CHEMISTRY COMMUNITY

How do we know when to write the equation for Kc vs. Kp when the question just asks to find the expression for K like in question 5G3?

Kp is only used when dealing with gases.

Kp is for partial pressures, so only involves gases

I would recommend you look at the values you're given in the problem first to determine if you solve for Kc or Kp. In the problem, if you're given values for the reactants and products and they're in units of bars, then that means you'll be solving for Kp since pressure is measured in bars. However, if the values for the reactants and products are in units of moles/L, then you solve for Kc since concentration has the units moles/L.

Hope this helps!

Like others said, Kp is specifically referring to the equilibrium constant when all relative molecules/atoms are in a gaseous phase, therefore you use partial pressures. Kc refers to the equilibrium constant with regards to the concentrations of reactants and products, so you would use molar concentrations.

Adding on, Kp is used for gases in the reaction, ignoring solids and liquids. When a reaction includes aqueous solutions, you can't use Kp because liquids can't have pressure, just concentrations.

Just for further clarification, when there are all gases, so you use Kp and partial pressure, the format for K would be Kp = and then use parenthesis with P and the molecule as the subscript, but for Kc and concentrations, the format for K would be Kc= and then use brackets?

for Kc, you use the square brackets to denote concentration