# When to use which Equilibrium Constant. (Kp vs. Kc)

Hello forum! My name's Patrick, and I'm taking Chemistry 212. We're learning about equilibrium and I'm constantly getting K's confused.
For example: In the equation ΔG=-RT*ln(K), I have no clue whether to use the Kp value or the Kc value. Could I have used either? I just need a very thorough explanation of when and why I should use the various equilibrium constants. If you could, don't just explain this example, and please try to predict some of my confusions, because it's impossible for me to know what I'm misunderstanding. I'm usually a very good chemistry student, but this material is hitting me like a ton of bricks!
Thanks so much in advance for all of your help and expertise.

## Answers and Replies

Use Kp when the substances are gases and use Kc when the substances are in aqueous state.
In equation you could use either kp or kc.
Any further queries?

DrDu
The ##\Delta ## G you are using refers to the standard state, i.e. it is ##\Delta G^o##. The definitions of the standard state depends on the state of the substance involved, for gases, it is the low pressure limit extrapolated to 1000 hPa, let's call it ##p^o## and so on, for solids, it is the pure solid, for solvents, the pure solvent and for solutes, the low concentration limit extrapolated to 1 mol/l or mol/kg. The concentrations/ pressures appear then in the equilibrium constant as ##p/p^o##, ##c/c^o## and so on.
See
http://en.wikipedia.org/wiki/Standard_state

The ##\Delta ## G you are using refers to the standard state, i.e. it is ##\Delta G^o##. The definitions of the standard state depends on the state of the substance involved, for gases, it is the low pressure limit extrapolated to 1000 hPa, let's call it ##p^o## and so on, for solids, it is the pure solid, for solvents, the pure solvent and for solutes, the low concentration limit extrapolated to 1 mol/l or mol/kg. The concentrations/ pressures appear then in the equilibrium constant as ##p/p^o##, ##c/c^o## and so on.
See
http://en.wikipedia.org/wiki/Standard_state
... Hmm. I have no idea what the difference between ##\Delta G^o## and ##\Delta G## is. And after you explain what those are, could you let me know how that tells me which K constant to use. Thanks so much!!!!!!

DrDu
Hm, ##\Delta G^o## is ##\Delta G## at the standard state. In general ##\Delta G=\Delta G^0 +RT \ln Q## where Q is the quotient of the pressures/ concentrations.
In equilibrium, ##\Delta G=0## and Q is named K.
To know which K or Q to use, you have to find the appropriate standard state for the reactands.