Enthalpy of Formation Reaction

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Semi Yoon
Posts: 60
Joined: Fri Sep 28, 2018 12:27 am

Enthalpy of Formation Reaction

Postby Semi Yoon » Sun Feb 10, 2019 5:07 pm

In the equation: 2Al (s) + 6HCl (g) ---> 2AlCl3 (s) + 3H2 (g), the enthalpy of formation for AlCl3 is given as -704kJ/mol. The enthalpy of the reaction is given as well as -856 kJ/mol.

To find the enthalpy of formation of gaseous hydrochloric acid, I know I would have to do the sum of the enthalpy of formation of products minus the enthalpy of reactants. As part of the answer, it says that for H2 and Al, the enthalpy of formation is zero. Why is it zero?

aisteles1G
Posts: 117
Joined: Fri Sep 28, 2018 12:15 am

Re: Enthalpy of Formation Reaction

Postby aisteles1G » Sun Feb 10, 2019 5:17 pm

The enthalpy of formation of all elements in their most stable states will always be zero bc when you think about it Al-->Al will have a(delta)Hf of 0 per mole since it is a pure element. H2 is also in it's most stable state even though it is not the single H, O2 would also be in the most stable state and 0


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