Rate And Order Of A Reaction

The knowledge of rate of chemical reactions is valuable to understand the chemistry and mechanism of reactions. The study of chemical kinetics includes the experimental determination of rate or speed of a reaction, the rate laws and the effects of temperature, pressure, concentration, catalyst on the rates of reaction. It is possible to deduce the mechanism of the reaction from the chemical kinetic data.

RATE AND ORDER OF A REACTION

For any reaction, the rate expression (or) the rate law equates the rate of the reaction to the product of its rate constant and the concentrations of the reactants raised to certain exponential powers. The rate law for a reaction and the power (exponent) of the concentration terms involved in it must be determined by experiment only.

In a general reaction, aA + bB -- >  products, the overall rate expression can be written as,

rate =     k[A]^p [B]^q               ... (.1)

           =a or less than a

           =b or less than b

where k = rate constant of the reaction, p and q are the exponents or also known as the orders with respect to reactants A and B respectively.

Therefore the term order can be defined as the sum of the powers of the exponential powers to which each concentration term is raised in the experimentally determined rate law of a chemical reaction.

In the rate expression (.1),

rate = k[A]^p, [B]^q, p and q are called the orders of the reaction with respect to reactants A and B respectively. The total order of the reaction, n = p + q. Order with respect to A only p and order with respect to B only is q.

It is found that the overall rate of a reaction, depends on the exponential powers to which each concentration term is raised in the rate law. For example, for a reaction with a rate law as rate = k[A]², when the concentration of A is doubled, the rate is increased by four times the initial rate. Thus, chemical reactions and the over all rates are classified and studied according to the magnitude of the order of their reactions.

Example

From the rate equations for the reactions given below determine the order with respect to the overall reaction

(i) 2HCrO₄^-  + 6I^-+ 14H⁺ -- > 2Cr³⁺ + 3I + 8H₂O

r = k [HCrO₄ ^- ] [I^- ]² [H⁺]²    Order = 1 + 2 + 2 = 5

(ii) H₂O + 2I^-  + 2H^{+    -- >} I2 + 2H₂O

r = k [H₂O₂^- ] [I^- ]    Order = 1 + 1 = 2

Characteristics of order of a reaction

The magnitude of order of a reaction may be zero, or fractional or integral values. For an elementary reaction, its order is never fractional since it is a one step process.

Order of a reaction should be determined only by experiments. It cannot be predicted interms of stoichiometry of reactants and products.

Simple reactions possess low values of order like n = 0,1,2, Reactions with order greater than or equal to 3.0 are called complex reactions. Higher order reactions are rare.

Some reactions show fractional order depending on rate.

Higher order reactions may be experimentally converted into simpler order (pseudo) reactions by using excess concentrations of one or more reactants.