For a first-order reaction, the rate constant k can be determined using the half-life t1/2 with the following equation:k = 0.693 / t1/2Given that the half-life t1/2 is 120 seconds, we can calculate the rate constant k as follows:k = 0.693 / 120k 0.005775 sThe rate constant for this first-order decomposition reaction is approximately 0.005775 s. Note that the initial concentration of the reactant is not needed to calculate the rate constant for a first-order reaction.