what is Enthalpy change here? + or -

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Dencology

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Guys what does this mean:

KOH(s) ----> KOH (aq) + 43KJ


does this mean this is an exothermic reaction or endothermic.

i thought if the energy is negative, then it is exothermic. and when it is + then it is endo.

so here it means that the energy is given off or energy is put in also what does it mean when we have this:


+ 43KJ KOH(s) ----> KOH (aq)


this means that energy is put in, right?

then for a problem like this:


22. The addition of 14g solid KOH pellets to water cause the following to take place;


KOH(s) ----> KOH (aq) + 43KJ


what is the enthalpy change for the dissolution of the 14g of KOH?

ans is -10.2KJ but why?

now we have to do the following:

14gKOH (1mole KOH/56g KOH) = 1/4 mole

then 1/4 (43KJ) or 1/4(-43KJ)

i don't get this part!

would someone explain. thanks.
 

zuma35

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When the heat is on the products side, it is being released and is exothermic (-H). If it's on the left, reaction is endo. In your problem, you correctly multiplied 43KJ by 1/4 to get 10.7. Since the heat is on the product side it's negative so the answer is -10.7.
 

Dencology

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When the heat is on the products side, it is being released and is exothermic (-H). If it's on the left, reaction is endo. In your problem, you correctly multiplied 43KJ by 1/4 to get 10.7. Since the heat is on the product side it's negative so the answer is -10.7.



thanks. i got you.
 

chessxwizard

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Enthalpy change is negative since heat is evolved by the reaction. In addition, entropy change is positive since an aqueous solution is being formed from a reactant that is a solid. Thus, the reaction should be spontaneous as written.
 

sciencegod

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Enthalpy change is negative since heat is evolved by the reaction. In addition, entropy change is positive since an aqueous solution is being formed from a reactant that is a solid. Thus, the reaction should be spontaneous as written.

Additionally, may I add, since a solute is being dissolved in a solvent, the freezing point would decrease, as would the vapor pressure. Conversely, we would observe an elevation in the boiling point. Thus, more heat would be required to boil the solution. The phase change that would occur in the circumstance that we would boil the aqueous solution would be called vaporization.
 

chessxwizard

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Good points, sciencegod. Gotta remember those colligative properties when you're dissolving a non-volatile solute! :thumbup:
 

Dencology

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yes guys, good point. Thanks to all of you. Just know the above cold. It will come on your dat. I hope it comes on mine. I want to nail this sob.
 

Dentista08

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So, the rxn is exothermic b/c the heat is evolved from the system and its on the right side, i got that. what if the reaction was written with double arrows and it said delta H= +50 KJ on the right side, that would mean its endothermic, even though the heat is written on the right ?
I think im confusing total enthalpy change and +43 KJ thing...
 

sciencegod

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So, the rxn is exothermic b/c the heat is evolved from the system and its on the right side, i got that. what if the reaction was written with double arrows and it said delta H= +50 KJ on the right side, that would mean its endothermic, even though the heat is written on the right ?
I think im confusing total enthalpy change and +43 KJ thing...
i dont know wht ur asking. u aint gonna see no double arrows or nothin.
 
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