If the rate constant for a first order reaction is k, the time (t) required for the completion of 99 % of the reaction is given by :

When initial concentration of the reactant is doubled, the half-life period of a zero order reaction

The correct difference between first and second order reactions is that

Mechanism of a hypothetical reaction
X₂ + Y₂ $$ \to $$ 2XY, is given below :
(i)   X₂ $$ \to $$ X + X (fast)
(ii)  X + Y₂ $$\rightleftharpoons$$ XY + Y (slow)
(iii) X + Y $$ \to $$ XY (fast)
The overall order of the reaction will be