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Old Apr 17th 2013, 11:31 AM   #1
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Smile Exothermic and Endothermic help?

I need some help with a question for chemistry. I'm not quite sure how to figure this out. Help is appreciated!

Identify each enthalpy change by name and classify each change as exothermic or endothermic.

1.1 mol NaCl (s) 1 mol NaCl(l)
2.C4H10 O(l) C4H10O (g)
3.H2O(g) H2O (l)
4.H2O(l) H2O (s)

A Molar heat of vaporization; endothermic
B Molar heat of condensation; exothermic
C Molar heat of solidification; exothermic
D Molar heat of fusion; endothermic
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Old Apr 17th 2013, 06:44 PM   #2
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I'm a student so I can't guarantee this is right, but I would assume you should focus on the phase changes.

First you need to know the names for phase changes
solid to liquid > fusion
liquid to gas > vaporization
gas to liquid > condensation
liquid to solid > solidification

and you also need to know that a reaction which releases heat is exothermic, while a reaction that absorbs heat is endothermic.

1. You're going from solid NaCl to liquid NaCl, so this is fusion. To determine if this is exothermic or endothermic, think about the heat required to go from solid H2O to liquid H2O. You need to add heat in order to melt ice, meaning the solid absorbs heat, so it is endothermic.

2. You're going from liquid to gas, so this is vaporization. Again if you think about going from liquid H2O to gas, you would need to add heat, so this is also endothermic.

3. gas to liquid, so condensation. Think of water droplets forming on the outside of a glass of water. For a gas to return to a liquid state it needs to release heat, so this is exothermic.

4. liquid to solid, so solidification. To go from liquid H2O to solid H2O you need to lose heat, so this is exothermic.

Also with noting:
Heat of fusion will always be endothermic.
Heat of vaporization will always be endothermic.
Heat of condensation will always be exothermic.
Heat of solidification will always be exothermic.

Hope that helps. Someone correct me if any of that is wrong.

Last edited by Unknown008; Apr 18th 2013 at 07:37 PM.
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Old Apr 18th 2013, 07:41 PM   #3
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I changed your 'h20' to the proper 'H2O' in your post. Try to type in the molecular formulae correctly! ^^

Otherwise, those are correct. I would have preferred if you tried first to give hints instead of giving the answers, because I'm sure that such things have been mentioned somewhere by the OP's tutor/teacher or in their textbook/manual, and remember that we are not a homework service site, but one that is willing to help others understand first. And what better way to tell if they use a bit of hint to get the right answer?
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chemistry, endothermic, enthalpy change, exothermic

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