# Chemistry - How to calculate the half time of a unimolecular reaction given the Arrhenius coefficient and the activation energy?

You calculated the high-pressure-limit half life, where the rate of collisions with M is so high that it has no influence on the rate constant. Now all you have to do is to consider that the pressure is 1 atm instead of at the high-pressure limit. That slows down the reaction such that the half-time is in the second instead of the micro second range.

Something similar happens with ozone. It is not very stable at sea level, but has a substantial half-time in the upper atmosphere (unless there are certain man-made substance up there).

To solve numerically, you could assume a Lindemann mechanism (see https://en.wikipedia.org/wiki/Lindemann_mechanism) and that $$k_1$$ is diffusion-limited (i.e. no activation energy, every collision leads to activated complex).

The textbook cited by the OP has:

Strangely, in an earlier table they show $$k_0$$ and $$k_\infty$$ as (along with pre-exponential temperature dependence $$T^n$$ and ?activation energy? E - units unknown):