Chapter 4: Chemical Kinetics

A condition under which the order of a chemical reaction can be altered is by taking the solvent or the reactant in the excess amount due to which its concentration does not change much.

Consider a bimolecular reaction

A + B → Product

Rate = k[A][B]

When concentration B is large rate becomes k’[A]

Where k’=k[B]

The order of the reaction will be equal to one.

The rate equation for the reaction 2A + B → C

Rate = k[A]⁰[B]⁰

The powers of the concentration of the reactants will be equal to zero. In the zero-order reactions, the rate of the equation is equal to the rate constant.

The rate of the reaction can be determined by

Rate = k[NO]²[O2]¹

It can be measured by the rate of the reaction as a function of initial concentration by keeping the concentration of one of the reactants constant and changing the other reactant.

For an elementary reaction, the order is the same as molecularity. Elementary reactions are those reactions which occur in a single step.

Consider a rate of a reaction with reactant A

Rate = k[A]^α

27 ˟ Rate = k[A]^α

By dividing the equations we get α = 3

Therefore the order of the reaction becomes three.

Consider a reaction,

R → P t=0

Here R is the reactant and P is the product.

Rate = k[R]0 t=t

(Instantaneous rate) –dR/dt = k , dR =-kdt

On integrating both sides,

ʃDr = -k ʃdt

At t = 0,

Here [R] = concentration of reactant at time ‘t’.

This reaction is known as the integrated rate equation of zero order.

After completion of zero order reaction [R]= 0

Using [R]= 0 in (i)

t=[R₀]/k

The reaction cannot be an elementary reaction because the order is different. For an elementary reaction, the molecularity and order should be the same.

The two most important conditions for a reaction to occur according to the collision theory are:

1. Energy is greater than activation energy and proper orientation of reactant molecules at the time of the collision.

So when the reaction occurs then molecules that don’t have a proper orientation decreases the rate of a reaction.

The molecularity of any reaction is the number of reacting species taking part in an elementary reaction. The zero molecularity means there is no reactant so the reaction does not occur. Therefore molecularity cannot be zero for a reaction.

(i) It is a zero-order reaction as the graph is satisfying the equation [A] = [A0] – kt.

(ii) The slope of the curve is the negative of the rate constant that is denoted by –k.

(iii) Unit of the rate constant is Ms-1 or mol L-1s-1.