The half-life of a reaction, t1/2 , is the time it takes for the reactant concentration [A] to decrease by half. For example, after one half-life the concentration falls from the initial concentration [A] to [A]/2, after a second half-life to [A]/4, after a third half-life to [A]/8, and so on. on.

For a first-order reaction, the half-life is constant. It depends only on the rate constant and not on the reactant concentration. It is expressed as

t1/2=0.693/k

For a second-order reaction, the half-life depends on the rate constant and the concentration of the reactant and so is expressed as
t1/2= 1/k[A]
1. A certain first-order reaction (A--> Products) has a rate constant of 7.20×10-3 s^-1 at 45 degrees celsius. How many minutes does it take for the concentration of the reactant, [A], to drop to 6.25% of the original concentration?

2. A certain second-order reaction (B--> Products) has a rate constant of 1.45×10-3 M^-1*s^-1 at 27 degrees celsius and an initial half-life of 282s . What is the concentration of the reactant B after one half-life?