8.99 Enthalpies of Formation  [ENDORSED]

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MCracchiolo 1C
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Joined: Sat Jul 22, 2017 3:00 am

8.99 Enthalpies of Formation

Postby MCracchiolo 1C » Sun Jan 21, 2018 12:34 pm

In the solution manual for 8.99, the calculated reaction enthalpy is found by the equation

Delta H (RXN) = -153.89 +2(-167.16)-2(-167.16)-0 = -153.89 kJ/mol

...but the balanced chemical equation is 2HCl(aq)+Zn(s) --> H2(g)+ZnCl2(aq)

Shouldn't the standard enthalpy of formation for H2 gas be 0? And why is the enthalpy of formation for Zn2+ used for ZnCl2?

Curtis Tam 1J
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Re: 8.99 Enthalpies of Formation

Postby Curtis Tam 1J » Mon Jan 22, 2018 9:38 am

I believe the solution manual says H2 is zero right? The enthalpy of formation for Zn2+ is used because ZnCl2 dissociates into Zn2+ and Cl- ions.

Esin Gumustekin 2J
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Joined: Thu Jul 27, 2017 3:01 am

Re: 8.99 Enthalpies of Formation

Postby Esin Gumustekin 2J » Mon Jan 22, 2018 9:51 am

For this question, how do we know that Zn is 2+. It is not stated anywhere in the problem so how could we determine this?

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Re: 8.99 Enthalpies of Formation  [ENDORSED]

Postby Chem_Mod » Mon Jan 22, 2018 9:58 am

ZnCl2 and halogens are always -1.
Therefore Zn must be 2+.

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