Answer to Question #16279 in Physical Chemistry for Ndongani

Question

A reaction is of first order in a reactant A and of second order in a reactant B . How is the rate of this reaction affected when
a) The concentration of B alone is increased to three times?
b)The concentration of A as well as B are doubled ?

Answer 1

Since it is given that a reaction is first order in reactant A and second order in reactant B; therefore,
r = k[A][B]² …………………………. (i)

Where r is the rate of reaction and k is the rate constant of the reaction.

a)
When concentration of B alone is increased three times, let the new rate be r₁
r₁ = k[A][3B]² = 9k[A][B]² …………………………. (ii)
Dividing eq.(ii) by eq.(i), we get
r₁ = 9r
Therefore, the rate of the reaction would become 9 times the initial rate when the concentration of B alone is increased three times.

b)
When the concentration of both the reactants is doubled, then the rate of the reaction would be r₂
r₂ = k[2A][2B]² = 8k[A][B]² …………………………. (iii)

Dividing eq.(iii) by eq.(i), we get
r₂ = 8r

Therefore, the rate of the reaction would become 8 times the initial rate when the concentration of both A and B is doubled.