## Quiz 2 Prep: Winter 2014 Number 9

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

alexjbednar
Posts: 4
Joined: Fri Sep 26, 2014 2:02 pm

### Quiz 2 Prep: Winter 2014 Number 9

Number nine gives the rate constant (8.39) and the initial and final concentration (.0840M and .0220, respectively). It asks for the time for the concentration to decrease by that much, but I am not sure how to decide whether this is a first or second order reaction.

KRai 1I
Posts: 6
Joined: Fri Sep 26, 2014 2:02 pm

### Re: Quiz 2 Prep: Winter 2014 Number 9

To figure out which order your reaction is, you look to the units provided on your rate constant (k). Here, in #9, we can see the units are 1/Ms, which correspond to second order reactions. So you use the second order reaction equation.

Niharika Reddy 1D
Posts: 127
Joined: Fri Sep 26, 2014 2:02 pm

### Re: Quiz 2 Prep: Winter 2014 Number 9

When a question does not specifically state what the order of the reaction is, but gives you the rate constant (k), the order of the reaction can be determined from the units of the given rate constant.

k is 8.39 M-1.s-1, so the reaction must be second order.

This is how to calculate the units for a second order reaction's rate constant.
rate=k[A]2
rate must be in the units M.s-1 since it is change in concentration over change in time, and [A]2 has the units of M2, so rearranging the above second order rate law to solve for k gives:
k=rate/[A]2
=(M.s-1)/M2
=M-1.s-1

The units for the rate constants of different order reactions can be calculated in a similar manner.