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3.
4.
1 Collision Theory
The collision theory believes that the collision between reactant molecules is a prerequisite for the reaction
.
The factors that affect the reaction rate mainly include the frequency of the collision of reactant molecules, the energy of the reactant molecules, and the orientation of the molecules when they collide
The higher the frequency (Z) of the collision of reactant molecules, the greater the reaction rate
.
Obviously, the greater the concentration of the reactant, the higher the collision frequency of the reactant molecules, so increasing the concentration of the reactant can increase the reaction rate
When reactant molecules collide with each other, only a few collisions are effective, that is, molecules that collide with each other may not react, and only molecules that collide effectively can react
.
Studies have shown that molecules that can react when they collide with each other must have enough energy to overcome the repulsive force when the molecules approach each other
In the formula, f is called the energy factor
.
The higher the activation energy E a , the smaller the fraction f of the number of collisions meeting the energy requirement in the total number of collisions
The reaction can only occur when each molecule in the activated molecule group adopts the appropriate orientation to collide
.
E.
g
NO 2 +CO=NO+CO 2
Only when the C in CO is close to the O in NO 2 is the proper orientation for the reaction to occur, as shown in Figure 3-2(a); other orientations are not suitable, so no reaction occurs, as shown in Figure 3.
-2(b), (c), (d) shown
.
Therefore, the real effective number of collisions Z* is
In the formula, Z is the number of collisions; f is the energy factor; P is the orientation factor; E a is the activation energy; T is the thermodynamic temperature; R is the molar gas constant
.
Figure 3-2 Schematic diagram of the orientation of molecular collisions
3.
4.
2 Transition state theory
The transition state theory believes that when two reactant molecules with sufficient energy approach each other, the reactant molecules first form an activated complex as the intermediate transition state of the reaction
.
In the activated complex, the bonds of the reactant molecules are partially broken, and the bonds of the product molecules are partially formed
The concentration of the activated complex, the probability of decomposing the activated complex into a product, and the rate of decomposing the activated complex into a product all affect the rate of the chemical reaction
.
The transition state theory often uses the reaction process-potential energy diagram to discuss the reaction process, as shown in Figure 3-3
.
The reaction process-potential energy diagram very intuitively describes the change of the potential energy of the system during the reaction process
In Figure 3-3, the difference between the potential energy of the activated complex and the average potential energy of the reactant molecules is the activation energy E a of the positive reaction ; the difference between the potential energy of the activated complex and the average potential energy of the product molecules is the activation energy E of the reverse reaction 'a
.
It can be seen that in the transition state theory, the activation energy represents an energy difference
.
Figure 3-3 Reaction process-potential energy diagram
It can be obtained from Figure 3-3
Reactant → activated complex △ r H m (1)=E a
Activated complex → product △ r H m (2)=-E' a
The total response is
Reactant → product △ r H m
Therefore, the molar reaction heat is
△ r H m =△ r H m (1)+△ r H m (2)=E a -E' a
Therefore, the difference between the activation energy of the positive reaction and the activation energy of the reverse reaction is the molar heat of reaction of the chemical reaction
.
Regardless of whether it is an exothermic reaction or an endothermic reaction, the reactant molecules must first pass through an energy barrier (activation energy)
.
From the potential energy diagram of the reaction process, it can be seen that the forward and reverse reactions pass through the same activated complex intermediate.
This is the principle of microscopic reversibility
.
The transition state theory combines the reaction rate with the microstructure of matter, which is an advanced aspect of the collision theory
.
Because the structure of the activated complex of many reactions cannot be determined experimentally, and the calculation method is complicated, the application of this theory is limited
.