TBR Example 3.13 (Equilibrium)

[California Bear]

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Which of the following is the result of cooling an endothermic reaction at equilibrium?
A. The amt of products decreases
B. The amt of reactants decreases
C. The equilibrium constant increases
D. All partial pressures increase

Now, I can see why A is the correct answer, but I was also confused by choice C. Shouldn't the equilibrium constant remain constant, as it is the reaction quotient Q that decreases? I thought the equilibrium constant expression only takes into account concentrations at equilibrium.

Thanks in advance!

 

[Teleologist]

This is a terrible question because the equilibrium constant is dependent on temperature and cooling a system at equilibrium will disturb the existing equilibrium and a new one may establish - the old equilibrium does not "shift" - it's gone.

However it's clear that this question is just trying to test you on Le Chatlier's principle, despite the terrible wording. Endothermic reactions have a heat as a reactant and it's clear you understood it. C could be true as we know the K value changes one way or another; whether it increases or decreases is another issue.

The question should have explicitly stated that cooling affected only the reactant(s). By reaction the cooling could be referring to the entire system - i.e. the entire reaction.

 

[California Bear]

Ah, okay. Very clear explanation, thank you!