A2: Kp and changing pressures and temperature

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blossomx
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So I understand the rule that only changing temperature changes Kp but I don't understand why...

I'll use the Haber process to explain my confusion
N2 + 3H2 ->/<- 2NH3 and it's exothermic (-veDeltaH)

So if you increase the temperature, the equilibrium is going to shift to the left and so Kp will decrease.
Now if you increase the pressure, the equilibrium is going to shift to the right so surely Kp will increase?

Both have a change in the position of equilibrium so surely Kp will change as doesn't it serve to show you were the equilibrium is positioned? I can't distinguish between why changing temperature changes Kp and why pressure doesn't when they both have the same effect - they change the position of equilibria via Le Chatlier's Principle.

Thank you.
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charco
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(Original post by LeaX)
So I understand the rule that only changing temperature changes Kp but I don't understand why...

I'll use the Haber process to explain my confusion
N2 + 3H2 ->/<- 2NH3 and it's exothermic (-veDeltaH)

So if you increase the temperature, the equilibrium is going to shift to the left and so Kp will decrease.
Now if you increase the pressure, the equilibrium is going to shift to the right so surely Kp will increase?

Both have a change in the position of equilibrium so surely Kp will change as doesn't it serve to show you were the equilibrium is positioned? I can't distinguish between why changing temperature changes Kp and why pressure doesn't when they both have the same effect - they change the position of equilibria via Le Chatlier's Principle.

Thank you.
I posted the answer to this last week!

http://www.thestudentroom.co.uk/show...9#post46843249
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Borek
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You are mistaking Kp and Q. Q is a reaction quotient, and can change. Kp is the equilibrium constant, and doesn't change. They are given by the same formula, but one described the equilibrium, while the other described system that is not necessarily at equilibrium. When you increase the pressure, Q can change, but the system will react to reach the equilibrium, and it will be reacting till Q becomes equal to Kp again.
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blossomx
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(Original post by charco)
I posted the answer to this last week!

http://www.thestudentroom.co.uk/show...9#post46843249

(Original post by Borek)
You are mistaking Kp and Q. Q is a reaction quotient, and can change. Kp is the equilibrium constant, and doesn't change. They are given by the same formula, but one described the equilibrium, while the other described system that is not necessarily at equilibrium. When you increase the pressure, Q can change, but the system will react to reach the equilibrium, and it will be reacting till Q becomes equal to Kp again.
I see! Thank you. So for an increase in temperature for an endothermic reaction what happens? Does the reaction quotient change and then is restablished to equal Kc or does Kc does increase?
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Borek
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When temperature changes the only reason Q could change would be if the volume changed as well. For gases at constant volume concentrations don't change, for water solutions concentration changes depend on the thermal expansion (which is rather low, and typically can be ignored).

Kc changes with the temperature always.

So, when you change the temperature it is either Q doesn't change, but Kc does, or both Kc and Q change. Afterwards Q drifts in the direction of Kc.
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