Enthelpy confusion (not a homework problem)

[Intrusionv2]

Hello, I have a question regarding enthalpy.

I know that when calculating ΔS(o), you just do all of the products minus all of the reactants.

However, when calculating ΔHf(o), I am confused as to what you include in the products and reactants? My book is sometimes disregarding some of the products/reactants, example below.

N2(g) + 2O2(g) --> 2NO2(g)
ΔH(o) = 2ΔHf(NO2)
ΔH(o) = 2mol*33.2kj/mol = 66.4 kJ

Why wouldn't it be ΔH(o) = 2ΔHf(NO2) - [ΔHf(N2) + 2ΔHf(O2)] ?

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Another example:
2KClO3 (s) --> 2KCl(s) + 3O2(g)
ΔH = 2ΔHf(KCl) - 2ΔHf(KClO3)

Why not ΔH = [2ΔHf(KCl) + 3ΔHf(O2)] - 2ΔHf(KClO3) ?
For ΔS everything IS included, so that's why I am confused.

Thanks!

 

[alxm]

Well, you have to measure the heat of formation relative some reference; it's the enthalpy of that molecule relative some other molecule.
The usual point of reference is the pure compounds (aka the standard state). So for e.g. hydrogen and oxygen it's the H2 and O2 molecules at STP,
in other cases it's not immediately obvious - e.g. for carbon it's graphite and not diamond.

So your heats of formation for O2 and N2 etc are included here, but they're just zero by convention. The other values you're using are relative them.

Another reference point that's sometimes used is the free, neutral atoms, in which case you talk about the heat of atomization.
But that's not a very practical reference point given that it's not very easy to measure.

 

[Intrusionv2]

Well, you have to measure the heat of formation relative some reference; it's the enthalpy of that molecule relative some other molecule.
The usual point of reference is the pure compounds (aka the standard state). So for e.g. hydrogen and oxygen it's the H2 and O2 molecules at STP,
in other cases it's not immediately obvious - e.g. for carbon it's graphite and not diamond.

So your heats of formation for O2 and N2 etc are included here, but they're just zero by convention. The other values you're using are relative them.

Another reference point that's sometimes used is the free, neutral atoms, in which case you talk about the heat of atomization.
But that's not a very practical reference point given that it's not very easy to measure.

Got it, thanks!