## Kc and Kp

EMurphy_2L
Posts: 128
Joined: Sat Sep 07, 2019 12:16 am

### Kc and Kp

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
Posts: 157
Joined: Fri Aug 09, 2019 12:15 am
Been upvoted: 1 time

### Re: Kc and Kp

EMurphy_2L wrote: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
Posts: 117
Joined: Fri Aug 02, 2019 12:16 am

### Re: Kc and Kp

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
Posts: 103
Joined: Sat Aug 24, 2019 12:16 am

### Re: Kc and Kp

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
Posts: 128
Joined: Sat Sep 07, 2019 12:16 am

### Re: Kc and Kp

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
Posts: 108
Joined: Thu Jul 25, 2019 12:17 am

### Re: Kc and Kp

EMurphy_2L wrote: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
Posts: 101
Joined: Sat Aug 24, 2019 12:16 am

### Re: Kc and Kp

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

Qiu Ya Wu 4I
Posts: 107
Joined: Fri Sep 20, 2019 12:17 am

### Re: Kc and Kp

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
Posts: 107
Joined: Fri Sep 20, 2019 12:17 am

### Re: Kc and Kp

EMurphy_2L wrote: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.