## deciding which order

$\frac{d[R]}{dt}=-k[R]^{2}; \frac{1}{[R]}=kt + \frac{1}{[R]_{0}}; t_{\frac{1}{2}}=\frac{1}{k[R]_{0}}$

Jose Torres
Posts: 31
Joined: Fri Sep 28, 2018 12:22 am

### deciding which order

How do we determine whether we are dealing with a first, second, or zero order reaction? Any easy hints in questions or chemical equations?

Andrea Zheng 1H
Posts: 61
Joined: Fri Sep 28, 2018 12:26 am

### Re: deciding which order

If you are given the units of K, you can determine the order of the reaction by looking at those units, as they should cancel out with the reactant's units to get $\frac{mol}{L*sec}$, which is the unit for the rate of the reaction. You know that the [reactant] is $\frac{mol}{L}$.

For example:
Rate constant (k) for a reaction has units $\frac{1}{sec}$. Using the equation {rate = k*[reactant]^order}, this means that $\frac{mol}{L*sec}$ = $\frac{1}{sec}$*[$\frac{mol}{L}$]^order. For this, the sec on both sides would cancel out, leaving you with $\frac{mol}{L}$ = [$\frac{mol}{L}$]^order, meaning it is 1st order.
Last edited by Andrea Zheng 1H on Thu Mar 07, 2019 12:05 am, edited 1 time in total.