020200
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Hi,

Can someone please explain why Kc only changes with temperature but not concentration? Also, if Kc does change with temperature, how does Kc change if you increase/decrease temperature?

Thanks!
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Reiner
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Kc gets affected by temperature because it affects either the exothermic/endothermic reaction. For example, if the forward reaction is endothermic and there is an increase in temperature it would favour the forward reaction because to counteract the systems increase in temperature it would favour this reaction because an endothermic reaction decreases temperature as it is taken in to drive the reaction.
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Anonymouspsych
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(Original post by DhylanP - CRGS)
Hi,

Can someone please explain why Kc only changes with temperature but not concentration? Also, if Kc does change with temperature, how does Kc change if you increase/decrease temperature?

Thanks!
Hi,

I presume you know about Le Chatelier's principle. Remember Kc is an equilibrium constant concerned with reversible reactions which will be exothermic in one way and endothermic the other way. So by changing the temperature, the equilibrium position will shift according to Le Chatelier's principle which will vary the concentrations of the chemicals and since Kc is an equation based on the concentrations of reactants and products, this means temperature consequently affects Kc.

Concentration doesn't affect Kc because if you try to vary the concentration of one of the chemicals then the system will restore the original concentrations (again via Le Chatelier) meaning the value of Kc stays the same.

To answer the last part of the question, in order to know how Kc changes by increasing/decreasing temperature you need to know if the forward reaction/backwards reaction is exothermic or endothermic. Then by using Le Chatelier's principle, you can work out how the equilibrium position changes and therefore what happens to the relative concentrations of the reactants and products.

Hope this helped
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charco
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(Original post by Anonymouspsych)
Hi,

I presume you know about Le Chatelier's principle. Remember Kc is an equilibrium constant concerned with reversible reactions which will be exothermic in one way and endothermic the other way. So by changing the temperature, the equilibrium position will shift according to Le Chatelier's principle which will vary the concentrations of the chemicals and since Kc is an equation based on the concentrations of reactants and products, this means temperature consequently affects Kc.

Concentration doesn't affect Kc because if you try to vary the concentration of one of the chemicals then the system will restore the original concentrations (again via Le Chatelier) meaning the value of Kc stays the same.

To answer the last part of the question, in order to know how Kc changes by increasing/decreasing temperature you need to know if the forward reaction/backwards reaction is exothermic or endothermic. Then by using Le Chatelier's principle, you can work out how the equilibrium position changes and therefore what happens to the relative concentrations of the reactants and products.

Hope this helped
Le Chatelier's principle does not actually explain anything. It is a guide to what happens.

For an endothermic reaction the activation energy of the forward reaction is greater than that of the reverse reaction.

An increase in temperature will allow relatively more particles to attain the required forward activation energy than for the reverse reaction.

The rate for both forward and reverse reactions increase BUT the rate forward increases MORE than for the reverse reaction and the position of equilbrium moves to the right and kc increases.
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charco
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(Original post by DhylanP - CRGS)
Hi,

Can someone please explain why Kc only changes with temperature but not concentration? Also, if Kc does change with temperature, how does Kc change if you increase/decrease temperature?

Thanks!
https://www.thestudentroom.co.uk/sho...ium%20pressure
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Anonymouspsych
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(Original post by charco)
Le Chatelier's principle does not actually explain anything. It is a guide to what happens.

For an endothermic reaction the activation energy of the forward reaction is greater than that of the reverse reaction.

An increase in temperature will allow relatively more particles to attain the required forward activation energy than for the reverse reaction.

The rate for both forward and reverse reactions increase BUT the rate forward increases MORE than for the reverse reaction and the position of equilbrium moves to the right and kc increases.
It does explain it and so does your explanation. It states that if there's an external influence to a system (like a change in temperature), the system will respond by counteracting that change by shifting the equilibrium position to try and restore the original conditions. So in the case of increasing temperature, the system will respond by shifting the equilibrium position to the side of the reaction that is more endothermic so some of that excess energy is used up so that the original temperature is restored. Of course all molecules will gain energy but the net effect is that the endothermic reaction is favored significantly more and hence occurs at a much greater rate than the exothermic one. Consequently, this will change the Kc value depending on whether the forward reaction is endothermic/exothermic.
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charco
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(Original post by Anonymouspsych)
It does explain it and so does your explanation. It states that if there's an external influence to a system (like a change in temperature), the system will respond by counteracting that change by shifting the equilibrium position to try and restore the original conditions. So in the case of increasing temperature, the system will respond by shifting the equilibrium position to the side of the reaction that is more endothermic so some of that excess energy is used up so that the original temperature is restored. Of course all molecules will gain energy but the net effect is that the endothermic reaction is favored significantly more and hence occurs at a much greater rate than the exothermic one. Consequently, this will change the Kc value depending on whether the forward reaction is endothermic/exothermic.
Le Chatelier's principle states that a system at equilibrium will respond to a change in conditions in such a way as to oppose that change.

You are explaining Le Chatelier once you start to enter into the thermodynamics of the system.

This may seem like semantics, BUT some examination boards do not accept Le Chatelier's principle as an explanation and, as such, it is important to highlight the difference.
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Anonymouspsych
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(Original post by charco)
Le Chatelier's principle states that a system at equilibrium will respond to a change in conditions in such a way as to oppose that change.

You are explaining Le Chatelier once you start to enter into the thermodynamics of the system.

This may seem like semantics, BUT some examination boards do not accept Le Chatelier's principle as an explanation and, as such, it is important to highlight the difference.
Right I see, thanks for pointing that out.
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