Homework 87

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Jade Fosburgh Discussion 2C
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Joined: Fri Sep 29, 2017 7:07 am

Homework 87

Postby Jade Fosburgh Discussion 2C » Fri Jan 19, 2018 12:31 pm

I was able to follow the math for the 5 steps all the way through the problem, I just had a question about the final signs of the delta H values you add at the end. Why are they all positive?

Are phase changes and heat capacity enthalpies always positive?

Hubert Tang-1H
Posts: 44
Joined: Thu Jul 27, 2017 3:00 am

Re: Homework 87

Postby Hubert Tang-1H » Fri Jan 19, 2018 12:40 pm

I think that phase changes are only positive if the process is endothermic (melting and vaporization). The reason they are all positive is that the problem asks to convert ice into gas, which requires that the ice absorb heat, making the enthalpy positive.

Cristina Sarmiento 1E
Posts: 52
Joined: Wed Nov 16, 2016 3:02 am

Re: Homework 87

Postby Cristina Sarmiento 1E » Fri Jan 19, 2018 2:32 pm

When a reaction is endothermic, delta H is positive. Reactions that are endothermic include vaporization, melting/fusion,and sublimation because heat is being absorbed. When a reaction is exothermic, delta H is negative. A reaction that exothermic is combustion because heat is released.

Return to “Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)”

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