Gen Chem Question.

[Slaughter421]

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I dont understand this question.

H2O(l) = H2O(l) ---> Oh + H3O
When the temp is decreased from 25C, it is found that the kw has decreased, which of the following is true?


Ans:
The Reaction must be endothermic.


The solutions say when you reduce heat, an exothermic reaction occurs and when u increase heat an endothermic rxn occurs. But for some reason the kw effects this answer to make it endothermic instead of exothermic.

Can some explain KW

 

[Rela89]

Ok, so Kw = [OH-][H+] (water dissociation constant)..and you only take into account solution that are aqueous (dissolve in water). H2O is a liquid, so it's not included in the equilibrium expression. If the Kw decreases due to the reduction of heat, then you form LESS products (less of H+ and OH-). Since H2O (l) --> OH-(aq) + H+ (aq) is an equilibrium expression/reaction, and the reverse direction is also possible, the formation of H2O(l) is preferred in this state.

Therefore, since Kw<1, products are favored, energy (heat) is needed to make the reaction go to the right, and hence it is endothermic!

Hope that helps 🙂

 

[sbdento]

Treat heat as product/reactant.

Since the Kw is decreasing [product]/[reactant], we know that more reactant is present or the reaction shifted left.

Now look at if heat was a reactant. Decreasing a reactant would shift left.
Then look at if heat was a product. Decreasing a product shifts it to the right.

So heat must have been a reactant since the equation shifted left.

Heat as a product=exo

Heat as a reactant=endo

 

[Slaughter421]

Thanks for all the help. I get understand it now.