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### Kc vs. Kp

Posted: Tue Nov 28, 2017 10:51 pm
When we are asked to write the equilibrium expression K for a reaction and we are not told whether to write that equation with respect to concentration or partial pressures, how do we know which way to write the expression? In question 11.3 on the homework, I originally answered it in terms of molar concentration but then changed it when I saw the solution manual write the answer in terms of partial pressure.

I understand that gases are typically referred to with respect to their partial pressures, but must we always assume to write equilibrium expressions K in terms of partial pressure and not molar concentration? Are there ever cases where we write K in terms of concentration and not pressure?

### Re: Kc vs. Kp

Posted: Tue Nov 28, 2017 11:02 pm
If the units are already in pressure and it just says k, then you should keep the units in pressure. If the units are already in molarity and it just says k, then you should keep the units in molarity. If it says kc, then make sure to have the units be in molarity. With Kp, have the units be in pressure.

### Re: Kc vs. Kp

Posted: Tue Nov 28, 2017 11:06 pm
when the problem only involves aq, K must be calculated from molar concentrations of the reactants and the products. when the problem involves both g and aq, you also have to use concentrations for all the things because aq can only have concentration while g can have both concentration and partial pressure. When there are only gases involved and you see K, consider using partial pressures for all; but if specified as Kc, use concentrations to calculate.

### Re: Kc vs. Kp

Posted: Wed Nov 29, 2017 8:16 pm
In general, you would use P when referring to gases. However, this does not mean you will use P whenever you have gases. If it's atm (bar, torr, Pa, etc.), then you would use Kp. If you have M or any other concentration units, then you would use Kc.