Textbook 4.29

[Tianna Nguyen 1I]

Hi, I'm confused how the equation ΔH_rxn=[2ΔH_S + 2ΔH_H2O] - [2ΔH_H2S + ΔH_O2] got reduced to ΔH_rxn=2ΔH_H2O - 2ΔH_H2S. Why did 2ΔH_S and ΔH_O2 get reduced to 0 and what's the general rule for that?

[Lauren Woodward 1I]

The standard enthalpy of formation for elements in their most stable form is 0, so that's why S and O2 were removed.

[Edwin Montalvo 1G]

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.

[Tianna Nguyen 1I]

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?

[Ethan Hung 2A]

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.