flow diagram

endothermic reaction

ABOUT
Typically, reactions between substances with strong covalent bonds are characterized by large Ea values ​​and proceed slowly. This applies to many interactions, like

whose speed under standard conditions is 0.

M
ionic interactions in solutions are characterized by low Ea values ​​and very high velocities

Reaction rate constant k in equation (72) there is a function of temperature; An increase in temperature generally increases the rate constant. The first attempt to take into account the influence of temperature was made by Van't Hoff, who formulated the following empirical (i.e., based on experimental data) rule: With an increase in temperature for every 10 degrees, the rate constant of an elementary chemical reaction increases by 2–4 times.

The value showing how many times the rate constant increases when the temperature increases by 10 degrees is van't Hoff temperature coefficient(γ). Mathematically, van't Hoff's rule can be written as follows:

Van't Hoff's rule is applicable only in a narrow temperature range, since the temperature coefficient of the reaction rate γ is itself a function of temperature; at very high and very low temperatures γ becomes equal to one(i.e. the rate of a chemical reaction ceases to depend on temperature).

The interaction of particles occurs during their collisions; however, not every collision results in a chemical interaction between the particles. Arrhenius postulated that collisions of molecules will be effective (i.e. will lead to a reaction) only if the colliding molecules have a certain amount of energy - activation energy. Activation energy E A – the necessary excess of energy (compared to the average energy of the reacting substances) that molecules must have in order for their collision to lead to a chemical interaction.

Consider the path of some elementary reaction

A ––> B

Since the chemical interaction of particles is associated with the rupture of old chemical bonds and the formation of new ones, it is believed that every elementary reaction passes through the formation of some unstable intermediate compound called activated complex:

A ––> K # ––> B

The formation of an activated complex always requires the expenditure of a certain amount of energy, which is caused, firstly, by the repulsion of electron shells and atomic nuclei when particles approach each other and, secondly, by the need to construct a certain spatial configuration of atoms in the activated complex and redistribute the electron density. Thus, on the way from the initial state to the final state, the system must overcome a kind of energy barrier (Fig. 26). The activation energy of a reaction is equal to the excess of the average energy of the activated complex over the average energy level of the reactants. Obviously, if the forward reaction is exothermic, then the activation energy of the reverse reaction E" A higher than the activation energy of the direct reaction E A. For an endothermic reaction, an inverse relationship is observed between E" A And E" A. The activation energies of the forward and reverse reactions are related to each other through the change in internal energy during the reaction - the thermal effect of the reaction ( D.U. in Fig. 26.).

Rice. 26. Energy profile of a chemical reaction. E ref– average energy of particles of starting substances, E cont– average energy of reaction product particles.

Since temperature is a measure of the average kinetic energy of particles, an increase in temperature leads to an increase in the proportion of particles whose energy is equal to or greater than the activation energy, which leads to an increase in the reaction rate constant (Fig. 27):

Fig.27. Energy distribution of particles. Here n E /N– fraction of particles with energy E; E 1 T 1, E 2- average particle energy at temperature T 2, E 3- average particle energy at temperature T 3 ;(T 1

The dependence of the rate constant on temperature is described by the Arrhenius equation:

Here A– pre-exponential factor. From equation (58) it is easy to show its physical meaning: the quantity A equal to the reaction rate constant at a temperature tending to infinity.

Let us take the logarithm of relation (88):

As can be seen from the last expression, the logarithm of the rate constant depends linearly on the inverse temperature (Fig. 28); activation energy value E A and the logarithm of the pre-exponential factor A can be determined graphically (respectively, the tangent of the angle of inclination of the straight line to the abscissa axis and the segment cut off by the straight line on the ordinate axis).

Fig.28. Dependence of the logarithm of the rate constant of a chemical reaction on the inverse temperature.

Knowing the activation energy of a reaction and the rate constant at a certain temperature T 1, using the Arrhenius equation, you can calculate the value of the rate constant at any temperature T 2.

Collision theory allows us to establish a mathematical relationship between the rate of reaction and the frequency of collisions, as well as the probability that the collision energy E exceeds the minimum energy Em required for the reaction to occur. This relation has the form

Reaction Rate = (Collision Frequency) (Probability that E > Em) From this relationship the following equation can be derived:

where k is the reaction rate constant; P-steric factor, having a value from 0 to 1 and corresponding to that part of the colliding molecules that have the necessary mutual orientation upon collision; Z-number of collisions, which is related to the frequency of collisions; Ea is the activation energy corresponding to the minimum collision energy that reacting molecules must have; L-gas constant; T-absolute temperature.

The two factors, P and Z, can be combined into one constant A, which is called the pre-exponential factor or Arrhenius constant. The result is the famous Arrhenius equation, which we already met in the previous section:

TRANSITION STATE THEORY

Transition state theory considers reacting molecules as a single system. It examines in detail the changes in the geometric arrangement of atoms in this system as it transforms reactants into products. The geometric position of the atoms in such a molecular system is called configuration. As the configuration of reactants turns into a configuration of products, there is a gradual increase in the potential energy of the system until it reaches a maximum. At the moment of reaching maximum energy, the molecules have a critical configuration, which is called a transition state or activated complex. Only those molecules that have sufficient total energy are able to achieve this critical configuration. As the configuration of this transition state changes to the product configuration, a decrease in potential energy occurs (Figure 9.12). The reaction coordinate in these two diagrams represents changes in the geometric arrangement of the atoms of the reacting molecules, considered as a single system, as that system undergoes a transformation starting with the reactant configuration, moving to the critical configuration, and ending with the product configuration. If intermediates are formed in a reaction, then the appearance of each intermediate corresponds to a minimum on the graph of the potential energy versus the reaction coordinate (Fig. 9.13).

Rice. 9.12. Energy profile of a reaction - a graph of potential energy versus reaction coordinate, a for an exothermic reaction; b-for an endothermic reaction.

Transition state theory can be used to predict the constants A and El in the Arrhenius equation. The use of this theory and modern computer technology makes it possible to establish an accurate picture of the occurrence of chemical reactions at the molecular level.