Units for reaction rates

Cristian Carrasco 1F
Posts: 52
Joined: Fri Sep 29, 2017 7:06 am

Units for reaction rates

What are the units for zero, first and second order reaction rates?

Nora Sharp 1C
Posts: 53
Joined: Sat Jul 22, 2017 3:00 am

Re: Units for reaction rates

Reaction rate units concerning one substance are always Molarity/time.

But for k:
zero order is M/time
first order is 1/time
second order is 1/(time*M)

Laura Riccardelli
Posts: 49
Joined: Fri Sep 29, 2017 7:04 am

Re: Units for reaction rates

And you can get these fom the equations for 0, 1st, and 2nd odrder reactions that are on the equations sheet.

Justin Quan 4I
Posts: 104
Joined: Sat Sep 14, 2019 12:17 am

Re: Units for reaction rates

A more general rule for the units of k is: 1/M1-nt-1 (n is the order of the reaction)

Kayli Choy 2F
Posts: 100
Joined: Sat Aug 24, 2019 12:17 am

Re: Units for reaction rates

Because the units for k are different for zero, first, and second order reactions, if you are only given k and concentrations and are looking for the reaction rate, you would automatically know the order of the reaction.

Emily Vainberg 1D
Posts: 102
Joined: Sat Jul 20, 2019 12:15 am

Re: Units for reaction rates

There are different units depending on which order of the reaction you have.

zero order= molL−1s−1
first order= s−1
second order= L mol−1s−1

Izzie Capra 2E
Posts: 103
Joined: Sat Sep 07, 2019 12:19 am

Re: Units for reaction rates

I always think about units with a quick rule:
- determine the overall order of the reaction
-take the overall order and subtract 1 and add a negative. That is the number that will go with molarity (M)
- also include time with -1.

This makes sense when you write out all the units and see what cancels, this is just an easier method.

805394719
Posts: 104
Joined: Wed Sep 11, 2019 12:16 am

Re: Units for reaction rates

The unit for k for a zeroth-order reaction is moles per liter per second since the power of the concentration of the reactant is zero and the rate value must have the rate in moles per liter per second. For a first-order reaction, the rate is equal to the rate constant x the concentration of the reactant which is in moles per liter. Since the overall rate must be in moles per liter per second, the units for the rate constant must be 1/sec. For a second-order reaction, the concentration of the reactant is to the power of two so the unit for the reactants is mol2 per liter2. So the rate constant must be in liters per moles per second where moles and liters will cancel out and the overall rate will be moles per liter per second.

Abigail_Hagen2G
Posts: 107
Joined: Fri Aug 09, 2019 12:17 am

Re: Units for reaction rates

And since the units for reaction rates are always molarity/ time, this will help you find out the k units.

SVajragiri_1C
Posts: 115
Joined: Thu Jul 11, 2019 12:16 am

Re: Units for reaction rates

The general units for k is M^(1-p)/s, where p is the order number. So for 0 order, p=0, M/s. For 1st order, p=1, s^-1. For 2nd order, p=2, M^-1/s.