Question

How do I determine the units of the rate constant for a zero order, first order, second order, and third order reaction?

Answer 1

You will need to be able to write a general rate law and solve for the units...

`r(t) = k[A]^n`

`r(t)` is the initial rate as a function of time `t`, `k` is the rate constant, `[A]` is the concentration of `A`, and `n` is the order of `A`.

Since `A` is assumed the only reactant for simplicity, its order IS the reaction order. If the units of time are `"s"`, and of concentration are `"M"`, then:

`"M"/"s" = [???] cdot "M"^n`

`=> [???]`

`=` `color(blue)("units of rate constant in general")`

`= color(blue)("M"^(1 - n)/"s")`

So,

• zero order should simply be `"M"^(1 - 0)/"s" = "M/s"`.
• first order should be `"M"^(1 - 1)/"s" = 1/"s"`.

Can you work out second order and third order?