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]0
a. a certain first-order reaction (a--> products) has a rate constant of 3.30×10^-3 s^-1 at 45 degrees c. how many minutes does it take for the concentration of the reactant, [a], to drop to 6.25% of the original concentration?
b. a certain second-order reaction (b--> products) has a rate constant of 1.70×10^-3 m^-1*s^-1 at 27 degrees c and an initial half-life of 296 s. what is the concentration of the reactant b after one half-life?