Chemical Kinetics

We know that the rate of reaction depends upon the concentration of reactants and temperature. But why does the change in the concentration and temperature affect the rate of reaction? To answer this question, let us understand the collision theory. Collision theory states that, for a reaction to occur the reacting particles must collide with each other. Particles must collide in proper orientation. This collision results in the formation of products.

In the context of collision theory, particles represent the atoms, molecules, or ions that participate in a chemical reaction. These particles are constantly in motion due to their kinetic energy. As they move, they have the potential to collide with other particles that are in the reaction mixture. The collision theory emphasizes that collisions between these particles are necessary for a reaction to occur.

However, not all collisions result in a reaction. The collision must satisfy two critical conditions. These conditions are sufficient energy and proper orientation. The colliding particles must possess enough kinetic energy to overcome the activation energy barrier. Activation energy refers to the minimum amount of energy required for a reaction to take place. It acts as a sort of hurdle that the particles must overcome to initiate the reaction.

When the colliding particles have an energy below the value indicated by the activation energy threshold, the particles are not able to disrupt the existing bonds in the reactant molecules. Instead, the particles simply bounce off each other due to the repulsive forces between them. This type of collision is considered ineffective for initiating a reaction. This is because the necessary energy is not reached to break the reactant bonds and initiate the formation of new bonds.

When particles collide with energy greater than or equal to the activation energy, the reactant bonds can be broken. This leads to the formation of new bonds and the generation of products. Such a collision is called an effective collision.

In order for a chemical reaction to occur, the reacting particles must not only collide with sufficient energy but also in the correct spatial orientation. This correct orientation is necessary for the necessary atomic or molecular rearrangements to take place. During a collision between reactant particles, energy is transferred from one particle to another. This energy-transfer can result in the breaking and the formation of bonds to create the desired products. However, for these processes to occur successfully, the reacting particles must have the proper alignment.

If the particles collide in an incorrect orientation, the necessary atomic or molecular rearrangements cannot occur efficiently. The energy transferred during the collision might not be utilized effectively to break the reactant bonds and form new bonds. As a result, the reaction is hindered. The formation of products becomes less likely.

Collision frequency refers to the number of collisions that occur per unit time in a chemical reaction. Collision frequency is directly proportional to the rate of reaction. Higher collision frequency means faster rate of reaction. If collision frequency is small then the rate of reaction will be slow.

The collision frequency depends on the concentration of reactant particles. A higher concentration of reactant particles means that there are more particles per unit volume available for collisions. As a result, the chances of collisions rises. This leads to a higher collision frequency.

The average velocities of the reacting particles also affect collision frequency. As the average velocities of the particles rises, their chances of colliding with other particles also rises. This results in a higher collision frequency. Moreover, the velocity of a particle is directly related to its kinetic energy. Kinetic energy is a measure of energy of motion of a particle. So a particle with high velocity would have high kinetic energy. This means that particle can overcome activation energy to have a successful collision.

Rate law is a mathematical equation that describes the relationship between the rate of a chemical reaction and the concentrations of its reactants. It represents how the concentrations of reactants affect the rate at which products are formed or reactants are consumed. Consider the reaction in which we have Reactant A and Reactant B. These reactants react to form product Reactant C. The general form of rate law equation is illustrated.

In the given equation Rate represents the reaction rate, which is the change in concentration of a reactant or product per unit of time. Letter A in square brackets represents concentration of Reactant A. Letter B in square brackets represents the concentration of Reactant B. Symbol K is called the rate constant. It is a proportionality constant that depends on factors like temperature, catalysts, and reaction mechanism. It is independent of the reactant concentrations.

The exponents x and y in the rate law equation represent the reaction orders with respect to the Reactant A and Reactant B, respectively. The reaction order indicates the sensitivity of the reaction rate to changes in the concentration of a particular reactant. It can be 0, positive, negative and possibly fractional. If the reaction order for a reactant is 0 then the reactant is called a zero order reactant. It means that the concentration of that reactant does not affect the reaction rate. The rate is independent of the concentration of that particular reactant. The rate law equation for a zero order reactant is illustrated.

If the reaction order for a reactant is one, then reactant is called first order reactant. It indicates that the reaction rate is directly proportional to the concentration of that reactant. Doubling the concentration of a first order reactant will result in doubling the reaction rate. The rate law equation for a first order reactant is illustrated.

If the reaction order for a reactant is two, then reactant is called second order reactant. It indicates that the reaction rate is proportional to the square of the concentration of that reactant. Doubling the concentration of a second order reactant will result in a four fold rise in the reaction rate. The rate law equation for a second order reactant is illustrated.