someone please explain this problem in destroyer

[spoog74]

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gives a reaction of h20 turning into OH and H30 + and says that the reaction temperature decreased from 25 celsius and the kw decreased. Which of the following is true?

Reaction endothermic
reaction exothermic


Answer is endothermic, but why? Whats the reasoning? Someone ?

 

[LetsGo2DSchool]

Think of the heat as being either a reactant or product.

H2O <---> H3O+ + OH-

If temperature decreased then it was a reactant and thus,

H2O + heat <---> H3O+ + OH-

If it's a reactant, then it's endothermic.

 

[flin5845]

My guess is because the temperature DECREASE that means the system was taking energy IN...therefore ENDOthermic...

I think

 

[rockclock]

le chatlier's principle (or however that's spelled):

if you "take away" heat (cool temp down from 25C) and the reaction equilibrium shifts to the left (Kw decreases, equilibrium shifts towards reactants) then "heat" must have been a "reactant"...which makes it an endothermic reaction.

 

[rockclock]

My guess is because the temperature DECREASE that means the system was taking energy IN...therefore ENDOthermic...

I think

pretty sure the question is meant to say that the system was cooled (an in, intentionally by the experimenter) down from 25C, not that the vessel cooled due to the reaction.

the consequence of that cooling was a decrease in Kw. you apply Le chatlier's reasoning to that fact and that will take you home.

BTW it has to be one or the other:
1. either you're at equilibrium at 25C and then cool the system (you'd have to have established an equilibrium to know that Kw decreased from there)
2. or you let the reaction proceed at an initial temp of 25C and note that the system cooled due to an endothermic reaction (here you can observe the temperature but there's only going to be the one final equilibrium so you can say anything about a change in Kw)

since this is water in a beaker, it's hard to "initiate" the reaction from pure undisociated H2O, so it's got to be #1

 

[LetsGo2DSchool]

And the other hint is that because Kw decreased there is less product than before, implying that the reaction shifted to the left, the endothermic side.

 

[spoog74]

im having trouble understanding... If the reaction cooled, meaning the temperature decreased, doesnt that mean that the reaction LET OFF heat?

 

[LetsGo2DSchool]

Try thinking of it like this. There's a system and surroundings.

In endothermic rxn, the heat actually is absorbed into the system from the surroundings, and thus the temperature in the surrounding decreases.

In exothermic rxn, the heat is released by the system into the surroundings, and thus the temperate in the surroundings increases.

Hope this makes sense.

 

[spoog74]

Try thinking of it like this. There's a system and surroundings.

In endothermic rxn, the heat actually is absorbed into the system from the surroundings, and thus the temperature in the surrounding decreases.

In exothermic rxn, the heat is released by the system into the surroundings, and thus the temperate in the surroundings increases.

Hope this makes sense.

That makes sense but when i try applying that logic into the problem i would get the answer WRONG. If the system temperature decreased, that would mean by your definition that the reaction is not gaining heat, but losing it, meaning exothermic wouldnt it?

 

[rockclock]

That makes sense but when i try applying that logic into the problem i would get the answer WRONG. If the system temperature decreased, that would mean by your definition that the reaction is not gaining heat, but losing it, meaning exothermic wouldnt it?

in an endothermic/exothermic problem, the system is the chemical reaction (specifically all the chemical bonds being broken and formed) and the surroundings is wherever the reaction takes place (in this case, the beaker of water).

 

[AllahIsGreater]

forget all this mumbo jumbo and remember this. If something gets cold (decreased temp...) than its endothermic. If something gets hot, then its exothermic. I am assuming you are not going for a phd in chemistry. This is all you need for the DAT.

may i add, that those are the exact words of Dr. Romano, with what he taught us.

 

[spoog74]

forget all this mumbo jumbo and remember this. If something gets cold (decreased temp...) than its endothermic. If something gets hot, then its exothermic. I am assuming you are not going for a phd in chemistry. This is all you need for the DAT.

may i add, that those are the exact words of Dr. Romano, with what he taught us.

ROFL thanks man !

 

[AllahIsGreater]

ROFL thanks man !

lol i dont want it to sound bad, but thats literally the level you need to understand it at for the DAT. good luck bro!