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### Kp vs Kc

Posted: Mon Jan 21, 2019 10:35 am
When you're calculating K and all the R and P are gases, does it really matter whether or not you use partial pressure or molar concentrations? You would get the same calculation right?

### Re: Kp vs Kc

Posted: Mon Jan 21, 2019 10:42 am
I think so..

### Re: Kp vs Kc

Posted: Mon Jan 21, 2019 12:21 pm
If all the reactants/products are gases, it doesn't matter which you use. However, Kc and Kp are not always equal! This is due to concentration and Pressure being related via the Ideal Gas Law (PV = nRT)

This link explains the technicalities of it well: https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Equilibria/Chemical_Equilibria/Calculating_An_Equilibrium_Concentrations/Writing_Equilibrium_Constant_Expressions_Involving_Gases/Gas_Equilibrium_Constants%3A_Kc_And_Kp

### Re: Kp vs Kc

Posted: Mon Jan 21, 2019 3:57 pm
Like the poster above noted, the equilibrium constant for the concentration of species in gaseous equilibrium and their partial pressures may be the same value, but this often not the case. Partial pressure and concentration within a system are directly proportional in value, meaning that while proportionally species have the same PP and concentration, their actual values may be multiple magnitudes larger or smaller.

For instance, take the hypothetical gaseous reaction $A + B\rightleftharpoons C$, at 200K in a 1.00 L reaction vessel. If there were 0.2 moles of A, 0.3 moles of B, and 0.5 moles of C at equilibrium, then 20% of the partial pressure in the system is due to A, 30% due to B, and 50% due to C. Likewise, the system is also 20% A, 30%B, and 50% C by concentration. However, this is not to state that concentration and PP are equal; using the ideal gas law will allow us translate concentration into PP and then find equilibrium concentration values.

### Re: Kp vs Kc

Posted: Mon Jan 21, 2019 4:27 pm
Kp applies to pressure and Kc applies to concentration

### Re: Kp vs Kc

Posted: Mon Jan 21, 2019 4:32 pm
To add to this, the question would usually give you either concentration values or partial pressure values and when given one or the other, you would know whether to use Kc or Kp.

### Re: Kp vs Kc

Posted: Mon Jan 21, 2019 8:15 pm
Kp has to do with the partial pressure while Kc is the concentration.

### Re: Kp vs Kc

Posted: Mon Jan 21, 2019 8:25 pm
Kp refers to pressure where Kc refers to concentration