A. the half-life for a first order reaction in 49 minutes. what is the rate constant? b. for a second order reaction, the half-life is 726 seconds. starting with a concentration of 0.600 m, how long will it take for the concentration to decrease to 0.150 m? c. the half-life for a first order reaction is 2768 years. starting with a concentration of 0.345 m, what will the concentration be after 11,072 years? d. the half-life for first order reaction a is 75 minutes. the half-life for a different first order reaction b is 322 minutes. which reaction has the faster rate? (a or b) which reaction has the larger rate constant, k? (a or b)