## Rate Constant Units

Alexia Joseph 2B
Posts: 56
Joined: Thu Jul 27, 2017 3:01 am

### Rate Constant Units

How do we tell what the units for the rate constant are based on the order of the reaction?

Andres Reynoso 1J
Posts: 30
Joined: Fri Sep 29, 2017 7:06 am

### Re: Rate Constant Units

You can use the rate law $rate=k[A]^{n}$ and plug in the units for each variable and simplify.
For example, for zero order reactions:
$\frac{mol A}{L\cdot s} = k[A]^{0}$

$\frac{mol A}{L\cdot s} = k$

For first order:
$\frac{mol A}{L\cdot s} = k[A]^{1}$

$\frac{mol A}{L\cdot s} = k(\frac{mol A}{L})$

$\frac{\frac{mol A}{L\cdot s} }{(\frac{mol A}{L})}=k$

$\frac{1}{s}=k$

etc. etc...

Alyssa Parry Disc 1H
Posts: 53
Joined: Sat Jul 22, 2017 3:01 am

### Re: Rate Constant Units

Or you could think of it like this units of k=M^(1-n).s-1 where n is the order.

David Minasyan 1C
Posts: 54
Joined: Thu Jul 13, 2017 3:00 am

### Re: Rate Constant Units

Know that zero order reactions are mol/(L*s) and then as you go up one order, multiply by L/mol each time.

RussellChin_3A
Posts: 42
Joined: Sat Jul 22, 2017 3:01 am

### Re: Rate Constant Units

The units of K must be able to cancel out with the units of the concentration in order for the rate units to always be M/s. so just add on however many M/s needed in order to get the rate to its proper units

Belle Calforda3f
Posts: 67
Joined: Fri Sep 29, 2017 7:07 am

### Re: Rate Constant Units

using either the integrated rate law or the half life eqn can help you figure out the units for the rate constant without having to memorize anything. If you plug in the units for everything but k, you can figure out what k has to be for the equations to work.

Erik Khong 2E
Posts: 50
Joined: Fri Sep 29, 2017 7:07 am

### Re: Rate Constant Units

Overall order and their units of k:

1: 1/s
2: L/mol*s
3: L^2/mol^2*s

David Zhou 1L
Posts: 61
Joined: Fri Sep 29, 2017 7:04 am

### Re: Rate Constant Units

Remember that multiplying my L/mol is the same thing as dividing by M, or multiplying by M-1.

William Xu Dis 1D
Posts: 31
Joined: Fri Sep 29, 2017 7:05 am

### Re: Rate Constant Units

Also, when solving for k through the equations, the units should cancel out to the proper units anyway, but it is useful to know what the units are for different orders anyway.

Michael Downs 1L
Posts: 29
Joined: Thu Jul 13, 2017 3:00 am

### Re: Rate Constant Units

What is the conceptual difference between a fast step and a slow step