Proper Units for the rate constant
Moderators: Chem_Mod, Chem_Admin
Proper Units for the rate constant
How do you identify the proper units for the rat constant, k, for each reaction order?
-
- Posts: 106
- Joined: Wed Sep 30, 2020 10:07 pm
- Been upvoted: 4 times
Re: Proper Units for the rate constant
If I can't remember, I use the rate law to solve for the unit. For example, if the reaction is second order rate = k[reactant]^2. The unit for the reactant is M and the unit for rate is M/S (or any unit of time) so only way to cancel out the M^2 is for the unit of k to be 1/MS
-
- Posts: 50
- Joined: Wed Nov 20, 2019 12:20 am
Re: Proper Units for the rate constant
Other than working through an equation to find the correct units, the easiest way is probably to just memorize them. Zero order units are M/s. First order units are 1/s. Second order units are 1/Ms.
-
- Posts: 96
- Joined: Wed Sep 30, 2020 10:09 pm
Re: Proper Units for the rate constant
I think the easiest way is to memorize them. We have only done 0-2 second order so it’s not a lot.
-
- Posts: 35
- Joined: Mon Jan 09, 2023 10:07 am
Re: Proper Units for the rate constant
The units for the rate constant are determined by the overall order of the reaction. For zero order, the units are M/s. For first order, the units are 1/s. For second order, the units are 1/M*s.
-
- Posts: 36
- Joined: Mon Jan 09, 2023 9:09 am
Re: Proper Units for the rate constant
Like other people have said, if I forget I use the units associated with the concentrations to find the k units. The k units have to cancel out the M valus so its left with reciprocal seconds. When in doubt, I just use that method.
-
- Posts: 98
- Joined: Fri Sep 24, 2021 5:50 am
Re: Proper Units for the rate constant
The units of the rate constant, k, depend on the overall order of the reaction. The overall order of a reaction is the sum of the orders with respect to each reactant.
For a zero-order reaction, the rate is independent of the concentration of the reactant. Therefore, the units of k are given by the units of concentration per unit time, or M/s.
For a first-order reaction, the rate is proportional to the concentration of the reactant. Therefore, the units of k are given by the inverse of time, or 1/s.
For a second-order reaction, the rate is proportional to the square of the concentration of the reactant. Therefore, the units of k are given by the inverse of concentration times time, or 1/Ms.
For a zero-order reaction, the rate is independent of the concentration of the reactant. Therefore, the units of k are given by the units of concentration per unit time, or M/s.
For a first-order reaction, the rate is proportional to the concentration of the reactant. Therefore, the units of k are given by the inverse of time, or 1/s.
For a second-order reaction, the rate is proportional to the square of the concentration of the reactant. Therefore, the units of k are given by the inverse of concentration times time, or 1/Ms.
-
- Posts: 35
- Joined: Mon Jan 09, 2023 8:23 am
Re: Proper Units for the rate constant
To find units, I solve for k units so I do not have to memorize. The rate of a reaction is always M/s and the rate of each reactants is M. So using this you can solve for k. For example: rate = k[A]^2[B] -> M/s = k[M]^2[M] -> M/s = kM^3 -> k = 1/M^2*s
-
- Posts: 35
- Joined: Mon Jan 09, 2023 9:11 am
Re: Proper Units for the rate constant
You can either remember it or solve for it mathematically:
Remember: Zero order= M S-1
First order: S-1
2nd order: M-1 S-1
3rd order: M-2 S-1 etc.
Or: Do rate / [X] (there could be more [] of other species) and solve via cancellation
Remember: Zero order= M S-1
First order: S-1
2nd order: M-1 S-1
3rd order: M-2 S-1 etc.
Or: Do rate / [X] (there could be more [] of other species) and solve via cancellation
-
- Posts: 40
- Joined: Mon Jan 09, 2023 8:32 am
Re: Proper Units for the rate constant
For the most part the units of the rate constant remains the same within in each reaction order, but if you are ever confused, just solve the rate law for k only looking at the units. Since 0 order means the reactant has no effect on the rate, it makes sense that k and the rate have the same units. Also from the 2nd order and on, M in the denominator is raised to the power of the order minus 1. For example, for the 3rd order, k = 1/M^2 *s ; 3-1=2 which is the exponent M is raised to.
-
- Posts: 41
- Joined: Mon Jan 09, 2023 8:54 am
Re: Proper Units for the rate constant
The units depend on the overall order of the reaction. So, for a zero-order reaction, it is M/s. For a first-order reaction, it is 1/s. For a second-order reaction, it is 1/Ms.
Return to “Zero Order Reactions”
Who is online
Users browsing this forum: No registered users and 6 guests