## Writing expression for K

Malia Shitabata 1F
Posts: 127
Joined: Sat Aug 17, 2019 12:17 am

### Writing expression for K

When a question asks you to write an expression for K, how do you know when to use Px or [X]?

Brian Tangsombatvisit 1C
Posts: 119
Joined: Sat Aug 17, 2019 12:15 am

### Re: Writing expression for K

You can use partial pressures when all the reactants and products of a reaction are in the gas phase and when the units are in the gas phase (bar, atm, mmHg, Torr, etc). You normally use concentrations when other phases other than the gas phase are involved in the reaction, and you can use PV=nRT to convert between concentrations and partial pressures for gases.

Malia Shitabata 1F
Posts: 127
Joined: Sat Aug 17, 2019 12:17 am

### Re: Writing expression for K

Brian Tangsombatvisit 1C wrote:You can use partial pressures when all the reactants and products of a reaction are in the gas phase and when the units are in the gas phase (bar, atm, mmHg, Torr, etc). You normally use concentrations when other phases other than the gas phase are involved in the reaction, and you can use PV=nRT to convert between concentrations and partial pressures for gases.

If all the reactants and products were in the gas phase would it be wrong to use concentrations?

Zoe Gleason 4F
Posts: 51
Joined: Tue Jul 23, 2019 12:15 am

### Re: Writing expression for K

Malia Shitabata 1F wrote:
Brian Tangsombatvisit 1C wrote:You can use partial pressures when all the reactants and products of a reaction are in the gas phase and when the units are in the gas phase (bar, atm, mmHg, Torr, etc). You normally use concentrations when other phases other than the gas phase are involved in the reaction, and you can use PV=nRT to convert between concentrations and partial pressures for gases.

If all the reactants and products were in the gas phase would it be wrong to use concentrations?

It would not be incorrect if the values for the concentrations were in moles. For example, the problem from lecture, $N_2 (g) + 3H_2(g) \rightleftharpoons 2NH_3 (g)$ is all in the gas phase, but $K_c$ uses concentrations, not pressures.

Minh Ngo 4G
Posts: 137
Joined: Thu Jul 25, 2019 12:17 am

### Re: Writing expression for K

If everything is gas then you use P but otherwise, use concentration […]

Michelle Xie 2B
Posts: 99
Joined: Sat Aug 17, 2019 12:18 am

### Re: Writing expression for K

Px is used for gases to represent their partial pressure. In other cases, you would use concentration.