Radioactive decay is a process that follows first-order kinetics. The half-life of 239 Pu is 2.436e+4 years; how long (in seconds) would it take for the amount of 239 Pu to decrease to 55.02% of its initial amount? Key Concept: Half-life for first order reaction. t1/2=0.693/k. Strategy: Determine rate constant and then calculate time using integrated first order rate law. For the reaction NO2(g)+CO(g)NO(g)+CO2(g) the activation energy is, Ea, is 134.0kJ/mol, and k=1.300s1 at 700.0K. At what temperature (Kelvin scale) is k=121.2s1 ? Key Concept: The Arrhenius equation, ln(k2/k1)=EA/R[1/T11/T2], explains how the rate of a reaction changes with temperature. Generally, we expect the reaction rate (rate constant) to increase when the temperature increases. Solution: 1. Substitute all given quantities into the Arrhenius equation, making sure the activation energy is expressed in J/mol; use R=8.3145J/molK. 2. Solve for the unknown T