Ethanol is a possible fuel. Use average bond energies to calculate ΔHrxn for the combustion of ethanol. CH3CH2OH(g) + 3 O2(g) → 2 CO2(g) + 3 H2O(g)

Answer 1

You can find the answer to this question online in a data table. The answer I found on Wikipedia is -1370.7 kJ/mol. So you can check the estimate you have calculated against the data table. In my calculation, I got an estimate that differed by just 7%.

How to estimate the ΔH_rxn of ethanol from average bond energies?

Again, you can find a table of average bond energies online in a data table, and you may have a table in your course textbook as well. I will note one thing very important: the average bond energy for C=O (double bond between carbon and oxygen) in CO2 has a significantly different value. The table I used gave 799 kJ/mol. Use this value.

You are going to need to sum the average bond energies of the reactants, and then of the products. When you have the sums, you will subtract the sum of reactants - sum of products, and use this value as the estimate for ΔH_rxn of ethanol.

Another suggestion: double check the number of each type of bond before completing the calculations. For example, on the reactants side of the equation, the oxygens will be double bonded as O=O. The bond energy is therefore significantly different than single bonded O-O. If this causes any confusion, reconsider the Lewis diagram for oxygen: 12 valence, 16 octet, 4 bonding electrons, 8 non-bonding electrons.