Chemical Kinetics || Class 12 Chapter 4 || Chemistry Part 1 ||

                   1. Introduction

Chemistry is full of changes. During any chemical reaction we have to notice these changes. For any chemical reaction, chemists try to find out:

(a) the feasibility of a chemical reaction which can be predicted by thermodynamics ( as we know that the reaction with ∆G < 0, at constant temperature and the pressure is feasible);

(b) the extent to which a reaction will be proceed can be determined from the chemical equilibrium;

(c) speed of a reaction i.e. time taken by a reaction to reach equilibrium.

Along with feasibility and extent, it is equally important to know the rate and the factors controlling the rate of a chemical reaction for its complete understanding. The word kinetics is derived from the Greek word ‘kinesis’ meaning movement. Thermodynamics tells only about the feasibility of a reaction whereas chemical kinetics tells about the rate of a chemical reaction. For example, thermodynamic data indicate that diamond shall convert to graphite but in reality the conversion rate is so slow that the change is not perceptible at all.

 Therefore, most people think that diamond is forever. Kinetic studies not only help us to determine the speed or rate of a chemical reaction but also helps us to describe the conditions by which the reaction rates can be altered. The factors such as concentration, temperature, pressure and catalyst affect the rate of a chemical reaction. At the macroscopic level, we are interested in amounts reacted or formed and the rates of their consumption or formation. At the molecular level, the reaction mechanisms involving orientation and energy of molecules undergoing collisions, are discussed. 

In this topic we shall be dealing with average(the sum total rate of a chemical reaction) and instantaneous rate(rate of speed at a particular instant of time) of reaction and the factors affecting these reactions. Some elementary ideas about the collision theory of reaction rates are also given.

      2. Rate of a Chemical reaction

We are aware that the speed of an automobile is expressed in terms of change in the position or distance covered by it in a certain period of time. Similarly, the speed of a reaction or the rate of a 

reaction can be defined as the change in concentration of a reactant or product in unit time. To be more specific, it can be expressed in 

terms of:

(i) the rate of decrease in concentration of any one of the reactants, 

(ii) the rate of increase in concentration of any one of the products.

Lets consider a hypothetical reaction, assuming that the volume of the system remains constant. 

                        A → B

One mole of the reactant A produces one mole of the product B. If [A]1 and [B]1 are the concentrations of R and P respectively at time t1 and [A]2 and [B]2 are their concentrations at time t2 then, 

                          ∆t = t 2 – t1 

                    ∆[A] = [A]2 – [A]1

                    ∆ [B] = [B]2 – [B]1 

The square brackets in the above expressions are used to express molar concentration. 

Rate of disappearance of R

= Decrease in concentration of R / Time taken t 

= − ∆[R] / ∆ t

3. Factors Influencing Rate of a Chemical Reaction

Rate of reaction depends upon the experimental conditions such as concentration of reactants (pressure in case of gases), temperature and catalyst.

1. Dependence of Rate on Concentration: The rate of a chemical reaction at a given temperature may depend on the concentration of one or more reactants and the products. The representation of rate of reaction in the terms of concentration of the reactants is known as rate law. It is also called as rate equation or rate expression.

2. Rate Expression and Rate Constant: The rate of a reaction decreases with the passage of time as the concentration of reactants decrease. Conversely, rates generally increase when reactant concentrations increase. So, rate of a reaction depends upon the concentration of reactants.

Consider a general reaction aA + bB → cC + dD where a, b, c and d are the stoichiometric coefficients of reactants and products. The rate expression for this reaction is 

              Rate ∝ [A]^x [B]^y

The rate law is the expression in which reaction rate is given in terms of molar concentration of reactants with each term raised to some power, which may or may not be same as the stoichiometric coefficient of the reacting species in a balanced chemical equation. 

3. Molecularity of a Chemical Reaction: Another property of a reaction called molecularity helps in understanding its mechanism. The number of reacting species (atoms, ions or molecules) taking part in an elementary reaction, which must collide simultaneously in order to bring about a chemical reaction is called molecularity of a reaction. The reaction can be uni molecular when one reacting species is involved, for example, decomposition of ammonium nitrite.

       4. Integrated Rate Equations

It is not always convenient to determine the instantaneous rate, as it is measured by determination of slope of the tangent at point ‘t’ in concentration vs time plot . This makes it difficult to determine the rate law and hence 

the order of the reaction. In order to avoid this difficulty, we can integrate the differential rate equation to give a relation between directly 

measured experimental data, i.e., concentrations at different times and rate constant. 

The integrated rate equations are different for the reactions of different reaction orders. We shall determine these equations only for zero and first order chemical reactions. 

Zero order reaction means that the rate of the reaction is proportional to zero power of the concentration of reactants.