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
rate = k[A]p [B]q ... (.1)
=a or less than a
=b or less than b
= 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.
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
Order = 1 + 2 + 2 = 5
(ii) H2O + 2I- + 2H+
-- > I2 +
r = k [H2O2- ] [I- ] Order = 1 + 1 = 2
Characteristics of order of a
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
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
Higher order reactions may be experimentally
converted into simpler order (pseudo) reactions by using excess concentrations
of one or more reactants.