtained from simple substances. Based on experience, students give the concept of synthesis.
3. Consolidation.
For reinforcement, I show a poster with drawings of the structures of complex and simple substances. Where students isolate complex substances. Next, students answer the question - what are complex substances and give examples. Based on the material studied, we conclude: complex substances have molecular (carbon dioxide) and non-molecular structures (manganese oxide).
Homework: pp. 4-6, exercise 4.
