Simple Enthalpy Question

[Neplina94]

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Hey Guys, please help me with this one

Given:

1 CH4(g) + 2 O2(g) --> 1 CO2(g) + 2 H2O(g)

And getting the formation eqns for each, and flipping each

CH4----> C + 2 H2

C + O2 --> CO2

2H2 + O2 --> H2O

The final eqn becomes:
CH4 + 2O2 --> CO2 + 2 H2O

Yet the standard enthalpy of combustion according to TBR is (Prod-reactants)
2ΔH H2O + ΔH CO2 - ΔH CH4

WHY isn't oxygen included in the final eqn if it doesn't cancel out on opposite sides of the eqn?

 

[Quinoline]

The standard enthalpy of formation of O2(g) is 0.
These enthalpies are the change in enthalpy from the compound, to the elements in the compound in their standard state. Like CH4 is composed of the standard C(s) and H2(g). O2(g) is already oxygen's standard state.

 

[Neplina94]

The standard enthalpy of formation of O2(g) is 0.
These enthalpies are the change in enthalpy from the compound, to the elements in the compound in their standard state. Like CH4 is composed of the standard C(s) and H2(g). O2(g) is already oxygen's standard state.

Makes perfect sense! Thank you very much