Destroyer Gen Chem #93

[Hope30]

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Consider the following:
water (l) + water (l) --> OH- (aq) + H3O+ (aq)
When the temperature is decreased from 25C, it is found that the Kw has decreased Which of the following is true?
a) The rxn must be exothermic
b) The rxn must be endothermic
c) The rxn is thermoneutral
d) Not possible. Kw = 1 x 10^-14 at all temperatures.
e) None of these
answer: b
I understand that as the temperature decreases, the rxn is exothermic. However, I'm not sure about the Kw. How does that work? It is said in the explanation that b is correct because Kw decreases. What if the temperature increases and Kw decreases, would the rxn still be endothermic?

THANKS FOR THE HELP!

 

[Demps]

Consider the following:
water (l) + water (l) --> OH- (aq) + H3O+ (aq)
When the temperature is decreased from 25C, it is found that the Kw has decreased Which of the following is true?
a) The rxn must be exothermic
b) The rxn must be endothermic
c) The rxn is thermoneutral
d) Not possible. Kw = 1 x 10^-14 at all temperatures.
e) None of these
answer: b
I understand that as the temperature decreases, the rxn is exothermic. However, I'm not sure about the Kw. How does that work? It is said in the explanation that b is correct because Kw decreases. What if the temperature increases and Kw decreases, would the rxn still be endothermic?

THANKS FOR THE HELP!

I don't know if I will do a good job explaining.... but I will give it a try.

Way I treat the the temperature or enthalpy... I treat it as a component as a product or reactant.

For endothermic, put T on the left side
For exothermic, put T on the right side. and follow along the Le Chatelier's principle.

so for endothermic reaction, increase temperature, reaction shifts right // decrease temperature, reaction shifts left vise versa for exothermic.

We are given on the question that Kw decreased... treat it like how Keq decreased in equilibrium problem, so that means the state is reactant favored - shifting left.

Two ways that the reaction shifts left is that:

1) reaction was exothermic and temperature was increased
2) reaction was endothermic and temperature was decreased

we have the situation #2, the answer is B. the reaction is endothermic.

 

[Hope30]

It took a while to absorb what you are saying because it is like PAT: there is all aspects you can apply the concepts. Yet, PERFECT MNEMONIC. I got it! 👍👍👍👍👍😀

 

[SN1]

Consider the following:
water (l) + water (l) --> OH- (aq) + H3O+ (aq)
When the temperature is decreased from 25C, it is found that the Kw has decreased Which of the following is true?
a) The rxn must be exothermic
b) The rxn must be endothermic
c) The rxn is thermoneutral
d) Not possible. Kw = 1 x 10^-14 at all temperatures.
e) None of these
answer: b
I understand that as the temperature decreases, the rxn is exothermic. However, I'm not sure about the Kw. How does that work? It is said in the explanation that b is correct because Kw decreases. What if the temperature increases and Kw decreases, would the rxn still be endothermic?

THANKS FOR THE HELP!

The answer is endothermic because it involves breaking the bonds in H20 into OH- and H30+. Remember

Endothermic = energy added, bond breaking
Exothermic = energy released, bond making

 

[Demps]

It took a while to absorb what you are saying because it is like PAT: there is all aspects you can apply the concepts. Yet, PERFECT MNEMONIC. I got it! 👍👍👍👍👍😀

glad I could help 😉