With an increase in temperature for every 10 degrees. Van't Hoff's rule. Temperature coefficient of reaction rate. Rate of heterogeneous reactions

where g is ttemperature coefficient, taking values ​​from 2 to 4.

An explanation for the dependence of the reaction rate on temperature was given by S. Arrhenius. Not every collision of reactant molecules leads to a reaction, but only the strongest collisions. Only molecules with excess kinetic energy are capable of chemical reactions.

S. Arrhenius calculated the fraction of active (i.e., leading to a reaction) collisions of reacting particles a, depending on temperature: - a = exp(-E/RT). and brought out Arrhenius equation for the reaction rate constant:

k = koe-E/RT

where ko and E d depend on the nature of the reagents. E is the energy that must be given to molecules in order for them to interact, called activation energy.

Van't Hoff's rule- an empirical rule that allows, as a first approximation, to estimate the effect of temperature on the rate of a chemical reaction in a small temperature range (usually from 0 °C to 100 °C). J. H. Van't Hoff, based on many experiments, formulated the following rule:

Activation energy in chemistry and biology - minimal amount energy that must be supplied to the system (in chemistry expressed in joules per mole) for a reaction to occur. The term was introduced by Svante August Arrhenius in. Typical notation for reaction energy Ea.

Activation entropy is considered as the difference between the entropy of the transition state and the ground state of the reactants. It is determined mainly by the loss of translational and rotational degrees of freedom of particles during the formation of an activated complex. Significant changes (vibrational degrees of freedom) can also occur if the activated complex is somewhat more tightly packed than the reactants.

The activation entropy of such a transition is positive.

Activation entropy depends on many factors. When in a bimolecular reaction two initial particles join together to form transition state, the translational and rotational entropy of two particles decreases to values ​​corresponding to a single particle; a slight increase in vibrational entropy is not enough to compensate for this effect.

Activation entropies essentially vary more depending on structure than enthalpies. The activation entropies agree well in most cases with the Price and Hammett rule. This series also has the particular significance that the increase in the entropy of the silap can probably be accurately calculated from the known absolute entropies of the corresponding hydrocarbons

The dependence of the reaction rate on temperature is approximately determined by the empirical Van't Hoff rule: With every 10 degree change in temperature, the rate of most reactions changes by 2-4 times.

Mathematically, van't Hoff's rule is expressed as follows:

where v(T2) and v(T1) are reaction rates, respectively, at temperatures T2 and T1 (T2> T1);

γ-temperature coefficient of reaction rate.

The value of γ for an endothermic reaction is higher than for an exothermic one. For many reactions, γ lies in the range of 2-4.

The physical meaning of the value γ is that it shows how many times the reaction rate changes with a change in temperature for every 10 degrees.

Since the reaction rate and the rate constant of a chemical reaction are directly proportional, expression (3.6) is often written in the following form:

(3.7)

where k(T2), k(T1) are reaction rate constants, respectively

at temperatures T2 and T1;

γ is the temperature coefficient of the reaction rate.

Example 8. How many degrees must the temperature be increased to increase the reaction rate by 27 times? Temperature coefficient reaction is 3.

Solution. We use expression (3.6):

We get: 27 = , = 3, DT = 30.

Answer: 30 degrees.

The speed of the reaction and the time during which it occurs are inversely proportional: the larger v, the more

less than t. Mathematically this is expressed by the relation

Example 9. At a temperature of 293 K, the reaction proceeds in 2 minutes. How long will this reaction take to occur at a temperature of 273 K if γ = 2.

Solution. From equation (3.8) it follows:

.

We use equation (3.6), since We get:

min.

Answer: 8 min.

Van't Hoff's rule applies to a limited number chemical reactions. The effect of temperature on the rate of processes is often determined using the Arrhenius equation.

Arrhenius equation . In 1889, the Swedish scientist S. Arre-1ius, based on experiments, derived an equation that is named after him

where k is the reaction rate constant;

k0 - pre-exponential factor;

e is the base of the natural logarithm;

Ea is a constant called activation energy, determined by the nature of the reagents:

R is the universal gas constant equal to 8.314 J/mol×K.

Ea values ​​for chemical reactions range from 4 to 400 kJ/mol.

Many reactions are characterized by a certain energy barrier. To overcome it, activation energy is necessary - some excess energy (compared to the harmful energy of molecules at a given temperature), which molecules must have in order for their collision to be effective, that is, to lead to the formation of a new substance. As the temperature rises, the number of active molecules increases rapidly, which leads to a sharp increase in the reaction rate.

In general, if the reaction temperature changes from T1 to T2, equation (3.9) after logarithm takes the form:

. (3.10)

This equation allows you to calculate the activation energy of a reaction as the temperature changes from T1 to T2.

The rate of chemical reactions increases in the presence of a catalyst. The effect of a catalyst is that it forms unstable intermediate compounds (activated complexes) with reagents, the decomposition of which leads to the formation of reaction products. In this case, the activation energy decreases, and molecules whose energy was insufficient to carry out the reaction in the absence of a catalyst become active. As a result, the total number of active molecules increases and the reaction rate increases.

The change in reaction rate in the presence of a catalyst is expressed by the following equation:

, (3.11)

where vcat, and Ea(cat) are the speed and activation energy of a chemical reaction in the presence of a catalyst;

v and Ea are the speed and activation energy of a chemical reaction without a catalyst.

Example 10. The activation energy of a certain reaction in the absence of a catalyst is 75.24 kJ/mol, with a catalyst - 50.14 kJ/mol. How many times does the reaction rate increase in the presence of a catalyst if the reaction occurs at a temperature of 298 K? Solution. Let's use equation (3.11). Substituting data into the equation

The law of mass action establishes the relationship between the masses of reacting substances in chemical reactions at equilibrium. The law of mass action was formulated in 1864-1867. K. Guldberg and P. Waage. According to this law, the rate at which substances react with each other depends on their concentration. The law of mass action is used in various calculations of chemical processes. It makes it possible to solve the question in which direction the spontaneous course of the reaction under consideration is possible at a given ratio of the concentrations of the reacting substances, what yield of the desired product can be obtained.

Question 18. Van't Hoff's rule.

Van't Hoff's rule is an empirical rule that allows, as a first approximation, to estimate the effect of temperature on the rate of a chemical reaction in a small temperature range (usually from 0 °C to 100 °C). Van't Hoff, based on many experiments, formulated the following rule: With every 10 degree increase in temperature, the rate constant of a homogeneous elementary reaction increases two to four times. The equation that describes this rule is:

V = V0 * Y(T2 − T1) / 10

where V is the reaction rate at a given temperature (T2), V0 is the reaction rate at temperature T1, Y is the temperature coefficient of the reaction (if it is equal to 2, for example, then the reaction rate will increase 2 times when the temperature increases by 10 degrees).

It should be remembered that Van't Hoff's rule has a limited scope of applicability. Many reactions do not obey it, for example, reactions occurring at high temperatures, very fast and very slow reactions. Van't Hoff's rule also does not apply to reactions involving bulky molecules, such as proteins in biological systems. The temperature dependence of the reaction rate is more correctly described by the Arrhenius equation.

V = V0 * Y(T2 − T1) / 10

Question 19. Activation energy.

Activation energy in chemistry and biology, the minimum amount of energy that must be supplied to the system (in chemistry expressed in joules per mole) for a reaction to occur. The term was introduced by Svante August Arrhenius in 1889. A typical designation for the reaction energy is Ea.

Activation energy in physics is the minimum amount of energy that electrons of a donor impurity must receive in order to enter the conduction band.

IN chemical model, known as the Theory of Active Collisions (TAC), there are three conditions necessary for a reaction to occur:

Molecules must collide. This is an important condition, but it is not sufficient, since a collision does not necessarily cause a reaction.

Molecules must have the necessary energy (activation energy). During a chemical reaction, the interacting molecules must pass through an intermediate state, which may have higher energy. That is, the molecules must overcome an energy barrier; if this does not happen, the reaction will not begin.

The molecules must be correctly oriented relative to each other.

At low (for a certain reaction) temperature, most molecules have energy less than the activation energy and are unable to overcome the energy barrier. However, in a substance there will always be individual molecules whose energy is significantly higher than the average. Even at low temperatures, most reactions continue to occur. Increasing the temperature allows you to increase the proportion of molecules with sufficient energy to overcome the energy barrier. This increases the reaction speed.

Mathematical description

The Arrhenius equation establishes the relationship between activation energy and reaction rate:

k is the reaction rate constant, A is the frequency factor for the reaction, R is the universal gas constant, T is the temperature in kelvins.

As the temperature rises, the probability of overcoming the energy barrier increases. General rule of thumb: a 10K increase in temperature doubles the reaction rate

Transition state

The relationship between the activation energy (Ea) and the enthalpy (entropy) of the reaction (ΔH) in the presence and absence of a catalyst. The highest point of energy represents an energy barrier. In the presence of a catalyst, less energy is required to start a reaction.

A transition state is a state of a system in which the destruction and creation of a connection are balanced. The system is in a transition state for a short time (10-15 s). The energy that must be expended to bring the system into a transition state is called activation energy. In multistep reactions that include several transition states, the activation energy corresponds to the highest energy value. After overcoming the transition state, the molecules scatter again with the destruction of old bonds and the formation of new ones or with the transformation of the original bonds. Both options are possible, since they occur with the release of energy (this is clearly visible in the figure, since both positions are energetically lower than the activation energy). There are substances that can reduce the activation energy for a given reaction. Such substances are called catalysts. Biologists call such substances enzymes. Interestingly, catalysts thus speed up the reaction without participating in it themselves.

At As the temperature increases, the rate of most chemical reactions increases significantly, and for homogeneous reactions When heated for every ten degrees, the reaction rate increases 2-4 times.

Total number particles in the system (N) is equal to the area under the curve. The total number of particles with energy greater than Ea is equal to the shaded area.

From Figure 2 it can be seen that as the temperature increases, the energy distribution of particles changes so that the proportion of particles with higher energy increases. Thus important concept for a chemical reaction is the activation energy.

Activation energy is the energy that particles must have in order for their interaction to lead to a chemical reaction. The activation energy is expressed in kJ/mol. For reactions occurring at a noticeable rate, the activation energy does not exceed 50 kJ/mol (for ion exchange reactions Ea » 0); if Ea > 100 kJ/mol, then the reaction rate is immeasurably low.

In 1889, S. Arrhenius gave an equation for the dependence of the rate constant of a chemical reaction on temperature:

k = Ae - Ea/RT

where, A - pre-expopotential factor, depending on the nature of the reacting substances;

R- gas constant = 8.314 J/(mol? K);

Ea- activation energy.

From the Arrhenius equation it follows that the higher the activation energy, the more it is necessary to increase the temperature to maintain the required reaction rate.

Figure 3 shows the dependence of the change in the potential energy of the reacting system on the reaction path. From the above figure it can be seen that for an exothermic reaction (which occurs with the release of heat), the loss of active molecules is replenished by the energy released during the reaction. In the case of an endothermic reaction, heat is required to maintain the required reaction rate.

Figure 10.3 Energy diagram of a chemical reaction

A - reactants, C - products.

2.4 Influence of foreign substances

Foreign substances, depending on the impact they have, can accelerate reactions - catalysts or slow them down - inhibitors.

Catalysts- these are substances that accelerate chemical reactions, but remain unchanged after the reaction.

Inhibitors - these are substances that slow down reactions. In practice, it is sometimes necessary to slow down reactions (corrosion of metals, etc.) this is achieved by introducing inhibitors into the reaction system. For example, sodium nitrite, potassium chromate and dichromate reduce the rate of corrosion of metals.

Promoters- substances that increase the activity of the catalyst. In this case, promoters themselves may not have catalytic properties.

Catalytic poisons- foreign impurities in the reaction mixture, leading to partial or complete loss of catalyst activity. Thus, traces of arsenic and phosphorus cause a rapid loss of activity by the V 2 O 5 catalyst during the contact method for producing H 2 SO 4.

3. Chemical equilibrium

In chemical reactions, the starting substances are not always completely converted into reaction products. This occurs because as reaction products accumulate, conditions may be created for the reverse reaction to occur. Most chemical reactions are reversible.

As an example, let us analyze the reversible reaction of ammonia synthesis from nitrogen and hydrogen, which is extremely important for industry:

direct reaction -2N 2 + 3H 2 →2NH 3 ,

reverse reaction - 2NH 3 →N 2 + 3H 2,

reversible reaction - 2N 2 + 3H 2« 2NH 3.

The forward and reverse reactions are separate reactions with their corresponding kinetic equations, pre-exposure factors, activation energies, etc.

An important quantitative characteristic of reversible reactions is the equilibrium constant, which is determined when the system reaches chemical equilibrium- a state in which the rates of forward and reverse reactions are equal. Examples of application of the law of mass action (LMA).

Let us derive the equilibrium constant using the example of the ammonia synthesis reaction.

Kinetic equation of forward reaction

N 2 +3H 2 →2NH 3

has the form Vpr = Kpr 3.

Kinetic equation of the reverse reaction

2NH 3 →N 2 + 3H 2

has the form Vobr = Cobr 2.

In a state of chemical equilibrium, Vpr = Vbr.

Substituting the expressions for the rates of direct and reverse reactions into the condition of chemical equilibrium, we obtain the following equality Kpr 3 = Cobr 2.

After transformation we get

.

4. Le Chatelier's principle

If a system in a state of chemical equilibrium is subject to any external influence, then the equilibrium as a result of processes occurring in the system will shift in such a way that the effect will decrease.

4.1 Effect of changing concentrations on equilibrium

When the concentration of any of the substances participating in the reaction increases, the equilibrium shifts towards the consumption of this substance, and when it decreases, towards the formation of this substance.

Example 1. If in an equilibrium system

2N 2 + 3H 2« 2NH 3

add N 2 or H 2 , then, in accordance with Le Chatelier’s principle, to reduce the concentrations of these substances, the equilibrium should shift to the right, the yield of NH 3 will increase. As the concentration of NH 3 increases, the equilibrium will correspondingly shift to the left.

4.2 Effect of pressure changes on equilibrium

The pressure in a closed reaction system is determined by the presence of gaseous substances in it: the more of them, the greater the pressure. Therefore, a change in external pressure will affect the equilibrium only in cases where gaseous substances are involved, and their quantity in the forward and reverse reactions is different.

If the pressure is increased in a system that is in a state of chemical equilibrium, then a reaction will predominantly occur, as a result of which the amount of gaseous substances decreases; when the pressure decreases, a reaction occurs preferentially, as a result of which the amount of gaseous products increases.

Example 1. Is it possible to increase the yield of products in a reaction by changing the pressure? CO 2 (g) + H 2 (g)« CO(g) + H 2 O(g).

Solution: The reaction mixture includes gaseous reagents, but their quantity in the reaction does not change: from one mole of CO 2 (g) and one mole of H2 (g), one mole of CO (g) and H 2 O (g) are obtained. For this reason, changes in pressure do not affect the equilibrium state.

Example 2. How will the equilibrium concentrations of reactants change with increasing pressure in the system? N 2 + 3H 2 « 2NH 3 ?

From the reaction equation it is clear that from 4 moles of the gas of the initial products, 2 moles of the gas of the reaction products are formed. Thus, with an increase in pressure, the equilibrium will shift from the forward reaction, since it leads to a decrease in pressure.

4.3 Effect of temperature changes on chemical equilibrium

Most chemical reactions occur with the release or absorption of heat. In the first case, the temperature of the mixture increases, in the second it decreases.

If a reaction mixture in a state of chemical equilibrium is heated, then, in accordance with Le Chatelier’s principle, a predominantly reaction should occur, as a result of which heat will be absorbed, i.e. endothermic reaction; When the mixture is cooled, a reaction should predominantly occur, as a result of which heat will be released, i.e. endothermic reaction.

If the temperature is increased in a system in a state of chemical equilibrium, then the equilibrium shifts towards an endothermic reaction, and when the temperature decreases, towards an exothermic reaction.

Example: 2N 2 + 3H 2« 2NH3,H0 = - 92 kJ

The reaction is exothermic, therefore, as the temperature increases, the equilibrium shifts to the left, and as the temperature decreases, it shifts to the right.

It follows from this that to increase the yield of ammonia the temperature must be lowered. In practice, they maintain a temperature of 500 0C, since at a lower temperature the rate of the direct reaction sharply decreases.

Chemical equilibrium is dynamic in nature: forward and reverse reactions do not stop at equilibrium.

The equilibrium constant depends on the temperature and the nature of the reactants. The greater the equilibrium constant, the more the equilibrium is shifted towards the formation of direct reaction products

Le Chatelier's principle is universal, since it is applicable not only to purely chemical processes, but also to physicochemical phenomena, such as crystallization, dissolution, boiling, and phase transformations in solids.

The effect of temperature on the number of molecular collisions can be shown using a model. To a first approximation, the effect of temperature on the rate of reactions is determined by the Van't Hoff rule (formulated by J. H. Van't Hoff on the basis of an experimental study of many reactions):

where g is ttemperature coefficient, taking values ​​from 2 to 4.

An explanation for the dependence of the reaction rate on temperature was given by S. Arrhenius. Not every collision of reactant molecules leads to a reaction, but only the strongest collisions. Only molecules with excess kinetic energy are capable of chemical reactions.

S. Arrhenius calculated the fraction of active (i.e., leading to a reaction) collisions of reacting particles a, depending on temperature: - a = exp(-E/RT). and brought out Arrhenius equation for the reaction rate constant:

k = k o e -E/RT

where k o and E d depend on the nature of the reagents. E is the energy that must be given to molecules in order for them to interact, called activation energy.