Define half-life of a reaction.

It the time required for the concentration of a reactant to decrease to half of its initial value.

First half reaction gets completed within 2 hours and next half reaction gets completed within next 2 hours. That means, t_1/2 is indepedent of concentration. Hence, the order of reaction is first order reaction.

Activation energy of a reaction is defined as the extra energy over and above the average potential energy of the reactants, which must be supplied to the reactants to enable them to cross-over the energy barrier between the reactants and products. It is denoted by E_A.

Unit of second order reaction is L mol^–1 s^–1