### Rate constant units

Posted:

**Sun Mar 18, 2018 9:20 am**What is a general rule of thumb to figure out the units of the rate constant and the initial rate? I'm having trouble figuring them out once we get past fourth order reactions...

Created by Dr. Laurence Lavelle

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

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=145&t=30374

Page **1** of **1**

Posted: **Sun Mar 18, 2018 9:20 am**

What is a general rule of thumb to figure out the units of the rate constant and the initial rate? I'm having trouble figuring them out once we get past fourth order reactions...

Posted: **Sun Mar 18, 2018 9:43 am**

for first order, M/s

for second order, 1/s

for third order, 1/Ms

in general:

for second order, 1/s

for third order, 1/Ms

in general:

Posted: **Sun Mar 18, 2018 10:07 am**

An easier way to think of it is just match up your concentrations and their exponents with the rate (which you know is M/s or mol/L*s)

Posted: **Sun Mar 18, 2018 5:10 pm**

write it all out and cancel whatever you can. whats left is your units

Posted: **Sun Mar 18, 2018 11:43 pm**

Another thing I do is whatever the order is, the liters and moles will have an exponent one less than the order.

For fourth order, the rate constant = L^3/ mol^3 * S

For third order, the rate constant = L^2/ mol^2 * s

For second order, the rate constant = L / mol * s

For fourth order, the rate constant = L^3/ mol^3 * S

For third order, the rate constant = L^2/ mol^2 * s

For second order, the rate constant = L / mol * s

Posted: **Sun Mar 18, 2018 11:59 pm**

In general the constant is M^1-n*time^-1

Posted: **Tue Feb 19, 2019 12:46 pm**

It's helpful to just write all your units out and then see which ones cancel!