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Joker88

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Lecture 8: Thermodynamics, Passage 9, Question: 62

What reaction, assuming all reactions are favorable, is most likely to result in a decrease in temperature?

A. Oxidation of a metal by oxygen
B. Combustion of a hydrocarbon
C. Precipitation of a Solute from Water
D. Dissolving a solute into a aqueous solution

I can narrow it down to C/D. I know that a decrease in temperature means that the reaction is endothermic. I think i might just be confusing myself but if someone could help me out id be grateful!
 
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MT Headed

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dG = dH - TdS
dS = (dH - dG)/T

This process is spontaneous, so dG is negative.
The process is endothermic, so dH is positive.
This process occurs at a temp above absolute zero, therefore the right side of the equation is positive.
Therefore, look for a reaction where dS is positive, i.e. a reaction that creates disorder from order.

You can eliminate C.
 
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MShopes

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Lecture 8: Thermodynamics, Passage 9, Question: 62

What reaction, assuming all reactions are favorable, is most likely to result in a decrease in temperature?

A. Oxidation of a metal by oxygen
B. Combustion of a hydrocarbon
C. Precipitation of a Solute from Water
D. Dissolving a solute into a aqueous solution

I can narrow it down to C/D. I know that a decrease in temperature means that the reaction is endothermic. I think i might just be confusing myself but if someone could help me out id be grateful!

Decrease in temperature is endothermic? depeneds decrease in temperature in what? It would lead to decrease in temperature in the sourrounding environment but an increase in temperature in the system itself since heat is taken in. Increase in the system's temperature (endothermic) is usually a change from solid to liquid to gas (just like if you try to melt ice, you would expose it to heat to change its state to liquid). I'm assuming the question asks which reaction have a decrease in temperature in itself not in the surrounding. In this case it would be any change from gas to liquid to solid (exothermic) so it would be C (precipitation is a change of liquid to solid).

I think C would be the right answer!
 

Joker88

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Im still a little confused. I thought that if dH was positive and ds was positive then at high temperatures the dG would be negative. Maybe i'm just missing something...

btw the answer is D
 

apex954

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Im still a little confused. I thought that if dH was positive and ds was positive then at high temperatures the dG would be negative. Maybe i'm just missing something...

btw the answer is D

yeah u're rite! that's y the question says '...assuming all reactions are favorable...' A favorable rxn has a negative dG. A decrease in temp, which is referring to the temp of the surroundings means dH is potive because d rxn is endothermic and the fact that a solute dissolving in an aqueous solution has a positive dS points to choice D as d answer...check 'MT Headed' explanation above, he/she nailed it as to why u can eliminate C
 

MShopes

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Im still a little confused. I thought that if dH was positive and ds was positive then at high temperatures the dG would be negative. Maybe i'm just missing something...

btw the answer is D

Like I said, it can be D if the question asks which reaction has a decrease in temperature in the surrounding. In this case, dissolving (change from solid to liquid) would require heat in the system and therefore less heat from surrounding (decrease in temperature).
 

salim271

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I get D just by process of elimination... for C, I can think back to more than one instance where precipitating a solid has increased the temp of the solution. If you can prove it wrong, you dont have to prove that the other one is right. For proof of D, i cant remember NaCl dissolving into water ever making it hotter... lol.
 

whiteshadodw

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I get D just by process of elimination... for C, I can think back to more than one instance where precipitating a solid has increased the temp of the solution. If you can prove it wrong, you dont have to prove that the other one is right. For proof of D, i cant remember NaCl dissolving into water ever making it hotter... lol.

page 90 in TBR Gen Chem book 2: "This is because the dissociation of salt into water is exothermic and the energy released causes localized pockets of boiling water"

Also, I don't know if you've ever diluted concentrated HCl, but it always heats up, that's why you can't just dump HCl into water, gotta dilute drop by drop.
 

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