Sapling #12 Week 9-10

[Aliya 1H]

Hi! For this question, I don't quite understand how to determine the order of the reaction. I know that we have the equations for t1/2 on our equation sheet and I know which ones go with a 1st, 2nd, and 0th order rxn, but how do I apply that to this question to determine the order when it gives two half lives at different concentrations? Thanks.

[John Pham 3L]

For this question, you need to understand the behavior of half-lives for the different orders of reaction.

Right away, we can figure out that it is not a 1st Order reaction
- 1st Order reactions have constant half-lives which do not depend on the initial concentration
- Since the half-live changes with concentration, this reactant isn't 1st Order

Now we can compare the 0th and 2nd Order half-lives
- For a 0th Order reaction, its half-life is [A]0/2k
- For a 2nd Order reaction, its half-life is 1/k[A]0

As the initial concentration increases, the half-life of a 0th order reaction will increase while the half-life of a 2nd order reaction decreases
- Since increasing the concentration from 0.262M to 0.388M decreased the half-life from 215 s to 145s, we can figure out that it is a 2nd Order reaction