gchem question from Top Score

[Isradoc1983]

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Hi I was wondering if someone can explain this easy concept to me from TS cause apparently I don't get in
The question was
Which one of the following reactions or processes isn't exothermic
a) Steam condensing
b)Natural gas burning
C)Water freezing
d)ice melting
3) molten lava cooling down

The answer is D ice melting. I don't really understand why can't it be C- water freezing, maybe I don't understand exo vs endo as well as i thought. Please help
Thanks

 

[JBarr29]

ok...so exothermic means the reaction gives off heat into the surroundings, where endothermic means heat is taken in.
A) Condensation gas-->liquid. Exothermic because we are releasing heat, going from a higher phase lower one.
B) Natural Gas burning--> releasing heat obviously
C) Water freezing --> also exothermic. Going from liquid to solid. Down the phase diagram. Releasing heat, because you do not want to absorb heat, If it absorbs heat it will go from liquid to gas.
D) Ice Melting --> Going UPPPP the phase diagram (see any patterns here?lol) In order for ice to melt to liquid, we must add heat ---common sense. The heat is ABSORBED by the ice, so it is endothermic.
E) Molten lava cooling--> releasing heat...exo

The way i remember it, which may be helpful for you is a block of ice (H20)
Solid -->liquid must absorb heat to melt
liquid-->gas even more heat to speed up the particle movement and have it evaporate is absorbed. so absorbing the heat is endothermic.

gas-->liquid..particles slow down must release heat
liquid-->solid must take even more heat out so it "freezes"

hope i didnt confuse you. post back if you have supplementary questions and if you understand. gl

 

[Isradoc1983]

thanks! it is very simple! the phases thing makes sense!
Thanks Again!

 

[JBarr29]

no problem. all this stuff is easier when you make it practical...thats what i try to do and i think it helps immensely. i have a ton of those stupid things ha

 

[supraman]

think about it man, you need heat to melt ice llol