Kc and Kp

[EMurphy_2L]

when a question simply asks for "the equilibrium constant" or "K" does that mean related to concentration or pressure? and why is this so easily interchangeable?

[Sebastian Lee 1L]

when a question simply asks for "the equilibrium constant" or "K" does that mean related to concentration or pressure? and why is this so easily interchangeable?

I believe that when a problem asks you to find the equilibrium constant, unless it specifies to find Kc or Kp, you should just use what it gives you. If it gives moles or molarity, find Kc; if it gives partial pressures, find Kp. It is also important to note that these certainly aren't interchangeable. You can only use Kp when the reaction is between gases with partial pressures (you can't find partial pressure of an aqueous solution). But anyways, I think it just depends on the context of the problem whether you find Kc or Kp.

[Brianna Becerra 1B]

You can identify when you should be using Kp as the reactants/products will be in the gas phase. Kc is used when the problem is given in the form of concentration or moles per liter. They seem interchangeable when solving for the equilibrium constant because the process is the same of dividing the products by the reactants to get the answer, however, the difference in this calculation can be distinguished based on the phases of the reactants and products.

[Abby Soriano 1J]

Whenever a question asks for the equilibrium constant, you have to figure out the phases of the reactants and products when you write out the reaction. If they are in the aqueous state, then use Kc. If they are in the gas phase, then use Kp. Writing out the entire reaction with all the phases will also help you in making sure you don't accidentally include any solids or liquids in the K expression.

[EMurphy_2L]

but can't you also use Kc for gases? so the phases don't really tell you anything but just go off of what you're given?

[Esha Chawla 2E]

when a question simply asks for "the equilibrium constant" or "K" does that mean related to concentration or pressure? and why is this so easily interchangeable?

When a question asks for the equilibrium constant, you have to look at the context of the question to determine if you will be using Kc or Kp. This is so easily interchangeable because, using PV = nRT, you can convert pressure to n/V. Similarly, you can covert n/V to pressure.

[005384106]

Why aren’t solids and liquids included in the equilibrium constant?

[Qiu Ya Wu 4I]

Solids and liquids aren't included in the equilibrium constant because there's no such thing as the "concentration" of a solid and liquids (solvents) can usually be expected to have nearly the same concentration on both sides of the equation so they would cancel out to 1 anyways if we divided a liquid's concentration before and after a reaction.

[Qiu Ya Wu 4I]

but can't you also use Kc for gases? so the phases don't really tell you anything but just go off of what you're given?

You can also use Kc for gases since a lot of the reactions we deal with have heterogeneous equilibria. If the units for the amount of gas are in L/mol, use concentration and if the units are given in bars, use partial pressure.