Question

Calculate the standard enthalpy of formation of So2

Answer 1

Well, we can always check our answer. To check, it should be (−296.81±0.20) kJ/mol. You should use NIST more often.

I actually got −310.17 kJ/mol though.

Supposedly, the ΔH∘rxn is −171.2 kJ for this reaction as-written:

2SO2(g)+O2(g)⇌2SO3(g)

You have to look up ΔH∘f for SO3(g) first. I got −395.77 kJ/mol from NIST.

Now, ΔH∘rxn can be calculated from tabulated enthalpies of formation.

ΔH∘rxn=∑PnPΔH∘f,P−∑RnRΔH∘f,R

where P stands for products, R for reactants, n for the mols of stuff, and ΔH∘f is the enthalpy of forming 1 mol of the substance from its elements in their elemental states at 25∘C and 1 bar.

We should know that ΔH∘f,O2(g)=0 kJ/mol... no energy is needed to form something by doing nothing to it. We have O2