## Rate constant units

$aR \to bP, Rate = -\frac{1}{a} \frac{d[R]}{dt} = \frac{1}{b}\frac{d[P]}{dt}$

Nicole Nikolov 1K
Posts: 50
Joined: Fri Sep 29, 2017 7:05 am

### Rate constant units

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...

SPandya1F
Posts: 51
Joined: Fri Sep 29, 2017 7:04 am
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### Re: Rate constant units

for first order, M/s
for second order, 1/s
for third order, 1/Ms
in general: $M^{1-n}$$time^{-1}$

Zane Mills 1E
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Joined: Fri Sep 25, 2015 3:00 am
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### Re: Rate constant units

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)

Phillip Tran
Posts: 54
Joined: Thu Jul 13, 2017 3:00 am

### Re: Rate constant units

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

Sarah Wax 1G
Posts: 29
Joined: Sat Jul 22, 2017 3:00 am

### Re: Rate constant units

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

Jimmy Zhang Dis 1K
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Joined: Fri Sep 29, 2017 7:05 am

### Re: Rate constant units

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

jane_ni_2d
Posts: 67
Joined: Fri Sep 28, 2018 12:23 am

### Re: Rate constant units

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