Kp vs Kc

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Katherine Grillo 1B
Posts: 69
Joined: Fri Sep 28, 2018 12:28 am

Kp vs Kc

Postby Katherine Grillo 1B » 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?

Gisselle Sainz 2F
Posts: 76
Joined: Wed Feb 21, 2018 3:00 am

Re: Kp vs Kc

Postby Gisselle Sainz 2F » Mon Jan 21, 2019 10:42 am

I think so..

Dhwani Krishnan 1G
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Joined: Fri Sep 28, 2018 12:17 am

Re: Kp vs Kc

Postby Dhwani Krishnan 1G » 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:

Luc Lorain 1L
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Joined: Fri Sep 28, 2018 12:18 am

Re: Kp vs Kc

Postby Luc Lorain 1L » 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 , 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.

Jason Ye 2I
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Joined: Fri Sep 28, 2018 12:22 am

Re: Kp vs Kc

Postby Jason Ye 2I » Mon Jan 21, 2019 4:27 pm

Kp applies to pressure and Kc applies to concentration

Kessandra Ng 1K
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Joined: Fri Sep 28, 2018 12:25 am

Re: Kp vs Kc

Postby Kessandra Ng 1K » 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.

Posts: 69
Joined: Fri Sep 28, 2018 12:25 am

Re: Kp vs Kc

Postby Yiting_Gong_4L » Mon Jan 21, 2019 8:15 pm

Kp has to do with the partial pressure while Kc is the concentration.

Ashe Chen 2C
Posts: 31
Joined: Mon Jan 07, 2019 8:23 am

Re: Kp vs Kc

Postby Ashe Chen 2C » Mon Jan 21, 2019 8:25 pm

Kp refers to pressure where Kc refers to concentration

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