### 6th edition 15.9

Posted:

**Mon Mar 04, 2019 9:03 pm**when trying to find the units for the rate constants, why is the units for k mol per liter per second when the units in the table for constants are liter per mol per second?

Created by Dr. Laurence Lavelle

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

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=145&t=43623

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Posted: **Mon Mar 04, 2019 9:03 pm**

when trying to find the units for the rate constants, why is the units for k mol per liter per second when the units in the table for constants are liter per mol per second?

Posted: **Tue Mar 05, 2019 9:20 am**

I'm not sure which question you're talking about (I have 7th edition), but generally, the units for rate constant k depend on the order of the reaction. The units change so that the rate in the rate law will always equal M/s. For example, for first order reactions where rate = k [A], [A] is in molars, so k must have units of s^-1 to make the rate have units of M/s.

Posted: **Tue Mar 05, 2019 2:33 pm**

I agree, the units for rate constant k depend on the order of reaction.

For zero order reactions, the unit for k is (mol x L^-1 x s^-1).

For first order reactions, the unit for k is (s^-1).

For second order reactions, the unit for k is (L x mol^-1 x s^-1).

Hope this helps!

For zero order reactions, the unit for k is (mol x L^-1 x s^-1).

For first order reactions, the unit for k is (s^-1).

For second order reactions, the unit for k is (L x mol^-1 x s^-1).

Hope this helps!

Posted: **Tue Mar 05, 2019 2:47 pm**

In general, the rule of the thumb for finding the units of k is to find what units will result in the rate having the units of mol/(L*s). First, look at the units that the concentrations give you and make the units of k cancel those concentrations leaving you with mol/(L*s).

Posted: **Tue Mar 05, 2019 3:03 pm**

i personally don't know the exact reason why but if you try it with the first reaction they give you in the chart (btw i have the seventh edition). the rate law = k[H2][I2] and if you're just considering the units for the problem the left side or the rate has units of M/s while the right side is L/mol*sec x mol/L x mol/l and if you cancel out the moles and liters you're left with mol/L*sec which is M/s the same as the left side