What does the change of enthalpy per mole of oxygen in a combustion reaction say about the amount of energy that can be released?

Question

For example, if -445.7 kJ is released per 1 mole of oxygen in the combustion of ethane, what does this say about the reaction?

Answer 1

Hello, Maddie,

I believe we need to find the balanced equation for this reaction to determine what "per mole of oxygen" means in the contect of the overall reaction. Ethane is C₂H₆.

C₂H₆ + O₂ = CO₂ + H₂O

The fact that there are an even number of oxygens on the left but an odd number on the right makes this slightly more difficult. I come up with:

2C₂H₆ + 7O₂ = 4CO₂ + 6H₂O

Please ask if balancing the equation is the part you had difficulty with, and I'll provide a more detained explanation.

We see that one mole of oxygen would only consume 2/7 mole of ethane. Since it is the ethane that is providing the majority of energy, it makes since to express the change of enthalpy per mole of ethane, not oxygen. Multiply the (-445 7 kJ/mole O₂) by (7 moles O₂/2 moles C₂H₆)/ That is -1560 kJ/mole ethane.

That number

1. sounds more impressive, and
2. provides key information on the more important reactant, ethane.

I am only assuming this is the response the question is asking for. I could say we know that -445.7 kJ is released per 1 mole of oxygen, and that would be true, but it was profoundly lacking in novelty.

Bob