Why the units of rate constant change with the change of order of reaction?

Question

Units of rate constant

Answer 1

Look at how the rate constant is calculated.  Let's use A ==> B as an example.

• First order in A:  rate = k[A] and k = rate/[A]
• Second order in A:  rate = k[A]² and k = rate/[A]²

 

If you use rate as mole/L/sec for example and concentration as moles/L, then...

• First order k = mole-L^-1s^-1/mole-L^-1 = s^-1
• Second order k = mole-L^-1s^-1/(mole-L^-1)² = L-mole^-1s^-1 = M^-1s^-1

 

You can apply the rationale/calculations for other orders.  The units of k will be different for each.