## Units of k

Artin Allahverdian 2H
Posts: 76
Joined: Fri Sep 28, 2018 12:26 am

### Units of k

Why are the units of k different between different reaction orders ?

Amy Lefley 1J
Posts: 60
Joined: Fri Sep 28, 2018 12:15 am

### Re: Units of k

Since rate=k*(concentration)^power of reaction, and we know that the units of rate are always mol.L-1.s-1, and that sometimes the concentration is to the zero, first, or second power depending on the order of the reaction, then the units of k must change.

Nathan Tran 4K
Posts: 92
Joined: Fri Sep 28, 2018 12:16 am

### Re: Units of k

The units are different because the reaction order affects the exponent--a first reaction order means an exponent of 1, a second reaction order means an exponent of 2, etc. Because we are raising the concentration (moles/Liter) to a different power, the units change accordingly. For example, by raising the concentration to the second power, we moles^2/Liters^2. After manipulating the equation to get this on the opposite side of k, we see that this would be different than concentration raised to a different power.

MariahClark 2F
Posts: 60
Joined: Wed Nov 15, 2017 3:04 am

### Re: Units of k

I was having trouble explaining out the equation and what makes the units different but I found a great explanation online that outlines it nicely.

The rate of a chemical reaction r (or c t^-1) is given by:

r = k [A]^a x [B]^b

where k is the rate constant, A and B are the concentrations of the two reactants and the exponents a and b are integers and give the order of the reaction. The overall kinetic order is the summation of a and b.

If we apply units where c is the concentration in moles or mass per litre, then,

c t^-1 = k x c^a x c^b,

or k = ct^-1/(c^a x c^b)

Therefore the units of the rate constant depend on the kinetic order of the reaction. Assuming the various possibilities of 0, 1, 2 or 3 for the orders will determine the units of k.

Destiny Diaz 4D
Posts: 51
Joined: Fri Sep 28, 2018 12:28 am

### Re: Units of k

The units of k will change depending on the order of the reaction so it is important to know which order your reaction is to have the correct units

Karishma_1G
Posts: 67
Joined: Fri Sep 28, 2018 12:18 am

### Re: Units of k

The units of k are different depending on reaction orders because the units for rate are always the same (mol x L-1 x s-1). Since the concentration of the reactants are raised to a power based on their particular order, the units may go up in exponents. For example since concentration is squared in a second order reaction the units would be (mol2 x L-2). The rate constant (k) must have units that help cancel and make it equal to the units for the rate. In this case, k would be mol-1 x L x s-1.

Mukil_Pari_2I
Posts: 87
Joined: Fri Sep 28, 2018 12:29 am

### Re: Units of k

The units of the rate are always M/s. Therefore, you always want to find a way to make the units equal to M/s. So the units of K would always be in relation to the units of the concentration. For example, for a zero order reaction, the units of K would be M/s.

Shibhon_Shepard
Posts: 62
Joined: Wed Nov 15, 2017 3:04 am

### Re: Units of k

they are different depending on the order.

so the 1st only has s^-1
2nd: mol*L^-1*s^-1
3rd: mol^2 *L^-2 *s^-2