1. Based on changes in the oxidation states of elements in molecules of reacting substances, all reactions are divided into:
a) redox reactions (reactions with electron transfer);
b) non-redox reactions (reactions without electron transfer).
2. By the sign of the thermal effect all reactions are divided into:
a) exothermic (going with the release of heat);
b) endothermic (coming with heat absorption).
3. Based on homogeneity of the reaction system reactions are divided into:
a) homogeneous (flowing in a homogeneous system);
b) heterogeneous (flowing in a heterogeneous system)
4. Depending on presence or absence of a catalyst reactions are divided into:
a) catalytic (going with the participation of a catalyst);
b) non-catalytic (going without a catalyst).
5. Based on reversibility all chemical reactions are divided into:
a) irreversible (flowing in only one direction);
b) reversible (flowing simultaneously in forward and reverse directions).
Consider another commonly used classification.
By the number and composition of the starting materials (reagents) and reaction products the following most important types of chemical reactions can be distinguished:
a) compound reactions; b) decomposition reactions;
v) substitution reactions; G) exchange reactions.
Compound reactions are reactions in the course of which one substance of a more complex composition is formed from two or more substances:
A + B + ... = B.
There are a large number of reactions of the combination of simple substances (metals with non-metals, non-metals with non-metals), for example:
Fe + S = FeS 2Na + H 2 = 2NaH
S + О 2 = SO 2 Н 2 + Сl 2 = 2HCl
The reactions of compounding simple substances are always redox reactions. Typically, these reactions are exothermic.
Complex substances can also participate in the reactions of the compound, for example:
CaO + SO 3 = CaSO 4 K 2 O + H 2 O = 2KON
CaCO 3 + CO 2 + H 2 O = Ca (HCO 3) 2
In the above examples, the oxidation states of the elements do not change during the course of the reactions.
There are also reactions of the combination of simple and complex substances that belong to redox reactions, for example:
2FеС1 2 + Сl 2 = 2FеСl 3 2SО 2 + О 2 = 2SO 3
· Decomposition reactions are reactions, during the course of which two or more simpler substances are formed from one complex substance: A = B + C + ...
Decomposition products of the initial substance can be both simple and complex substances, for example:
2Fе (ОН) 3 = Fe 2 О 3 + 3Н 2 О ВаСО 3 = ВаО + СО 2
2АgNO 3 = 2Аg + 2NO 2 + О 2
Decomposition reactions usually occur when substances are heated and are endothermic reactions. Like compound reactions, decomposition reactions can proceed with or without a change in the oxidation states of the elements.
Substitution reactions are reactions between simple and complex substances, during which the atoms of a simple substance replace the atoms of one of the elements in the molecule of a complex substance. As a result of the substitution reaction, a new simple and new complex substance is formed:
A + BC = AC + B
These reactions are almost always redox reactions. For example:
Zn + 2HCl = ZnCl 2 + H 2
Ca + 2H 2 O = Ca (OH) 2 + H 2
Fe + CuSO 4 = FeSO 4 + Cu
2Аl + Fe 2 О 3 = 2Fе + Аl 2 О 3
2KBr + Cl 2 = 2KCl + Br 2
There are a small number of substitution reactions in which complex substances are involved and which occur without changing the oxidation states of the elements, for example:
CaCO 3 + SiO 2 = CaSiO 3 + CO 2
Ca 3 (PO 4) 2 + 3SiO 2 = 3CaSiO 3 + P 2 O 5
Exchange reactions are reactions between two complex substances, the molecules of which exchange their constituent parts:
AB + CB = AB + CB
Exchange reactions always proceed without electron transfer, that is, they are not redox reactions. For example:
HNO 3 + NaOH = NaNO 3 + H 2 O
BaCl 2 + H 2 SO 4 = BaSO 4 + 2HCl
As a result of exchange reactions, a precipitate (↓), or a gaseous substance (), or a weak electrolyte (for example, water) is usually formed.
During chemical reactions, other substances are obtained from some substances (not to be confused with nuclear reactions, in which one chemical element is converted into another).
Any chemical reaction is described by a chemical equation:
Reagents → Reaction Products
The arrow indicates the direction of the reaction.
For example:
In this reaction, methane (CH 4) reacts with oxygen (O 2), resulting in the formation of carbon dioxide (CO 2) and water (H 2 O), or rather, water vapor. This is what happens in your kitchen when you light your gas burner. The equation should be read like this: one molecule of methane gas reacts with two molecules of oxygen gas, resulting in one molecule of carbon dioxide and two molecules of water (water vapor).
The numbers in front of the components of a chemical reaction are called reaction coefficients.
Chemical reactions are endothermic(with energy absorption) and exothermic(with the release of energy). Combustion of methane is a typical example of an exothermic reaction.
There are several types of chemical reactions. The most common:
- compound reactions;
- decomposition reactions;
- single substitution reactions;
- double substitution reactions;
- oxidation reactions;
- redox reactions.
Compound reactions
In compound reactions, at least two elements form one product:
2Na (t) + Cl 2 (g) → 2NaCl (t)- the formation of table salt.
Attention should be paid to the essential nuance of the reactions of the compound: depending on the conditions of the reaction or the proportions of the reactants entering into the reaction, different products may result. For example, under normal conditions of combustion of coal, carbon dioxide is obtained:
C (t) + O 2 (g) → CO 2 (g)
If the amount of oxygen is insufficient, then deadly carbon monoxide is formed:
2C (t) + O 2 (g) → 2CO (g)
Decomposition reactions
These reactions are, as it were, essentially opposite to the reactions of the compound. As a result of the decomposition reaction, the substance decomposes into two (3, 4 ...) simpler elements (compounds):
- 2H 2 O (l) → 2H 2 (g) + O 2 (g)- decomposition of water
- 2H 2 O 2 (l) → 2H 2 (g) O + O 2 (g)- decomposition of hydrogen peroxide
Single substitution reactions
As a result of single substitution reactions, the more active element replaces the less active one in the compound:
Zn (t) + CuSO 4 (p-p) → ZnSO 4 (p-p) + Cu (t)
The zinc in the copper sulfate solution displaces the less active copper, resulting in a zinc sulfate solution.
The degree of activity of metals by increasing activity:
- The most active are alkali and alkaline earth metals.
The ionic equation of the above reaction will be:
Zn (t) + Cu 2+ + SO 4 2- → Zn 2+ + SO 4 2- + Cu (t)
The ionic bond of CuSO 4, when dissolved in water, decomposes into a copper cation (charge 2+) and anion of sulfate (charge 2-). As a result of the substitution reaction, a zinc cation is formed (which has the same charge as the copper cation: 2-). Note that the sulfate anion is present on both sides of the equation, so it can be abbreviated by all the rules of mathematics. As a result, we get the ion-molecular equation:
Zn (t) + Cu 2+ → Zn 2+ + Cu (t)
Double substitution reactions
In double substitution reactions, two electrons are already substituted. Such reactions are also called exchange reactions... Such reactions take place in solution with the formation of:
- insoluble solid (precipitation reaction);
- water (neutralization reaction).
Precipitation reactions
When mixing a solution of silver nitrate (salt) with a solution of sodium chloride, silver chloride is formed:
Molecular Equation: KCl (p-p) + AgNO 3 (p-p) → AgCl (t) + KNO 3 (p-p)
Ionic equation: K + + Cl - + Ag + + NO 3 - → AgCl (t) + K + + NO 3 -
Molecular ion equation: Cl - + Ag + → AgCl (s)
If the compound is soluble, it will be ionic in solution. If the compound is insoluble, it will precipitate forming a solid.
Neutralization reactions
These are the reactions of interaction of acids and bases, as a result of which water molecules are formed.
For example, the reaction of mixing a solution of sulfuric acid and a solution of sodium hydroxide (lye):
Molecular Equation: H 2 SO 4 (p-p) + 2NaOH (p-p) → Na 2 SO 4 (p-p) + 2H 2 O (g)
Ionic equation: 2H + + SO 4 2- + 2Na + + 2OH - → 2Na + + SO 4 2- + 2H 2 O (g)
Molecular ion equation: 2H + + 2OH - → 2H 2 O (l) or H + + OH - → H 2 O (l)
Oxidation reactions
These are reactions of interaction of substances with gaseous oxygen in the air, in which, as a rule, a large amount of energy is released in the form of heat and light. A typical oxidation reaction is combustion. At the very beginning of this page, the reaction of the interaction of methane with oxygen is given:
CH 4 (g) + 2O 2 (g) → CO 2 (g) + 2H 2 O (g)
Methane refers to hydrocarbons (compounds of carbon and hydrogen). When a hydrocarbon reacts with oxygen, a lot of thermal energy is released.
Redox reactions
These are reactions in which there is an exchange of electrons between the atoms of the reactants. The reactions discussed above are also redox reactions:
- 2Na + Cl 2 → 2NaCl - compound reaction
- CH 4 + 2O 2 → CO 2 + 2H 2 O - oxidation reaction
- Zn + CuSO 4 → ZnSO 4 + Cu - single substitution reaction
The most detailed redox reactions with a large number of examples of solving equations by the electronic balance method and the half-reaction method are described in the section
Department of Education of the Ivanovo Region
Regional state budgetary professional educational institution
Southern College of Technology
METHODOLOGICAL DEVELOPMENT
OPEN LESSON ON CHEMISTRY
On the topic:
« Classification of chemical reactions»
Teacher: Vdovin Yu.A.
Well:I
Group: 39-40
Yuzha - 2017
| Lesson topic: | Classification of chemical reactions |
| Lesson objectives: | Expand and deepen knowledge of chemical reactions, compare them with other types of phenomena. Learn to highlight the essential features that can be used as the basis for the classification of chemical reactions. Consider the classification of chemical reactions according to various criteria. |
| Lesson Objectives: | 1. Educational - to systematize, generalize and deepen the knowledge of students about chemical reactions and their classification, develop the skills of independent work, the ability to write down reaction equations and arrange coefficients, indicate the types of reactions, draw conclusions and generalizations. 2. Developing - to develop a culture of speech using chemical terms and formulas, the development of cognitive abilities, thinking, attention. 3. Educational - education of independence, perseverance, attentiveness, tolerance. |
| Lesson type: | Combined |
| Equipment and reagents: | Reagents: Ammonium nitrate, sodium hydroxide, ammonium hydroxide, copper (II) sulfate, sodium carbonate, hydrochloric acid, potassium hexacyanoferrate (III), iron (III) chloride, potassium permanganate, sulfuric acid, ethanol. Equipment: Test tubes, flasks with solutions, pipettes, racks, Petri dish, porcelain dish for evaporation, glass rod, cotton wool, metal baking sheet. |
| Teaching methods | Verbal (conversation, explanation) Problem-based learning methods, laboratory experience. |
| Forms of work: | individual, frontal. |
Lesson plan:
During the classes:
1. Organizational moment (1 min)
A) Greetings;
B) Safety precautions;
2. Motivation (2 min)
Introduction:
A huge number of reactions take place in the world around us. So we just sit, stand, go somewhere, and in every cell of our body every second there are tens and hundreds of thousands of transformations of some substances into others.
Almost not inferior to a living organism and inanimate matter. Somewhere now, at this very moment, a chemical cycle is taking place: some molecules disappear, others arise, and these processes never stop.
If they all stopped overnight, the world would become silent. How to keep in memory the variety of chemical processes, how to navigate them in practice? How do biologists manage to navigate the diversity of living organisms? (creating a problem situation).
Supposed answer: In any science, a classification technique is used, which allows to divide the entire set of objects into groups based on common criteria.
Let's formulate the topic of the lesson: Classification of chemical reactions.
Any lesson should have a purpose.
Let's formulate the objectives of today's lesson?
What should we consider?
What should you learn?
Consider possible classifications of chemical reactions.
Learn to highlight the signs by which the classification of reactions is made.
And what is the use of classifying chemical reactions?
Supposed answer: It helps to generalize, structure knowledge about chemical processes, highlight something in common and predict, on the basis of existing knowledge, something else unknown, but similar to the known.
And where can knowledge of the classification of chemical reactions be applied in your practice?
Supposed answer: some classes of chemical reactions can be useful to us in practice. For example, such an important phenomenon for you as electroplating is based on redox processes. I think the concept of "Galvanic cells" is painfully familiar to you!
In addition, knowledge of the class of chemical reaction of a particular process can help in controlling this process.
3. Knowledge update (6 min)
A) Task with cards on the difference between physical processes and chemical reactions (2 min).
The task is performed by a student on a magnetic board and in parallel with a group with a presentation.
Take a look at these phenomena known to all of you. Divide them into groups. Name the groups and define each group.
B) Repeat safety precautions
Laboratory experiments (3 min)
How can you know that we are having a chemical reaction?
Estimated answer # 1: by criteria.
Supposed answer # 2: By precipitation, gas evolution, etc.
And now I suggest you plunge into the atmosphere of empiricism and be experimenters. There are test tubes and bottles with reagents in front of you. In the working field, in task No. 2, the methods of the experiment are indicated. Do these experiments. Enter the results of your experiments into the "Signs of chemical reactions" table.
| Sign of leak | Reaction scheme |
|
| Odor | ||
| Precipitation | ||
| Dissolution of sediment | ||
| Gas evolution | ||
| Color change | ||
| Light emission | ||
| Highlighting or heat absorption |
4 ... Learning new material (15 min)
We have seen that chemical reactions are often accompanied by effects. Some of these effects are used as a basis for different types of classification ...
Yes, chemical reactions are classified according to different types, so the same chemical reaction can be viewed and classified in different ways.
A) Classification by the number and composition of reagents and their products:
Connections
Expansions
Substitutions
One slide shows examples of chemical reactions.
The guys compare the reaction equations and formulate class definitions based on this comparative analysis. The same happens with other types.
B) By thermal effect
Exothermic
Endothermic
C) By a change in the oxidation state
Redox
No change in oxidation state
D) By phase composition
Homogeneous
Heterogeneous
E) On the use of the catalyst
Catalytic
Non-catalytic
E) In the direction:
Reversible
Non reversible
5. Application and consolidation of knowledge (15 min)
Now it's time to apply our knowledge.
The guys perform tasks 3-5 of the working field.
3. In front of each term belonging to a class of chemical reactions, paste in the required definition.
| Compound reactions | Reactions resulting in one complex substance from two or more substances |
| Decomposition reactions | Reactions as a result of which several new substances are formed from a complex substance. |
| Substitution reactions | Reactions as a result of which the atoms of a simple substance replace the atoms of one of the elements in a complex substance. |
| Exchange reactions | Reactions in which two complex substances exchange their constituent parts. |
| Exothermic reactions | Reactions proceeding with the release of heat. |
| Endothermic reactions | Reactions involving heat absorption. |
| Catalytic reactions | Reactions involving a catalyst. |
| Non-catalytic reactions | Reactions without a catalyst. |
| Redox | Reactions proceeding with a change in the oxidation states of the elements that form the substances involved in the reaction. |
| Reversible reactions | Chemical reactions that take place simultaneously in two opposite directions - forward and backward. |
| Irreversible reactions | Chemical reactions, as a result of which the starting materials are almost completely converted into final products. |
| Homogeneous reactions | Reactions that take place in a homogeneous medium, such as a mixture of gases or solutions. |
| Heterogeneous reactions | Reactions that take place between substances in an inhomogeneous medium. |
The work is checked on the slide of the presentation.
4. Assign chemical reactions to their class:
| Compound reactions | ||
| Decomposition reactions | ||
| Substitution reactions | ||
| Exchange reactions | ||
| Exothermic reactions |
DEFINITION
Chemical reaction called the transformation of substances in which there is a change in their composition and (or) structure.
Most often, chemical reactions are understood as the process of converting initial substances (reagents) into final substances (products).
Chemical reactions are written using chemical equations containing the formulas of the starting materials and reaction products. According to the law of conservation of mass, the number of atoms of each element on the left and right sides of the chemical equation is the same. Usually, the formulas of the starting materials are written on the left side of the equation, and the formulas for the products are on the right. The equality of the number of atoms of each element in the left and right sides of the equation is achieved by placing integer stoichiometric coefficients in front of the formulas of substances.
Chemical equations may contain additional information about the features of the reaction: temperature, pressure, radiation, etc., which is indicated by the corresponding symbol above (or “below”) the equal sign.
All chemical reactions can be grouped into several classes, which have certain characteristics.
Classification of chemical reactions by the number and composition of the starting and resulting substances
According to this classification, chemical reactions are subdivided into reactions of combination, decomposition, substitution, exchange.
As a result compound reactions one new substance is formed from two or more (complex or simple) substances. In general terms, the equation for such a chemical reaction will look like this:
For example:
CaCO 3 + CO 2 + H 2 O = Ca (HCO 3) 2
SO 3 + H 2 O = H 2 SO 4
2Mg + O 2 = 2MgO.
2FеСl 2 + Сl 2 = 2FеСl 3
The reactions of the compound are in most cases exothermic, i.e. proceed with the release of heat. If simple substances are involved in the reaction, then such reactions are most often redox reactions (ORR), i.e. proceed with a change in the oxidation states of the elements. It is impossible to say unequivocally whether the reaction of a compound between complex substances belongs to the OVR.
Reactions as a result of which several other new substances (complex or simple) are formed from one complex substance are referred to as decomposition reactions... In general, the chemical decomposition equation will look like this:
For example:
CaCO 3 CaO + CO 2 (1)
2H 2 O = 2H 2 + O 2 (2)
CuSO 4 × 5H 2 O = CuSO 4 + 5H 2 O (3)
Cu (OH) 2 = CuO + H 2 O (4)
H 2 SiO 3 = SiO 2 + H 2 O (5)
2SO 3 = 2SO 2 + O 2 (6)
(NH 4) 2 Cr 2 O 7 = Cr 2 O 3 + N 2 + 4H 2 O (7)
Most decomposition reactions occur on heating (1,4,5). Decomposition by electric current possible (2). The decomposition of crystalline hydrates, acids, bases and salts of oxygen-containing acids (1, 3, 4, 5, 7) proceeds without changing the oxidation states of the elements, i.e. these reactions do not belong to OVR. The decomposition reactions include the decomposition of oxides, acids, and salts formed by elements in higher oxidation states (6).
Decomposition reactions are also found in organic chemistry, but under other names - cracking (8), dehydrogenation (9):
C 18 H 38 = C 9 H 18 + C 9 H 20 (8)
C 4 H 10 = C 4 H 6 + 2H 2 (9)
At substitution reactions a simple substance interacts with a complex substance, forming a new simple and new complex substance. In general terms, the equation for the chemical reaction of substitution will look like this:
For example:
2Аl + Fe 2 O 3 = 2Fе + Аl 2 О 3 (1)
Zn + 2HCl = ZnCl 2 + H 2 (2)
2KBr + Cl 2 = 2KCl + Br 2 (3)
2KSlO 3 + l 2 = 2KlO 3 + Сl 2 (4)
CaCO 3 + SiO 2 = CaSiO 3 + CO 2 (5)
Ca 3 (PO 4) 2 + 3SiO 2 = 3CaSiO 3 + P 2 O 5 (6)
CH 4 + Cl 2 = CH 3 Cl + HCl (7)
Substitution reactions are mostly redox reactions (1 - 4, 7). Examples of decomposition reactions in which no change in oxidation states occurs are few (5, 6).
Exchange reactions call the reactions that occur between complex substances, in which they exchange their constituent parts. Usually this term is used for reactions involving ions in aqueous solution. In general, the equation of the chemical exchange reaction will look like this:
AB + CD = AD + CB
For example:
CuO + 2HCl = CuCl 2 + H 2 O (1)
NaOH + HCl = NaCl + H 2 O (2)
NaHCO 3 + HCl = NaCl + H 2 O + CO 2 (3)
AgNO 3 + KBr = AgBr ↓ + KNO 3 (4)
СrСl 3 + ЗNаОН = Сr (ОН) 3 ↓ + ЗNаСl (5)
Metabolic reactions are not redox. A special case of these exchange reactions are neutralization reactions (reactions of interaction of acids with alkalis) (2). Exchange reactions proceed in the direction where at least one of the substances is removed from the reaction sphere in the form of a gaseous substance (3), a precipitate (4, 5), or a low-dissociating compound, most often water (1, 2).
Classification of chemical reactions by changes in oxidation states
Depending on the change in the oxidation states of the elements that make up the reagents and reaction products, all chemical reactions are subdivided into redox (1, 2) and proceeding without a change in the oxidation state (3, 4).
2Mg + CO 2 = 2MgO + C (1)
Mg 0 - 2e = Mg 2+ (reducing agent)
C 4+ + 4e = C 0 (oxidizing agent)
FeS 2 + 8HNO 3 (conc) = Fe (NO 3) 3 + 5NO + 2H 2 SO 4 + 2H 2 O (2)
Fe 2+ -e = Fe 3+ (reducing agent)
N 5+ + 3e = N 2+ (oxidizing agent)
AgNO 3 + HCl = AgCl ↓ + HNO 3 (3)
Ca (OH) 2 + H 2 SO 4 = CaSO 4 ↓ + H 2 O (4)
Thermal classification of chemical reactions
Depending on whether heat (energy) is released or absorbed during the reaction, all chemical reactions are conventionally divided into exo - (1, 2) and endothermic (3), respectively. The amount of heat (energy) released or absorbed during the reaction is called the heat effect of the reaction. If the amount of released or absorbed heat is indicated in the equation, then such equations are called thermochemical.
N 2 + 3H 2 = 2NH 3 +46.2 kJ (1)
2Mg + O 2 = 2MgO + 602.5 kJ (2)
N 2 + O 2 = 2NO - 90.4 kJ (3)
Classification of chemical reactions according to the direction of the reaction
In the direction of the reaction, reversible (chemical processes whose products are capable of reacting with each other under the same conditions in which they were obtained, with the formation of initial substances) and irreversible (chemical processes whose products are not able to react with each other with the formation of initial substances ).
For reversible reactions, the equation in general form is usually written as follows:
A + B ↔ AB
For example:
CH 3 COOH + C 2 H 5 OH↔ H 3 COOC 2 H 5 + H 2 O
Examples of irreversible reactions include the following reactions:
2KSlO 3 → 2KSl + 3O 2
С 6 Н 12 О 6 + 6О 2 → 6СО 2 + 6Н 2 О
Evidence of the irreversibility of the reaction can be the release of a gaseous substance, a precipitate or a low-dissociating compound, most often water, as the reaction products.
Classification of chemical reactions by the presence of a catalyst
From this point of view, catalytic and non-catalytic reactions are distinguished.
A catalyst is a substance that accelerates the course of a chemical reaction. Reactions involving catalysts are called catalytic. Some reactions are generally impossible without the presence of a catalyst:
2H 2 O 2 = 2H 2 O + O 2 (catalyst MnO 2)
Often, one of the reaction products serves as a catalyst that accelerates this reaction (autocatalytic reactions):
MeO + 2HF = MeF 2 + H 2 O, where Me is a metal.
Examples of problem solving
EXAMPLE 1
All chemical reactions can be classified according to various criteria:
1. Classification of reactions according to the change in the number of starting substances and reaction products
- addition reactions - a new substance is formed from several substances: C + O 2 = CO 2;
- decomposition reactions - from one substance several others are formed: CaCO 3 = CaO + CO 2;
- substitution reactions - as a result of the reaction of a simple and complex substance, new complex and simple substances are formed: 2Na + 2H 2 O = 2NaOH + H 2;
- exchange reactions - reagents exchange components: MgS + 2HCl = MgCl 2 + H 2 S;
- neutralization reactions (they are a special case of exchange reactions) - the initial substances of the reaction are acid and base, and the products are water and salt: NaOH + HCl = NaCl + H 2 O
2. Classification of reactions according to the release / absorption of energy
- exothermic reactions (with heat evolution): C + O 2 = CO 2 + Q
- endothermic reactions (with heat absorption): N 2 + O 2 = 2NO - Q
3. Classification of reactions by the presence of a catalyst
Catalysts are substances that do not directly participate in the reaction itself, but change the rate of its course.
- catalytic reactions: CO + H 2 O = CO 2 + H (FeO catalyst)
- non-catalytic reactions
4. Classification of reactions based on the reversibility of the reaction
- reversible reactions - can spontaneously proceed under these conditions, both in the forward and in the opposite direction: N 2 (g) + 3H 2 (g) ↔ 2NH 3 (g)
- irreversible reactions - proceed only in one direction with almost complete conversion of the starting materials into reaction products (one of the products is a gaseous or weakly dissociating substance): CaCO 3 = CaO + CO 2
5. Classification of reactions by particle type
- molecular;
- ionic;
- radical.
6. Classification of reactions by the type of energy impact
- thermochemical reactions - proceed at elevated temperatures: N 2 + O 2 = 2NO;
- photochemical reactions - proceed under the influence of light: H 2 + Cl 2 = 2HCl;
- electrochemical reactions - occur under the influence of an electric current: 2NaCl = 2Na + Cl 2.
7. Classification of reactions by the state of aggregation of the environment
- homogeneous reactions - take place in a homogeneous medium, for example, in a gaseous or liquid medium:
C + O 2 = CO 2
KOH + HCl = H 2 O + KCl; - heterogeneous reactions - occur at the interface of two substances in different phases (solid-gaseous; liquid-gaseous, solid-liquid; liquid-liquid; solid-solid):
- CaCO 3 (t) = CaO (t) + CO 2 (g)
- FeO (t) + CO (g) = Fe (g) + CO 2 (t)
- Zn (s) + H 2 SO 4 (g) = H 2 (g) + ZnSO 4 (g)
Homogeneous and heterogeneous reactions, in turn, are subdivided into simple(only one reaction occurs in the system, usually irreversible) and complex(several simple reactions occur simultaneously in the system).
Types of simple chemical reactions:
- Monomolecular reactions: only one type of molecules of the parent substance is involved in such reactions:
I 2 ↔ 2I - Bimolecular reactions: consist of one stage, two particles are involved in each act:
H 2 + I 2 = 2HI - Trimolecular reactions: three particles take part in the act simultaneously:
2NO + H 2 = N 2 O + H 2 O
Types of complex chemical reactions:
- Parallel reactions: the starting materials interact simultaneously in several different directions;
- Consecutive reactions: initial substances undergo changes, going through several successive stages, forming intermediate reaction products;
- Conjugate reactions: two reactions take place in the same environment, while the course of one reaction depends on the other, or the course of both reactions affects each other.
Any chemical reaction is accompanied by the release or absorption of heat (see Heat effect of reaction).
The progress of the reaction can be judged by the formation of reaction products, the disappearance or change of the starting substances: change in the color of substances; the formation or disappearance of sediment; release or absorption of gas; appearance, disappearance, change of smell; the release or absorption of heat, etc.
In conclusion, mention should be made of such an important nuance as the conditions of the reaction, not all chemical reactions proceed under the so-called normal conditions. In some cases, the interaction of substances requires a high temperature, pressure, a certain acidity of the medium, the presence of a catalyst, etc.
