Question

ASSERTION: For a free radical combination, K=A.
REASON: ${{\text{E}}_{\text{a}}}$is zero for free radical combination.
(A)Both assertion and reason are correct and reason is the correct explanation for assertion
(B)Both assertion and reason are correct and reason is not the correct explanation for assertion
(C)Assertion is correct but reason is incorrect
(D)Assertion is incorrect and reason is correct

Answer 1

${{\text{E}}_{\text{a}}}$is the activation energy required for the products to from the reactants, the maximum activation energy of a reaction is when a transition state is formed. The relation between rate of the reaction and activation energy with temperature is given by Arrhenius equation.

Complete answer:
Some reactions such as free radical combinations of odd electron molecules have zero activation energy. Examples for odd electron species which disobey octet rule are oxides of nitrogen, nitrogen oxide and nitrogen dioxide.
Dimerization of nitrogen oxide: Nitrogen dioxide dies with another nitrogen dioxide to form dimer is due to the achieved stability. As nitrogen dioxide is unstable due to lack of electrons, it gets dimerises to stable octet configuration. The chemical equation for nitrogen oxide dimerization is ${\text{2N}}{{\text{O}}_{\text{2}}} \rightleftharpoons {{\text{N}}_{\text{2}}}{{\text{O}}_{\text{4}}}$.
The Arrhenius equation for the temperature dependence of the reaction rate is given by $k = A.{e^{ - \dfrac{{{E_a}}}{{RT}}}}$.
Where k is the rate constant, A is the Arrhenius factor or frequency factor, R is the gas constant, T is temperature and ${E_a}$is the activation energy.
If activation energy is zero, the whole term ${e^{\dfrac{{ - {E_a}}}{{RT}}}}$ becomes equal to one, so then k=A i.e. the rate constant equals to Arrhenius factor.

So the answer is option (A).

Note:
Generally chemical reactions do not possess zero activation energy. This means when there is no energy is given to the reactants to form products they already have achieved potential energy so the reactants are at equilibrium. But here is the example of reaction where activation energy is zero.