TBR Chemistry Chapter 8 Thermo Question 43

[StretchDoe]

The question states which of the following is the free energy change of combustion for ethanol under standard conditions? A chart is given in the passage with the different values for free energy

I get this is a simple implementation of Hess' Law. But my question is don't we need to write out a balanced equation first and the calculate the free energy value. The answer key when making the balanced equation disregards O2. It simply writes ethanol --> 2H2O + 2CO2

Why didn't they put the O2 in the reactants side? This would change the coefficients in the balanced equation thus changing the free energy value.

Thanks

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[aldol16]

I get this is a simple implementation of Hess' Law. But my question is don't we need to write out a balanced equation first and the calculate the free energy value. The answer key when making the balanced equation disregards O2. It simply writes ethanol --> 2H2O + 2CO2

Looks like their equation is simply wrong. EtOH --> 2H2O + 2CO2 doesn't balance. Mass balance of the carbons is fine but not the oxygens. You have four O atoms on the right side and only one on the left side.

You need oxygen, since combustion is, by defintion, oxidation of substrates. So the carbons in ethanol are in relatively reduced forms. The carbon in CO2 is in the fully-oxidized form. Therefore, there needs to be an oxidizing agent on the left side of the equation (unless it's a disproportionation, which this is not).