Question

A friend is calculating the energy of reaction for a combustion reaction using bond energies, and the calculations are returning a positive ΔH. What, if anything, has your friend done incorrectly?

A) Bond energies don't apply to combustion reactions and should not be used.
B) He or she used the wrong table of bond energies.
C) Nothing, a combustion reaction should have a positive ΔH.
D) He or she may have subtracted the bond energies for the reactants from the products.

Answer 1

If the calculations for a combustion reaction using bond energies return a positive ΔH, it means that the bonds in the products are weaker than the bonds in the reactants. The friend may have subtracted the bond energies for the reactants from the products instead of the other way around.

Step-by-step explanation:

To calculate the energy of reaction for a combustion reaction using bond energies, it is important to remember that an exothermic reaction (ΔH negative, heat produced) occurs when the bonds in the products are stronger than the bonds in the reactants. On the other hand, an endothermic reaction (ΔH positive, heat absorbed) occurs when the bonds in the products are weaker than those in the reactants.

Therefore, if your friend's calculations are returning a positive ΔH for the combustion reaction, it means that the bonds in the products are weaker than the bonds in the reactants.

It is possible that your friend may have made an error by subtracting the bond energies for the reactants from the products, instead of the other way around (subtracting the bond energies for the products from the reactants).