⚛️ Reaction Kinetics · Collision theory

Rate of reaction is the speed at which the reaction takes place. Chemical reactions are useful only when they occur at a reasonable rate. In this chapter we shall study the rates at which chemical reactions proceed. We shall also try to understand the factors that govern the rate of reaction. The studies eventually lead us to come to know how reactions occur at the molecular level.

17.1 Collision Theory of Reaction Rate

A reaction takes place only when the participating particles (atoms, molecules or ions) collide with one another. It has been observed that a very small fraction of collisions lead to the formation of products. In the majority of collisions the reactant particles simply bounce back without any change. Collisions which lead to reaction are called effective collisions.

The rate of a reaction depends upon the number of effective collisions which, in turn, depends upon the following factors.

1. The number of particles per unit volume
The larger the number of particles per unit volume, larger is the possibility of effective collisions. In other words, higher the concentration of the reactants, more the number of molecules and hence more will be the number of collisions between them.

2. The frequency of Collisions
A greater number of collisions per second will also result in the increase in the number of effective collisions. At higher temperature the velocities of the molecules increase resulting in the increase in the frequency of collisions. This increase in the frequency of collisions will also increase the number of successful collisions.

3. The kinetic energy of the particles
The kinetic theory of particles says that all matter is composed of tiny constant moving particles. Their motion is due to kinetic energy they possess. As the temperature increases, the kinetic energy of the particles also increases. As a result the particles start moving fast leading to more vigorous motion. The fast moving particles will have more chances to collide with each other.

4. Activation energy
According to collision theory, the colliding particles will lead to effective collisions only when they possess sufficient energy to break the bonds present in the reactant molecules. This minimum energy required to start a chemical reaction is called its activation energy.

Generally, at a particular temperature, most of the molecules of the reactant possess average energy. A fraction of total molecules will, however have more than the average energy. Particles will react only when average energy of this small fraction of molecules exceeds the energy needed to activate molecules so that they can undergo a chemical reaction.

 Potential      ╱‾‾‾‾‾‾‾ high energy state
   energy      ╱       ╲
              ╱         ╲  Activation
  Reactants  ╱           ╲   energy
    ____    ╱             ╲____
        ╲  ╱                    ╲ Products
         ╲╱                      ╲
          └────────────────────────▶
                 Reaction coordinate
        

When the reactant particles come close to each other at the time of collision, they slow down and the kinetic energy they possess, is converted to increase their potential energy. This process can be understood with the help of a graph drawn between the path of reaction and the potential energy of the system as shown in Figure 17.1. When the reactant particles possessing activation energy collide they form high energy state with the corresponding increase in the potential energy of the system. After this stage the reactants are converted into the stable products and the potential energy of the system falls down.

The information provided by the energy of activation is very important in understanding the mechanism of the reaction.