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A zeroth-order reaction is one where the rate does not depend on the concentration of the species. For a general unimolecular reaction,

A -> products
the reaction rate expression for a zeroth order reaction is
rate = k[A]0 = k
The integrated rate law for a 0th order reaction can be easily shown to be
[A] = [A]0 -kt

Example: The decomposition of HI into hydrogen and iodine on a gold surface is 0th order in HI. The rate constant for the reaction is 0.050 1/s. If you begin with a 0.500 M concentration of HI, what is the concentration of HI after 5 seconds?

Solution: Use the integrated rate law above

[HI] = [HI]0 -kt
The intial concentration is 0.500 M, and the reaction runs for 5 seconds:
[HI] = 0.500 - 0.050 1/s * 5 s = 0.250 M


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