Equilibrium shift under the influence of various factors. Reversible and irreversible chemical reactions

Chemically irreversible reactions under these conditions, they go almost to the end, until the complete consumption of one of the reacting substances (NH4NO3 → 2H2O + N2O - no attempt to obtain nitrate from H2O and N2O leads to a positive result).

Chemically reversible reactions flow simultaneously under given conditions both in the forward and in the reverse direction. There are fewer irreversible reactions than reversible ones. An example of a reversible reaction is the interaction of hydrogen with iodine.

After some time, the rate of formation of HI will become equal to the rate of its decomposition.

In other words, there will be a chemical equilibrium.

chemical equilibrium called the state of the system in which the rate of formation of reaction products is equal to the rate of their transformation into the original reagents.

Chemical equilibrium is dynamic, that is, its establishment does not mean the termination of the reaction.

Law of acting masses:

The mass of substances involved in the reaction is equal to the mass of all products of the reaction.

Law of acting masses establishes the ratio between the masses of reactants in chemical reactions at equilibrium, as well as the dependence of the rate of a chemical reaction on the concentration of the starting substances.

Signs of true chemical equilibrium:

1. the state of the system remains unchanged in time in the absence of external influences;

2. the state of the system changes under the influence of external influences, no matter how small they may be;

3. The state of the system does not depend on which side it approaches equilibrium from.

At steady state, the product of the concentrations of the reaction products, divided by the product of the concentrations of the starting materials, in powers equal to the corresponding stoichiometric coefficients, for a given reaction at a given temperature is a constant value, called the equilibrium constant.

The concentrations of reactants at steady state equilibrium are called equilibrium concentrations.

In the case of heterogeneous reversible reactions, the expression for Kc includes only the equilibrium concentrations of gaseous and dissolved substances. So, for the reaction CaCO3 ↔ CaO + CO2

Under constant external conditions, the equilibrium position is maintained for an arbitrarily long time. When external conditions change, the equilibrium position may change. A change in temperature, concentration of reagents (pressure for gaseous substances) leads to a violation of the equalities of the rates of forward and reverse reactions and, accordingly, to an imbalance. After some time, the equality of speeds will be restored. But the equilibrium concentrations of the reagents under the new conditions will be different. The transition of a system from one equilibrium state to another is called shift or balance shift . Chemical equilibrium can be compared to the position of a balance beam. Just as it changes with the pressure of a load on one of the cups, the chemical equilibrium can shift towards a forward or reverse reaction, depending on the process conditions. Each time a new equilibrium is established, corresponding to new conditions.

The numerical value of the constant usually changes with temperature. At a constant temperature, the values ​​of Kc do not depend on pressure, volume, or concentrations of substances.

Knowing the numerical value of Kc, it is possible to calculate the values ​​of the equilibrium concentrations or pressures of each of the participants in the reaction.

Direction displacement of the position of chemical equilibrium as a result of changes in external conditions is determined Le Chatelier's principle:

If an external influence is exerted on an equilibrium system, then the equilibrium shifts in the direction that counteracts this influence.

Dissolution as a physical and chemical process. solvation. Solvates. Special properties of water as a solvent. Hydrates. Crystal hydrates. Solubility of substances. Dissolution of solid, liquid and gaseous substances. Influence of temperature, pressure and nature of substances on solubility. Methods for expressing the composition of solutions: mass fraction-la, molar concentration, equivalent concentration and mole fraction.

There are two main theories of solutions: physical and chemical.

Physical theory of solutions was proposed by the Nobel Prize winners the Dutchman J. Van't Hoff (1885) and the Swedish physical chemist S. Arrhenius (1883). The solvent is considered as a chemically inert medium in which the particles (molecules, ions) of the dissolved substance are evenly distributed. It is assumed that there is no intermolecular interaction, both between the particles of the solute and between the molecules of the solvent and the particles of the solute. The particles of the solvent and the solute are evenly distributed in the volume of the solution due to diffusion. Subsequently, it turned out that the physical theory satisfactorily describes the nature of only a small group of solutions, the so-called ideal solutions, in which the particles of the solvent and the solute do not really interact with each other. Many gas solutions are examples of ideal solutions.

Chemical (or solvate) theory of solutions proposed by D.I. Mendeleev (1887). For the first time, on a huge experimental material, he showed that a chemical interaction occurs between the particles of a solute and the molecules of a solvent, as a result of which unstable compounds of variable composition are formed, called solvates or hydrates ( if the solvent is water). DI. Mendeleev defined a solution as a chemical system in which all forms of interaction are associated with the chemical nature of the solvent and solutes. Leading role in education solvates unstable intermolecular forces and hydrogen bond play.

Dissolution process cannot be represented by a simple physical model, such as the statistical distribution of a solute in a solvent as a result of diffusion. It is usually accompanied by a noticeable thermal effect and a change in the volume of the solution, due to the destruction of the structure of the solute and the interaction of the solvent particles with the particles of the solute. Both of these processes are accompanied by energy effects. To destroy the structure of the dissolved substance, it is required energy consumption , while the interaction of the particles of the solvent and the solute releases energy. Depending on the ratio of these effects, the dissolution process can be endothermic or exothermic.

When copper sulfate is dissolved, the presence of hydrates is easily detected by a color change: an anhydrous white salt, dissolving in water, forms a blue solution. Sometimes hydration water It binds strongly to the solute and, when it is separated from the solution, enters into the composition of its crystals. Crystalline substances containing water called crystalline hydrates , and the water included in the structure of such crystals is called crystallization water. The composition of crystalline hydrates is determined by the formula of the substance, which indicates the number of crystallization water molecules per one of its molecule. So, the formula of crystalline copper sulfate (copper sulfate) CuSO4 × 5H2O. The preservation of the color characteristic of the corresponding solutions by crystalline hydrates is direct evidence of the existence of similar hydrate complexes in solutions. The color of the crystalline hydrate depends on the number of molecules of water of crystallization.

There are various ways to express the composition of a solution.. Most commonly used mass fraction solute, molar and normal concentration.

In general, the concentration can be expressed as the number of particles per unit volume or as the ratio of the number of particles of a given type to the total number of particles in solution. The amount of a solute and solvent is measured in units of mass, volume, or moles. Generally, solution concentration - this is the amount of a dissolved substance in a condensed system (mixture, alloy or in a certain volume of solution). There are different ways of expressing the concentration of solutions, each of which has a predominant application in a particular field of science and technology. Usually, the composition of solutions is expressed using dimensionless (mass and mole fractions) and dimensional quantities (molar concentration of a substance, molar concentration of a substance - equivalent and molality).

Mass fraction- a value equal to the ratio of the mass of the dissolved substance (m1) to the total mass of the solution (m).

Video lesson 2: Shift in chemical equilibrium

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

In the previous lesson, you learned what the rate of a chemical reaction is and what factors affect it. In this lesson, we will look at how these reactions proceed. It depends on the behavior of the initial 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 again converted into starting substances, then the reaction is reversible. Considering this, we formulate the definitions:

reversible reaction is a certain reaction that proceeds under the same conditions in the forward and reverse directions.

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

CO 2 + H 2 O<->H2CO3

irreversible reaction is a certain chemical reaction that goes to the end in one specific direction.

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

One of the evidence of the irreversibility of the reaction is the precipitation or evolution of gas.

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

That is, in reversible reactions, equilibrium mixtures of reactants and products are formed. Let's see an example of how chemical equilibrium is formed. Take the reaction for the formation of hydrogen iodine:

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

We can heat a mixture of gaseous hydrogen and iodine or already prepared 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 iodine, a direct reaction takes place at a rate ( v etc ). We express it by the kinetic equation v pr \u003d k 1, where k 1 is the rate constant of the direct 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 \u003d k 2 2, where v arr is the rate of the reverse reaction, k 2 is the rate constant of the reverse reaction. The moment when HI is enough to equalize v at v a chemical equilibrium is reached. The number of substances in equilibrium, in our case, it is H 2 , I 2 and HI does not change with time, but only if there are no external influences. From what has been said, it follows that the chemical equilibrium is dynamic. In our reaction, hydrogen iodine is either formed or consumed.

Remember, changing the reaction conditions allows you to shift the equilibrium in the right direction. If we increase the concentration of iodine or hydrogen, then the v pr, there will be a shift to the right, more hydrogen iodide will be formed. If we increase the concentration of hydrogen iodine, the 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 is scientifically called principle of Le Chatelier. It says:

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

Remember, the catalyst is not able to shift the balance. He can only hasten its advance.

Concentration change . Above, we considered how this factor shifts the balance 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 the concentration is reduced, it shifts to the side where this substance is formed. Remember, the reaction is reversible, and the reactants can be substances on the right side or on the left, depending on which reaction we are considering (direct or reverse).

Influencet . Its growth provokes a shift of equilibrium towards an endothermic reaction (- Q), and a decrease towards an exothermic reaction (+ Q). The reaction equations indicate the thermal effect of the direct reaction. The thermal effect of the reverse reaction is opposite to it. This rule applies only to 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 equilibrium emergence.

Pressure influence . This factor can be used in reactions involving gaseous substances. If the moles of the gas are equal to zero, there will be no change. 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 in front of gaseous substances in the reaction equation.

Reversible reactions are reactions that occur simultaneously in two opposite directions.

Irreversible reactions - reactions in which the taken substances are completely converted into reaction products that do not react with each other under given conditions, for example, the decomposition of explosives, the combustion of hydrocarbons, the formation of low-dissociating compounds, precipitation, the formation of gaseous substances.

32. Chemical balance. Le Chatelier's principle.

Chemical equilibrium is a state of a chemical system in which one or more chemical reactions reversibly proceed, and the rates in each pair of forward-reverse reactions are equal to each other. For a system in chemical equilibrium, the concentrations of reagents, temperature, and other parameters of the system do not change with time.

33. Le Chatelier's principle. Conditions for shifting chemical equilibrium.

Le Chatelier's principle: if an external influence is exerted on a system in a state of equilibrium, then the equilibrium shifts in the direction of weakening the external influence.

Factors affecting chemical equilibrium:

1) temperature

As the temperature increases, the chemical equilibrium shifts towards an endothermic (absorption) reaction, and as it decreases, towards an exothermic (isolation) reaction.

CaCO 3 \u003d CaO + CO 2 -Q t →, t↓ ←

N 2 +3H 2 ↔2NH 3 +Q t ←, t↓ →

2) pressure

When the pressure increases, the chemical equilibrium shifts towards a smaller volume of substances, and when it decreases, towards a larger volume. This principle only applies to gases, i.e. if solids are involved in the reaction, they are not taken into account.

CaCO 3 \u003d CaO + CO 2 P ←, P↓ →

1mol=1mol+1mol

3) concentration of starting substances and reaction products

With an increase in the concentration of one of the starting substances, the chemical equilibrium shifts towards the reaction products, and with a decrease in the concentration of the reaction products, towards the starting substances.

S 2 +2O 2 \u003d 2SO 2 [S], [O] →, ←

Catalysts do not affect the shift of chemical equilibrium!

End of work -

This topic belongs to:

Basic concepts of chemistry

Chemistry is the science of substances and the laws of their transformation, the object of study of chemistry is the chemical elements and their compounds, a chemical element, calling the type of atoms .. law .. the order in which orbitals are filled with electrons ..

If you need additional material on this topic, or you did not find what you were looking for, we recommend using the search in our database of works:

What will we do with the received material:

If this material turned out to be useful for you, you can save it to your page on social networks:

All topics in this section:

Law of Equivalents
Substances interact with each other in amounts proportional to their equivalents. m(a)/m(b)=E(a)/E(b). An equivalent is a real or conditional particle of a substance that is equivalent to one ion

electrode cloud. quantum numbers
An electron cloud is a visual model that reflects the distribution of electron density in an atom or molecule. To characterize the behavior of an electron in an atom, quantum numbers are introduced: chap.

Quantum-mechanical model of the structure of the atom
The QMM is based on the quantum theory of the atom, according to which the electron has both the properties of a particle and the properties of a wave. In other words, the location of an electron at a certain point can

Periodic law and periodic system D.I. Mendeleev
The discovery of the Periodic Law by D.I. Mendeleev The Periodic Law was discovered by D.I. Mendeleev while working on the text of the textbook "Fundamentals of Chemistry", when he encountered difficulties

inorganic compounds
Acids are complex chemicals. compounds consisting of H ions and an acid residue. They are divided into single-component and multi-component, oxygen-containing and oxygen-free. The bases are

Salts and their chem. properties
Salts are a class of chemical compounds consisting of cations and anions. Chemical properties are determined by the properties of the cations and anions that make up their composition. Salts interact with

covalent bond. Saturation and Directionality
A covalent bond is a chemical communication between atoms, carried out by socialized electrons. Kov. The bond is either polar or non-polar. Nonpolar cov. connection n. in molecules where each atomic nucleus with

The main provisions of the theory of VS. Hybridization
The main provisions of the theory of VS: A) the chemical bond between two atoms arises as a result of overlapping AO with the image. electronic pairs. B) atoms entering the chemical. communication, exchange

hydrogen bond
A hydrogen bond is a form of association between an electronegative atom and an H hydrogen atom bonded covalently to another electronegative atom. As electronegative atoms, you can

Donor-acceptor bond. Complex compounds
Mechanism image. covalent bond due to two electrons of one atom (donor) and a free orbital of another atom (acceptor) called. donor-acceptor. Complex compounds are compounds

complex compounds. Chemical bond in a complex compound
A complex compound is a chemical substance that contains complex particles. Chem. bond-In crystalline complex compounds with charged complexes, the bond between the complex and in

Dissociation of complex compounds. Stability constants of complex ions
The dissociation of a complex compound proceeds in two stages: a) dissociation into complex and simple ions with preservation of the inner sphere of the complex and b) dissociation of the inner sphere, drive

First law of thermodynamics. Hess' law
1st start t/d: in any process, the change in the internal energy U of the system is equal to the sum of the amount of heat transferred and the work done. ΔU=Q – W If the system is in

1 and 2 laws of thermodynamics. Calculation of thermal effects of chemical reactions
The formulation of the I law of t / d: energy is not created or destroyed, but only passes from one form to another in an equivalent ratio. Formulation of the second law of t/d: in an isolated system

Hess's law and consequences from it
Hess' law: the heat of a chemical reaction is equal to the sum of the heats of any series of successive reactions with the same initial substances and final products. The calculations use the consequences of the law

The concept of the standard state and standard heats of formation. Calculation of thermal effects of chemical reactions
Standard states - in chemical thermodynamics, conditionally accepted states of individual substances and components of solutions in the assessment of thermodynamic quantities. under standard heat

Gibbs free energy. Direction of a chemical reaction
Gibbs free energy (or simply Gibbs energy, or Gibbs potential, or thermodynamic potential in the narrow sense) is a quantity that shows the change in energy during a chemical reaction.

The rate of a chemical reaction. Law of acting masses
Chemical kinetics is a branch of chemistry that studies the rate of chemical reactions and the mechanism of chemical reactions. The rate of a chemical reaction is the number of favorable collisions

Arrhenius equation. The concept of activation energy
lnk=lnA-Ea/2.3RT Activation energy is the minimum energy that particles must have in order to enter into a chemical interaction.

Catalysts. Homogeneous and heterogeneous catalysis
Catalyst - a substance that changes the rate of a chemical reaction, but does not enter into chemical interaction and is excreted at the end of the reaction in its pure form. The process of accelerating the reaction in the presence

Colligative properties of solutions
The colligative properties of solutions are those properties that, under given conditions, turn out to be equal and independent of the chemical nature of the solute; properties of solutions that depend

Raoult's laws. Boiling and freezing points of solutions
A vapor in equilibrium with a liquid is called saturated. The pressure of such a vapor over a pure solvent (p0) is called the pressure or elasticity of the saturated vapor of pure pa

Osmosis and osmotic pressure
Diffusion is the process of mutual penetration of molecules. Osmosis is the process of one-way diffusion through a semi-permeable membrane of solvent molecules towards a higher concentration of the solution.

Dissolution of gases in liquids. Henry's Law
The solubility of substances is affected by temperature and pressure. Their influence on the equilibrium in solution obeys the Le Chatelier principle. The solubility of gases is accompanied by: A) the release of heat

Degree and constant of electrolytic dissociation. Ostwald's breeding law
Electrolytic dissociation is the disintegration of a molecule into ions under the action of polar solvent molecules. E.d. implies the ionic conductivity of the solution. Degree ed. - a value equal to the ratio

Ionic product of water. Hydrogen index of the environment
The ionic product of water - a value equal to the product of hydrogen cations and hydroxide ions is a constant value at a given temperature (25 ° C) and is equal to 10-14. kw=

Electrolytic dissociation of water. Hydrogen index of the environment
Water is a weak amphoteric electrolyte. Water molecules can both donate and add H+ cations. As a result of the interaction between molecules in aqueous solutions, there is always and

Degree and constant of hydrolysis of salts
The degree of hydrolysis refers to the ratio of the part of the salt undergoing hydrolysis to the total concentration of its ions in solution. Denoted α (or hhydr); α = (chydr

Activity and ionic strength of solutions. Relationship between activity coefficient and ionic strength of a solution
The activity of the components of the solution is the effective (apparent) concentration of the components, taking into account the various interactions between them in the solution. a=f*c Ionic strength of a solution - measure of intensity

The concept of electrode potential
Electrode potential - the difference in electrical potentials between the electrode and the electrolyte in contact with it (most often between the metal and the electrolyte solution). WHO

Electrode potential. Nernst equation
Electrode potential - the difference in electrical potentials between the electrode and the electrolyte in contact with it (most often between the metal and the electrolyte solution). Pin

gas electrodes. Nernst equation for calculating the potentials of gas electrodes
Gas electrodes consist of a conductor of the 1st kind, which is in contact simultaneously with a gas and a solution containing ions of this gas. The conductor of the 1st kind serves for the supply and removal of electrons and, in addition to

Galvanic cell. Calculation of the EMF of a galvanic cell
GALVANIC CELL - a chemical current source in which electrical energy is generated as a result of direct conversion of chemical energy by a redox reaction. In co

Concentration and electrochemical polarization
concentration polarization. A change in the electrode potential due to a change in the concentration of reagents in the near-electrode layer during the passage of current is called concentration polarization. In my

Electrolysis. Faraday's laws

Electrolysis. current output. Electrolysis with insoluble and soluble anodes
Electrolysis is a physical and chemical process consisting in the release of constituents of dissolved substances or other substances on the electrodes, which are the result of secondary reactions on the electrodes,

The main types of corrosion. Methods for protecting metals from corrosion
Corrosion is the process of destruction of metals under the influence of electrochemical or chemical environmental factors. Accordingly, two types of corrosion are distinguished, depending on the method of interaction

chemical corrosion. Chemical corrosion rate
Chemical corrosion - corrosion caused by the interaction of Me with dry gases or liquids that do not conduct electric current. The rate of chemical corrosion depends on many factors.

Stray current corrosion
Stray currents emanating from electrical installations operating on direct current, trams, subways, electric railways, cause the appearance of a patch on metal objects (cables, rails).

All chemical reactions can be divided into two groups: irreversible and reversible reactions. Irreversible reactions proceed to the end - until one of the reactants is completely consumed. Reversible reactions do not proceed to the end: in a reversible reaction, none of the reactants is completely consumed. This difference is due to the fact that an irreversible reaction can only proceed in one direction. A reversible reaction can proceed both in the forward and reverse directions.

Let's consider two examples.

Example 1. The interaction between zinc and concentrated nitric acid proceeds according to the equation:

With a sufficient amount of nitric acid, the reaction will end only when all the zinc has dissolved. In addition, if you try to carry out this reaction in the opposite direction - to pass nitrogen dioxide through a solution of zinc nitrate, then metallic zinc and nitric acid will not work - this reaction cannot proceed in the opposite direction. Thus, the interaction of zinc with nitric acid is an irreversible reaction.

Example 2. The synthesis of ammonia proceeds according to the equation:

If one mol of nitrogen is mixed with three mols of hydrogen, conditions favorable for the reaction to occur in the system, and after a sufficient time the gas mixture is analyzed, the analysis results will show that not only the reaction product (ammonia) will be present in the system, but also the initial substances (nitrogen and hydrogen). If now, under the same conditions, not a nitrogen-hydrogen mixture, but ammonia, is placed as the starting substance, then it will be possible to find that part of the ammonia decomposes into nitrogen and hydrogen, and the final ratio between the quantities of all three substances will be the same as in that case when starting from a mixture of nitrogen and hydrogen. Thus, the synthesis of ammonia is a reversible reaction.

In the equations of reversible reactions, arrows can be used instead of the equal sign; they symbolize the flow of the reaction in both forward and reverse directions.

On fig. 68 shows the change in the rates of forward and reverse reactions over time. Initially, when the starting materials are mixed, the rate of the forward reaction is high, and the rate of the reverse reaction is zero. As the reaction proceeds, the starting materials are consumed and their concentrations fall.

Rice. 63. Change in the rate of forward and reverse reactions over time.

As a result, the rate of the forward reaction decreases. At the same time, reaction products appear and their concentration increases. As a result, a reverse reaction begins to take place, and its rate gradually increases. When the rates of the forward and reverse reactions become equal, chemical equilibrium occurs. So, in the last example, an equilibrium is established between nitrogen, hydrogen and ammonia.

Chemical equilibrium is called dynamic equilibrium. This emphasizes that at equilibrium, both forward and reverse reactions occur, but their rates are the same, as a result of which changes in the system are not noticeable.

A quantitative characteristic of chemical equilibrium is a quantity called the constant of chemical equilibrium. Consider it using the example of the iodine-hydrogen synthesis reaction:

According to the law of mass action, the rates of forward and reverse reactions are expressed by the equations:

At equilibrium, the rates of the forward and reverse reactions are equal to each other, whence

The ratio of the rate constants of the forward and reverse reactions is also a constant. It is called the equilibrium constant of this reaction (K):

Hence finally

On the left side of this equation are those concentrations of interacting substances that are established at equilibrium - equilibrium concentrations. The right side of the equation is a constant (at constant temperature) value.

It can be shown that in the general case of a reversible reaction

the equilibrium constant is expressed by the equation:

Here, capital letters denote the formulas of substances, and small letters denote the coefficients in the reaction equation.

Thus, at a constant temperature, the equilibrium constant of a reversible reaction is a constant value showing the ratio between the concentrations of the reaction products (numerator) and starting substances (denominator), which is established at equilibrium.

The equilibrium constant equation shows that under equilibrium conditions, the concentrations of all substances participating in the reaction are interconnected. A change in the concentration of any of these substances entails a change in the concentrations of all other substances; as a result, new concentrations are established, but the ratio between them again corresponds to the equilibrium constant.

The numerical value of the equilibrium constant in the first approximation characterizes the yield of this reaction. For example, at , the reaction yield is large, because at the same time

i.e., at equilibrium, the concentrations of the reaction products are much higher than the concentrations of the starting materials, and this means that the reaction yield is high. At (for a similar reason), the yield of the reaction is small.

In the case of heterogeneous reactions, the expression of the equilibrium constant, as well as the expression of the law of action of masses (see § 58), includes the concentrations of only those substances that are in the gas or liquid phase. For example, for the reaction

the equilibrium constant has the form:

The value of the equilibrium constant depends on the nature of the reactants and on the temperature. It does not depend on the presence of catalysts. As already mentioned, the equilibrium constant is equal to the ratio of the rate constants of the forward and reverse reactions. Since the catalyst changes the activation energy of both the forward and reverse reactions by the same amount (see § 60), it does not affect the ratio of their rate constants.

Therefore, the catalyst does not affect the value of the equilibrium constant and, therefore, can neither increase nor decrease the yield of the reaction. It can only speed up or slow down the onset of equilibrium.

Chemical reactions are reversible and irreversible.

those. if some reaction A + B = C + D is irreversible, this means that the reverse reaction C + D = A + B does not occur.

i.e., for example, if a certain reaction A + B = C + D is reversible, this means that both the reaction A + B → C + D (direct) and the reaction C + D → A + B (reverse) proceed simultaneously ).

In fact, because both direct and reverse reactions proceed, reagents (starting substances) in the case of reversible reactions can be called both substances on the left side of the equation and substances on the right side of the equation. The same goes for products.

For any reversible reaction, a situation is possible when the rates of the forward and reverse reactions are equal. Such a state is called state of equilibrium.

In a state of equilibrium, the concentrations of both all reactants and all products are unchanged. The concentrations of products and reactants at equilibrium are called equilibrium concentrations.

Shift in chemical equilibrium under the influence of various factors

Due to such external influences on the system as a change in temperature, pressure or concentration of starting substances or products, the equilibrium of the system may be disturbed. However, after the cessation of this external influence, the system will pass to a new state of equilibrium after some time. Such a transition of a system from one equilibrium state to another equilibrium state is called shift (shift) of chemical equilibrium .

In order to be able to determine how the chemical equilibrium shifts with a particular type of exposure, it is convenient to use the Le Chatelier principle:

If any external influence is exerted on a system in a state of equilibrium, then the direction of the shift in chemical equilibrium will coincide with the direction of the reaction that weakens the effect of the impact.

The influence of temperature on the state of equilibrium

When the temperature changes, the equilibrium of any chemical reaction shifts. This is due to the fact that any reaction has a thermal effect. In this case, the thermal effects of the forward and reverse reactions are always directly opposite. Those. if the forward reaction is exothermic and proceeds with a thermal effect equal to +Q, then the reverse reaction is always endothermic and has a thermal effect equal to -Q.

Thus, in accordance with Le Chatelier's principle, if we increase the temperature of some system that is in a state of equilibrium, then the equilibrium will shift towards the reaction, during which the temperature decreases, i.e. towards an endothermic reaction. And similarly, if we lower the temperature of the system in a state of equilibrium, the equilibrium will shift towards the reaction, as a result of which the temperature will rise, i.e. towards an exothermic reaction.

For example, consider the following reversible reaction and indicate where its equilibrium will shift as the temperature decreases:

As you can see from the equation above, the forward reaction is exothermic, i.e. as a result of its flow, heat is released. Therefore, the reverse reaction will be endothermic, that is, it proceeds with the absorption of heat. By condition, the temperature is lowered, therefore, the equilibrium will shift to the right, i.e. towards a direct reaction.

Effect of concentration on chemical equilibrium

An increase in the concentration of reagents in accordance with the Le Chatelier principle should lead to a shift in equilibrium towards the reaction in which the reagents are consumed, i.e. towards a direct reaction.

Conversely, if the concentration of the reactants is lowered, then the equilibrium will shift towards the reaction that results in the formation of the reactants, i.e. side of the reverse reaction (←).

A change in the concentration of reaction products also affects in a similar way. If you increase the concentration of products, the equilibrium will shift towards the reaction, as a result of which the products are consumed, i.e. towards the reverse reaction (←). If, on the contrary, the concentration of products is lowered, then the equilibrium will shift towards the direct reaction (→), in order for the concentration of products to increase.

Effect of pressure on chemical equilibrium

Unlike temperature and concentration, a change in pressure does not affect the equilibrium state of every reaction. In order for a change in pressure to lead to a shift in chemical equilibrium, the sums of the coefficients in front of gaseous substances on the left and right sides of the equation must be different.

Those. from two reactions:

a change in pressure can affect the state of equilibrium only in the case of the second reaction. Since the sum of the coefficients in front of the formulas of gaseous substances in the case of the first equation on the left and right is the same (equal to 2), and in the case of the second equation it is different (4 on the left and 2 on the right).

From this, in particular, it follows that if there are no gaseous substances among both the reactants and the products, then a change in pressure will not affect the current state of equilibrium in any way. For example, pressure will not affect the equilibrium state of the reaction:

If the amount of gaseous substances is different on the left and on the right, then an increase in pressure will lead to a shift in equilibrium towards the reaction, during which the volume of gases decreases, and a decrease in pressure will lead to a shift in the direction of the reaction, as a result of which the volume of gases increases.

Effect of a catalyst on chemical equilibrium

Since a catalyst equally accelerates both the forward and reverse reactions, its presence or absence does not affect to a state of equilibrium.

The only thing that a catalyst can affect is the rate of transition of the system from a non-equilibrium state to an equilibrium one.

The impact of all the above factors on chemical equilibrium is summarized below in a cheat sheet, which at first you can peek at when performing balance tasks. However, she will not be able to use it in the exam, therefore, after analyzing several examples with her help, she should be taught and trained to solve tasks for balance, no longer peeping into her:

Designations: T - temperature, p - pressure, with – concentration, – increase, ↓ – decrease