Kp and Kc

[Rachel Yu 1G]

When do we use Kp vs Kc? I understand that Kp is for pressure and Kc is for concentration but for gases do we use Kp or Kc? Also, just to clarify, do we use PV=nRT to convert between equilibrium concentrations of Kp and Kc?

[Brian Tangsombatvisit 1C]

Typically, if you are given a reaction with all reactants and products in the gas phase (and their units are given in partial pressures, not concentration) then it's assumed you're supposed to use Kp. If you do, just denote it. You could technically still calculate Kc in this scenario (where all the molecules are in the gas phase and have partial pressure units), but this is just a lot of extra work and you would have to use the equation PV=nRT to solve for n/V, which is mol/L, so concentration. (n/V = P/RT)

However, the standard equilibrium constant is Kc since most reactions don't involve all gases so, when in doubt, calculate Kc (but make sure all the reactants and products have concentrations, not partial pressures before plugging in)

[Mariah]

You use them when referring to what information is given or asked for. Yes, we use the ideal gas law to switch between the two if needed.

[Altamash Mahsud 1I]

Depending on the units they provide to you in the problem, you will use either Kc or Kp. For example, if they were to give you the partial pressure of reactants that were in the gas phase, say in the unit bar, then you would use Kp. But if they were to give you the molar concentrations of those reactant gases, in mol L^-1, then you would use Kc.

[Dina Marchenko 2J]

When it comes to gases, partial pressure (Kp) is more accurate for finding the equilibrium constant than Kc. For aqueous solutions obviously you can't use partial pressures so here you can use Kc. If concentrations are provided for gases rather than partial pressures, it's fine to use Kc and deal with molarity.