How do you know when to use Kc vs Kp?

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Abril Guanes 2A
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How do you know when to use Kc vs Kp?

Postby Abril Guanes 2A » Wed Jan 06, 2021 10:03 pm

In lecture today, Dr. Lavelle said that most problems should specific whether they want Kc or Kp; however, none of the Module problems specified whether they wanted use to solve Kc or Kp. I understand that the problem will hint at whether it's Kc or Kp when giving partial pressure or concentration, but I was wondering if anyone can provide a more specific way to determine if it's Kc or Kp in a problem, and thus need to convert concentration to partial pressure or vice versa? Thank you!

Kailani_Dial_2K
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Re: How do you know when to use Kc vs Kp?

Postby Kailani_Dial_2K » Wed Jan 06, 2021 10:22 pm

I would say that if all of the information that you are given is in concentration then use Kc if everyhting is given is partial pressure use Kp or if you are given concentation and the question asks for the answer in partial pressure, then I would convert with the gas law and answer using Kp and visa versa if you are given partial pressure and the questions asks for Kc. If anyone has a better way I would also like to know, but I hope this helps

Nicole Huang 3F
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Re: How do you know when to use Kc vs Kp?

Postby Nicole Huang 3F » Wed Jan 06, 2021 10:23 pm

From what I understand, both are valid, but usually the question has hints embedded stating if it wants Kc or Kp (like if it gives you a concentration of a solution you would find Kc, and if they gave you a gas pressure you would find Kp). You should probably only convert if the question asks you to or if theres both an aqueous solution and gas in that reaction and both are given in two different standard units so you would have to convert one to the other. Hope this kinda helps

Mikayla Kwok 3K
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Re: How do you know when to use Kc vs Kp?

Postby Mikayla Kwok 3K » Wed Jan 06, 2021 10:47 pm

I believe if at least one of the species involved in the reaction is aqueous, you should use Kc, because you cannot convert the concentration of an aqueous species to partial pressure. However, if the species are all in the gas phase, you can use either Kc or Kp, and the problem probably would specify.

Madison Muggeo 3H
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Re: How do you know when to use Kc vs Kp?

Postby Madison Muggeo 3H » Thu Jan 07, 2021 1:27 pm

I would say if the reactants or products are in the gaseous phase you can use Kp or Kc (but the question should specify) and if the reactants and products are in the aqueous state (aq) then we would use Kc because you cannot convert the concentration of an aq solution to partial pressure. Hope this helps! :)

KhanTran3K
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Re: How do you know when to use Kc vs Kp?

Postby KhanTran3K » Thu Jan 07, 2021 1:34 pm

I believe that, depending on context, we can decide to use Kc or Kp. When a question is given in all partial pressures, then it would be safe to assume to use Kp, while if given in aq, then use Kc. I would not convert anything unless the question asks, since it is possible to convert the Kp to Kc. Hope this helps!

Rahul Sobti 1E
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Re: How do you know when to use Kc vs Kp?

Postby Rahul Sobti 1E » Thu Jan 07, 2021 1:36 pm

If the reactants and products are all gasses, then I would use Kp because then they are measured in their partial pressures. If the reactants and products are aqueous (dissolved in water), I would use Kc because then they are measured in their concentrations.

SalmaQ
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Re: How do you know when to use Kc vs Kp?

Postby SalmaQ » Thu Jan 07, 2021 5:15 pm

I believe that if you are asked to use K to calculate the concentration of a species at equilibrium, and if the reaction is in a gas phase, we would need to use its partial pressure (Kp). After this, we can use PV=nRT, or once solved P=n/V(RT) ---> P=conc(RT) to covert Kp to find the concentration Kc.

Jason_Glass_2L
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Re: How do you know when to use Kc vs Kp?

Postby Jason_Glass_2L » Thu Jan 07, 2021 5:28 pm

It will usually say in the question which K they want, but if it isn't stated, then just use what ever information is given. For example, If moles and volume are given you would use Kc and if a partial pressure is given then you use Kp.

Lilly Catarozoli 1B
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Re: How do you know when to use Kc vs Kp?

Postby Lilly Catarozoli 1B » Thu Jan 07, 2021 6:20 pm

I think the question would specify if it wanted a concentration (Kc) or partial pressure (Kp). If only gases are involved, I think it would most likely be Kp though. Hope this helps!


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