### units of k

Posted:

**Wed Mar 04, 2020 11:38 pm**I know the units of k is dependent on the order of the rate equation, but how do we use that knowledge to determine the units of k?

Created by Dr. Laurence Lavelle

https://lavelle.chem.ucla.edu/forum/

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=146&t=61365

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Posted: **Wed Mar 04, 2020 11:38 pm**

I know the units of k is dependent on the order of the rate equation, but how do we use that knowledge to determine the units of k?

Posted: **Wed Mar 04, 2020 11:43 pm**

Since the unit of rate is M/s, if the reaction is zero-order, the unit of k is the same as the unit of rate. If the reaction is first order, then the reactant will have the unit M and the unit for k would be 1/s since the unit of rate is always M/s. If the reaction is second order, the reactant will have the unit M^2, which means that the unit of k has to be 1/M*s since the unit of rate is always M/s.

Posted: **Thu Mar 05, 2020 9:54 pm**

If the reaction is zero-order, K has units of M/s

If the reaction is first-order, K has units of 1/s

If the reaction is second-order, K has units of 1/M.s

If the reaction is first-order, K has units of 1/s

If the reaction is second-order, K has units of 1/M.s

Posted: **Thu Mar 05, 2020 11:20 pm**

For first order, the units are just s^-1

For second order, the units are L x mol^-1 x s^-1

For third order, the units are L^2 x mol^-2 x s^-2

The pattern continues in this way.

For second order, the units are L x mol^-1 x s^-1

For third order, the units are L^2 x mol^-2 x s^-2

The pattern continues in this way.

Posted: **Fri Mar 06, 2020 6:25 pm**

For the units of k, the zeroth order is M/s, the first order is 1/s, and the second order is 1/M*s. A good way to remember this is to divide by M when going from zeroth to first to second order.

zeroth order: M/s --> divide by M --> first order: 1/s --> divide by M --> second order:1/M*s

zeroth order: M/s --> divide by M --> first order: 1/s --> divide by M --> second order:1/M*s

Posted: **Fri Mar 06, 2020 9:01 pm**

When you are given a problem sometimes they may give you a rate constant to solve for something else like concentration. For example, a given rate constant can be written with the following units: L/mol*s. This is the same as 1/M*s because M=mol/L so if we have L/mol then this is the inverse of mol/L so it will equal 1/M. Then we multiplied 1/M by s to get 1/M*s. This indicates that we will be using rate laws or half-life equations for the second order.

Posted: **Fri Mar 06, 2020 9:44 pm**

I found this video to be very helpful in visualizing why the units are different for zeroth, first, and second-order reactions.

https://www.khanacademy.org/science/che ... constant-k

https://www.khanacademy.org/science/che ... constant-k

Posted: **Fri Mar 06, 2020 11:41 pm**

units of k can be determined by (M/s) / (M^n) where n is the total reaction order

Posted: **Sun Mar 08, 2020 5:04 pm**

for zero-order reactions, K has units of M/s

for first-order reactions, K has units of 1/s

for second-order reactions, K has units of 1/M.s

for first-order reactions, K has units of 1/s

for second-order reactions, K has units of 1/M.s

Posted: **Sun Mar 08, 2020 6:39 pm**

Always go based on the order of the reaction:

If the reaction is zero-order, K has units of M/s

If the reaction is first-order, K has units of 1/s

If the reaction is second-order, K has units of 1/M.s

If the reaction is zero-order, K has units of M/s

If the reaction is first-order, K has units of 1/s

If the reaction is second-order, K has units of 1/M.s