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Postby Meachelle_Lum_1I » Mon Feb 04, 2019 2:05 pm

When finding deltaH of the rxn, when are our units kJ and when are the units kJ/mol? I thought Hf is kJ/mol, but in some cases Hrxn overall is kJ/mol as well?

Kayla Vo 1B
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Re: Units

Postby Kayla Vo 1B » Mon Feb 04, 2019 2:27 pm

I believe by definition that the standard enthalpy of formation (Hf) represents the standard reaction's enthalpy per mole for the formation of a substance from its elements in their most stable form. Hf therefore is represented in units of kJ/mol. In addition, bond dissociation energies (aka bond enthalpies) are the amount of energy required to break one mole of a bond and therefore are also represented as kJ/mol.

Since we know that enthalpy (delta H) equals to qp (heat, constant pressure), delta H can represented as kJ. However, if you are dealing with molar enthalpy, delta H is in kJ/mol, and if you are looking at the standard enthalpy of reaction, the delta H rxn is in kJ/mol as well. The units therefore basically depend on what the problem is asking you to solve.

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Re: Units

Postby BenJohnson1H » Tue Feb 05, 2019 10:05 pm

Sometimes one is needed to find the other, it depends on what units the answer must be in and sometimes conversions must be made accordingly

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