Destroyer GC question

[Troyvdg]

Consider the following:

H20 (l) + H20 (l) --> OH^- (aq) H3O ^+

when the temperature is decreased from 25C, it is found that the Kw has decreased. which of the following is true:

answer: the reaction must be endothermic.

I understand why the reaction is endothermic because temp decreases, but I don't understand why Kw decreases, could someone please explain

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[mario0923]

the reaction is endothermic so
think the equation in this way

HEAT+ H2O(l) -> H+(aq) + OH-(aq)

so if the temperature is decreasing the reaction will shift to the left
where Kw=[H+][OH-] less product therefore Kw will decrease.

remember nothing can change the K (equilibrium constant) except the temperature.

 

[NDPitch]

Consider the following:

H20 (l) + H20 (l) --> OH^- (aq) H3O ^+

when the temperature is decreased from 25C, it is found that the Kw has decreased. which of the following is true:

answer: the reaction must be endothermic.

I understand why the reaction is endothermic because temp decreases, but I don't understand why Kw decreases, could someone please explain

The way I understood this problem was as follows...

Think back to the auto ionization of water, which is what this problem is concerning.

I believe at 25 C, we have the following....

H2O (l) + H2O (l) -------->>> OH- + H3O+

The equilibrium constant for this reaction is Kw = [OH-][H3O+] = 1 x 10 ^-14. So at 25C, OH and H3O+ are each 10^-7.

This Kw constant is how pH is defined. So now, in this question, it's saying that when the temperature was decreased, Kw decreased. If Kw decreased, what had to have happened to the concentrations of OH- and H3O+? Well, let's say Kw went from 10^-14 to 10^-16. OH- and H3O+ decreased to 10^-8. So the product concentrations decreased!

Now you need to understand equilibrium. If the products of the original reaction decreased, where did the reaction shift? It shifted left. So if something shifted left, that means that the reactant concentrations decreased, and the reaction had to shift left to compensate. In this case, the only thing that decreased was heat, so we can think of heat as a reactant. So, heat decreased, reaction shifted left, heat is a reactant, so this is ENDOTHERMIC.