Reversible and irreversible reactions - Knowledge Hypermarket. Reversibility of chemical reactions. Chemical equilibrium Reversible reactions concept of chemical equilibrium

All chemical reactions can be divided into two groups: irreversible and reversible e reactions. Irreversible reactions proceed to the end (until complete consumption of one of the reagents), and in reversible None of the reactants are completely consumed because a reversible reaction can occur in either the forward or reverse direction.

An example of an irreversible reaction:

Zn + 4HNO 3 → Zn(NO 3) 2 + 2NO 2 + 2H 2 O

Example of a reversible reaction:

Initially, the rate of the forward reaction v pr is high, and the rate of the reverse reaction v about equal to zero

Dependence of the rates of forward and reverse reactions on time τ. When these rates are equal, chemical equilibrium occurs.

As the reaction proceeds, the starting substances are consumed and their concentrations fall. At the same time, reaction products appear and their concentrations increase. As a result, a reverse reaction begins to occur, and its speed gradually increases. When the rates of forward and reverse reactions become equal, chemical equilibrium occurs. It is dynamic because, although the concentrations of substances in the system remain constant, the reaction continues to occur in both the forward and reverse directions.

If there is equality v at v we can equate their expressions according to the law of mass action*. For example, for the reversible interaction of hydrogen with iodine:

k pr ··= k rev · 2 or

Attitude rate constants for forward and reverse reactions (K) is called the equilibrium constant. At a constant temperature, the equilibrium constant is a constant value showing the relationship between the concentrations of products and starting substances that is established at equilibrium. Magnitude K depends on the nature of the reactants and temperature.

The system is in a state of equilibrium as long as external conditions remain constant. When the concentration of any of the substances participating in the reaction increases, the equilibrium shifts towards the consumption of this substance; When the concentration of any substance decreases, the equilibrium shifts towards the formation of this substance.

Among the numerous classifications of types of reactions, for example those that are determined by the thermal effect (exothermic and endothermic), by changes in the oxidation states of substances (redox), by the number of components participating in them (decomposition, compounds) and so on, reactions occurring in two mutual directions, otherwise called reversible . An alternative to reversible reactions are reactions irreversible, during which the final product (precipitate, gaseous substance, water) is formed. Among these reactions are the following:

Exchange reactions between salt solutions, during which either an insoluble precipitate is formed - CaCO 3:

Ca(OH) 2 + K 2 CO 3 → CaCO 3↓ + 2KON (1)

or a gaseous substance - CO 2:

3 K 2 CO 3 + 2H 3 RO 4 →2K 3 RO 4 + 3 CO 2+ 3H 2 O (2)

or a slightly dissociable substance is obtained - H 2 O:

2NaOH + H 2 SO 4 → Na 2 SO 4 + 2 H 2O(3)

If we consider a reversible reaction, then it proceeds not only in the forward direction (in reactions 1,2,3 from left to right), but also in the opposite direction. An example of such a reaction is the synthesis of ammonia from gaseous substances - hydrogen and nitrogen:

3H 2 + N 2 ↔ 2NH 3 (4)

Thus, a chemical reaction is called reversible if it proceeds not only in the forward direction (→), but also in the reverse direction (←) and is indicated by the symbol (↔).

The main feature of this type of reaction is that reaction products are formed from the starting substances, but at the same time, the starting reagents are formed from the same products. If we consider reaction (4), then in a relative unit of time, simultaneously with the formation of two moles of ammonia, their decomposition will occur with the formation of three moles of hydrogen and one mole of nitrogen. Let us denote the rate of direct reaction (4) by the symbol V 1, then the expression for this rate will take the form:

V 1 = kˑ [Н 2 ] 3 ˑ , (5)

where the value “k” is defined as the rate constant of a given reaction, the values ​​[H 2 ] 3 and correspond to the concentrations of the starting substances raised to powers corresponding to the coefficients in the reaction equation. In accordance with the principle of reversibility, the rate of the reverse reaction will take the expression:

V 2 = kˑ 2 (6)

At the initial moment of time, the rate of the forward reaction takes on the greatest value. But gradually the concentrations of the starting reagents decrease and the reaction rate slows down. At the same time, the rate of the reverse reaction begins to increase. When the rates of forward and reverse reactions become the same (V 1 = V 2), state of equilibrium , at which there is no longer a change in the concentrations of both the initial and the resulting reagents.

It should be noted that some irreversible reactions should not be taken literally. Let us give an example of the most frequently cited reaction of a metal with an acid, in particular, zinc with hydrochloric acid:

Zn + 2HCl = ZnCl 2 + H 2 (7)

In fact, zinc, when dissolved in acid, forms a salt: zinc chloride and hydrogen gas, but after some time the rate of the direct reaction slows down as the concentration of salt in the solution increases. When the reaction practically stops, a certain amount of hydrochloric acid will be present in the solution along with zinc chloride, so reaction (7) should be given in the following form:

2Zn + 2HCl = 2ZnНCl + H2 (8)

Or in the case of the formation of an insoluble precipitate obtained by merging solutions of Na 2 SO 4 and BaCl 2:

Na 2 SO 4 + BaCl 2 = BaSO 4 ↓ + 2NaCl (9)

the precipitated salt BaSO 4, albeit to a small extent, will dissociate into ions:

BaSO 4 ↔ Ba 2+ + SO 4 2- (10)

Therefore, the concepts of irreversible and irreversible reactions are relative. But nevertheless, both in nature and in the practical activities of people, these reactions are of great importance. For example, combustion processes of hydrocarbons or more complex organic substances, such as alcohol:

CH 4 + O 2 = CO 2 + H 2 O (11)

2C 2 H 5 OH + 5O 2 = 4CO 2 + 6H 2 O (12)

are completely irreversible processes. It would be considered a happy dream of humanity if reactions (11) and (12) were reversible! Then it would be possible to synthesize gas and gasoline and alcohol again from CO 2 and H 2 O! On the other hand, reversible reactions such as (4) or oxidation of sulfur dioxide:

SO 2 + O 2 ↔ SO 3 (13)

are basic in the production of ammonium salts, nitric acid, sulfuric acid, and other inorganic and organic compounds. But these reactions are reversible! And in order to obtain the final products: NH 3 or SO 3, it is necessary to use such technological methods as: changing the concentrations of reagents, changing pressure, increasing or decreasing the temperature. But this will already be the subject of the next topic: “Shift in chemical equilibrium.”

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Chemical reactions based on reversibility are divided into irreversible and reversible. Irreversible reactions include those reactions that proceed until one of the reactants is completely consumed. Signs of irreversible reactions occurring in solutions are: a) precipitation, b) gas formation, c) formation of a weak electrolyte.

Reversible reactions are those reactions that occur simultaneously in two mutually opposite directions. For such reactions, opposite arrows (-) are used instead of the equal sign.

Over time, the rate of any reaction, measured by the decreasing concentrations of the starting substances, will decrease, since as the substances interact, their concentrations decrease (the rate of the direct reaction). If the reaction is reversible, then as the concentration of products increases, its speed will increase (the speed of the reverse reaction). As soon as the rates of forward and reverse reactions become equal, chemical equilibrium is established in the system and further changes in the concentrations of all substances in the system stop.

A quantitative characteristic of the equilibrium state is the chemical equilibrium constant K, which is determined by the ratio of the rate constants of the forward and reverse reaction

In the vast majority of cases, the rate constants of the forward and reverse reactions are not equal. The equilibrium constant is a constant value at a given temperature and determines the relationship between the equilibrium concentrations of reaction products and starting substances, raised to the power of their stoichiometric coefficients. For example, for the process

The square bracket indicates the concentration of each substance at the moment of equilibrium, the so-called equilibrium concentration.

The equilibrium constant depends on the nature of the reactants and temperature. The catalyst does not affect the equilibrium state. The presence of a catalyst in the system only changes the time it takes to reach it. The system can remain in a state of equilibrium until at least one of the external influences changes: temperature, concentration of one of the reagents, pressure (for gases). Changes occurring in an equilibrium system as a result of external influences are determined by the principle of moving equilibrium (Le Chatelier's principle): an external influence on a system in a state of equilibrium leads to a shift in this equilibrium in the direction in which the effect of the effect produced is weakened.

The balance shift is influenced by:

• 1) temperature change: the endothermic process accelerates to a greater extent with increasing temperature and, conversely, with decreasing temperature the exothermic process accelerates;
• 2) change in pressure (for reactions occurring in the gas phase): with increasing pressure, the equilibrium of the reaction shifts towards the formation of substances that occupy a smaller volume, and, conversely, a decrease in pressure promotes a process accompanied by an increase in volume. If the reaction proceeds without a change in volume, then a change in pressure in the system does not affect the chemical equilibrium.
• 3) change in concentration: an increase in the concentration of the starting substances leads to an increase in the rate of the forward reaction, while the process occurring in the system will end when the rates of the forward and reverse reactions become equal and a new equilibrium is established. A decrease in the concentration of one of the reaction products (removal from the system) leads to a shift in the equilibrium towards its formation.

Codifier Topics: reversible and irreversible reactions. Chemical balance. Shift in chemical equilibrium under the influence of various factors.

If a reverse reaction is possible, chemical reactions are divided into reversible and irreversible.

Reversible chemical reactions - these are reactions whose products under given conditions can interact with each other.

For example, ammonia synthesis is a reversible reaction:

N2 + 3H2 = 2NH3

The process takes place at high temperature, under pressure and in the presence of a catalyst (iron). Such processes are usually reversible.

Irreversible reactions - these are reactions whose products cannot interact with each other under given conditions.

For example, combustion reactions or reactions that occur with an explosion are most often irreversible. Carbon combustion proceeds irreversibly:

C + O 2 = CO 2

More details about classification of chemical reactions can be read.

The likelihood of product interaction depends on the process conditions.

So, if the system open, i.e. exchanges both matter and energy with the environment, then chemical reactions in which, for example, gases are formed, will be irreversible.

For example , when calcining solid sodium bicarbonate:

2NaHCO 3 → Na 2 CO 3 + CO 2 + H 2 O

carbon dioxide gas is released and evaporates from the reaction zone. Therefore, this reaction will be irreversible under these conditions.

If we consider closed system , which can not exchange a substance with the environment (for example, a closed box in which the reaction occurs), then carbon dioxide will not be able to escape from the reaction zone, and will interact with water and sodium carbonate, then the reaction will be reversible under these conditions:

2NaHCO 3 ⇔ Na 2 CO 3 + CO 2 + H 2 O

Let's consider reversible reactions. Let the reversible reaction proceed according to the scheme:

aA + bB ⇔ cC + dD

The rate of direct reaction according to the law of mass action is determined by the expression:

v 1 =k 1 ·C A a ·C B b

Feedback speed:

v 2 =k 2 ·C С с ·C D d

Here k 1 And k 2 are the rate constants of the forward and reverse reactions, respectively, C A, C B, C C, C D– concentrations of substances A, B, C and D, respectively.

If at the initial moment of the reaction there are no substances C and D in the system, then particles A and B collide and interact predominantly, and a predominantly direct reaction occurs.

Gradually, the concentration of particles C and D will also begin to increase, therefore, the rate of the reverse reaction will increase. At some point the rate of the forward reaction will be equal to the rate of the reverse reaction. This state is called chemical equilibrium .

Thus, chemical equilibrium is a state of the system in which the rates of forward and reverse reactions are equal .

Since the rates of forward and reverse reactions are equal, the rate of formation of reagents is equal to the rate of their consumption, and the current concentrations of substances do not change . Such concentrations are called equilibrium .

Please note that at equilibrium Both forward and reverse reactions occur, that is, the reactants interact with each other, but the products also interact with each other at the same rate. At the same time, external factors can influence displace chemical equilibrium in one direction or another. Therefore, chemical equilibrium is called mobile, or dynamic .

Research in the field of mobile equilibrium began in the 19th century. The works of Henri Le Chatelier laid the foundations of the theory, which was later generalized by the scientist Karl Brown. The principle of mobile equilibrium, or the Le Chatelier-Brown principle, states:

If a system in a state of equilibrium is influenced by an external factor that changes any of the equilibrium conditions, then processes in the system aimed at compensating for the external influence are intensified.

In other words: When there is an external influence on the system, the equilibrium will shift so as to compensate for this external influence.

This principle, which is very important, works for any equilibrium phenomena (not just chemical reactions). However, we will now consider it in relation to chemical interactions. In the case of chemical reactions, external influences lead to changes in the equilibrium concentrations of substances.

Chemical reactions in a state of equilibrium can be influenced by three main factors - temperature, pressure and concentrations of reactants or products.

1. As is known, chemical reactions are accompanied by a thermal effect. If the direct reaction occurs with the release of heat (exothermic, or +Q), then the reverse reaction occurs with the absorption of heat (endothermic, or -Q), and vice versa. If you raise temperature in the system, the equilibrium will shift so as to compensate for this increase. It is logical that in an exothermic reaction the temperature increase cannot be compensated. Thus, as the temperature increases, the equilibrium in the system shifts towards heat absorption, i.e. towards endothermic reactions (-Q); with decreasing temperature - towards an exothermic reaction (+Q).

2. In the case of equilibrium reactions, when at least one of the substances is in the gas phase, the equilibrium is also significantly affected by a change pressure in system. As pressure increases, the chemical system tries to compensate for this effect and increases the rate of reaction, in which the amount of gaseous substances decreases. As the pressure decreases, the system increases the rate of reaction, which produces more molecules of gaseous substances. Thus: with an increase in pressure, the equilibrium shifts towards a decrease in the number of gas molecules, and with a decrease in pressure - towards an increase in the number of gas molecules.

Note! Systems where the number of molecules of reactant gases and products are the same are not affected by pressure! Also, changes in pressure have virtually no effect on the equilibrium in solutions, i.e. on reactions where there are no gases.

3. Also, equilibrium in chemical systems is affected by changes concentrations reactants and products. As the concentration of reactants increases, the system tries to use them up and increases the rate of the forward reaction. As the concentration of reagents decreases, the system tries to produce them, and the rate of the reverse reaction increases. As the concentration of products increases, the system also tries to consume them and increases the rate of the reverse reaction. When the concentration of products decreases, the chemical system increases the rate of their formation, i.e. rate of forward reaction.

If in a chemical system the rate of forward reaction increases right , towards the formation of products And reagent consumption . If the rate of reverse reaction increases, we say that the balance has shifted left , towards food consumption And increasing the concentration of reagents .

For example, in the ammonia synthesis reaction:

N 2 + 3H 2 = 2NH 3 + Q

An increase in pressure leads to an increase in the rate of reaction, in which fewer gas molecules are formed, i.e. direct reaction (the number of molecules of reactant gases is 4, the number of gas molecules in products is 2). As pressure increases, the equilibrium shifts to the right, towards the products. At temperature rise the balance will shift in the opposite direction of the endothermic reaction, i.e. to the left, towards the reagents. An increase in the concentration of nitrogen or hydrogen will shift the equilibrium towards their consumption, i.e. to the right, towards the products.

Catalyst does not affect balance, because accelerates both forward and reverse reactions.

Video tutorial 2: Chemical equilibrium shift

Lecture: Reversible and irreversible chemical reactions. Chemical balance. Shift in chemical equilibrium under the influence of various factors

From the previous lesson, you learned what the rate of a chemical reaction is and what factors influence it. In this lesson we will look at how these reactions occur. This depends on the behavior of the starting substances participating in the reaction - the reagents. If they are completely converted into final substances - products, then the reaction is irreversible. Well, if the final products are converted back into the original substances, then the reaction is reversible. Taking this into account, let us formulate the definitions:

Reversible reaction- this is a certain reaction that occurs under the same conditions in forward and reverse directions.

Remember, in chemistry lessons you were shown a clear example of a reversible reaction for the production of carbonic acid:

CO 2 + H 2 O<->H2CO3

Irreversible reaction- this is a certain chemical reaction that goes to completion in one specific direction.

An example is the phosphorus combustion reaction: 4P + 5O 2 → 2P 2 O 5

Some evidence of the irreversibility of a reaction is the formation of a precipitate or the release of gas.

When the rates of forward and reverse reactions are equal, it occurs chemical equilibrium.

That is, in reversible reactions, equilibrium mixtures of reactants and products are formed. Let us see with an example how a chemical equilibrium is formed. Let's take the reaction of hydrogen iodide formation:

H 2 (g) + I 2 (g)<->2HI(g)

We can heat a mixture of gaseous hydrogen and iodine or ready-made hydrogen iodine, the result in both cases will be the same: the formation of an equilibrium mixture of three substances H 2, I 2, HI.

At the very beginning of the reaction, before the formation of hydrogen iodide, a direct reaction occurs at a rate of ( v etc ). Let us express it by the kinetic equation v pr = k 1, where k 1 is the rate constant of the forward reaction. The product HI is gradually formed, which, under the same conditions, begins to decompose into H 2 and I 2. The equation for this process is as follows: v arr = k 2 2, where v rev – reverse reaction rate, k 2 – reverse reaction rate constant. At the moment when HI is sufficient for leveling v at v chemical equilibrium occurs. The amount of substances in equilibrium, in our case these are H 2, I 2 and HI, does not change over time, but only if there are no external influences. From the above it follows that chemical equilibrium is dynamic. In our reaction, hydrogen iodide is either formed or consumed.

Remember, changing the reaction conditions allows you to move the equilibrium in the desired direction. If we increase the concentration of iodine or hydrogen, it will increase v Thus, there will be a shift to the right, more hydrogen iodide will be formed. If we increase the concentration of hydrogen iodide, it will increase v arr, and the shift will be to the left. We can get more/less reagents and products.

Thus, chemical equilibrium tends to resist external influences. The addition of H 2 or I 2 ultimately leads to an increase in their consumption and an increase in HI. And vice versa. This process in science is called Le–Chatelier principle. It reads:

If a system that is in stable equilibrium is influenced from the outside (by changing temperature, or pressure, or concentration), then a shift will occur in the direction of a process that weakens this influence.

Remember, a catalyst cannot shift the equilibrium. He can only speed up its onset.

Change in concentration . Above, we looked at how this factor shifts the equilibrium either in the forward or in the opposite direction. If the concentration of reactants is increased, the equilibrium shifts to the side where this substance is consumed. If you reduce the concentration, it shifts to the side where this substance is formed. Remember, the reaction is reversible, and the reactants can be substances on both the right and left sides, depending on which reaction we are considering (direct or reverse).

Influencet . Its increase provokes a shift in equilibrium towards the endothermic reaction (- Q), and a decrease towards the exothermic reaction (+ Q). The reaction equations indicate the thermal effect of the forward reaction. The thermal effect of the reverse reaction is the opposite. This rule is only suitable for reactions with a thermal effect. If it is not there, then t is not capable of shifting the equilibrium, but its increase will accelerate the process of the emergence of equilibrium.

Effect of pressure . This factor can be used in reactions involving gaseous substances. If the moles of gas are zero, there will be no changes. As pressure increases, the equilibrium shifts towards smaller volumes. As the pressure decreases, the equilibrium will shift towards larger volumes. Volumes - look at the coefficients of gaseous substances in the reaction equation.