Textbook 4.29
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Textbook 4.29
Hi, I'm confused how the equation ΔHrxn=[2ΔHS + 2ΔHH2O] - [2ΔHH2S + ΔHO2] got reduced to ΔHrxn=2ΔHH2O - 2ΔHH2S. Why did 2ΔHS and ΔHO2 get reduced to 0 and what's the general rule for that?
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Re: Textbook 4.29
The standard enthalpy of formation for elements in their most stable form is 0, so that's why S and O2 were removed.
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Re: Textbook 4.29
This is because the standard enthalpy of formation of 1 mole of a substance is defined as the energy required/released to form the substance from its elements in their most stable form. O2, for example, has a standard enthalpy of formation of 0 because to form O2 from its most stable form (O2) requires/releases 0 kJ of energy since it is already is in its most stable form.
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Re: Textbook 4.29
Thanks for your responses! Is there a way to know the most stable form of an element? Or is it just something we have to memorize?
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Re: Textbook 4.29
It seems like something that is memorized, though I would imagine unusual cases (i.e. not the common nonmetals like hydrogen, oxygen, carbon, etc.) would be stated in the problem.
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