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8.99 Enthalpies of Formation

Posted: Sun Jan 21, 2018 12:34 pm
by MCracchiolo 1C
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?

Re: 8.99 Enthalpies of Formation

Posted: Mon Jan 22, 2018 9:38 am
by Curtis Tam 1J
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.

Re: 8.99 Enthalpies of Formation

Posted: Mon Jan 22, 2018 9:51 am
by Esin Gumustekin 2J
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?

Re: 8.99 Enthalpies of Formation  [ENDORSED]

Posted: Mon Jan 22, 2018 9:58 am
by Chem_Mod
ZnCl2 and halogens are always -1.
Therefore Zn must be 2+.