Reaction for Combustion of Hydrogen Gas

[Nathan Min 1I]

One question asked me to write the reaction for the combustion of hydrogen gas, H2.

I wrote this balanced equation: 2H2 + O2 -> 2H2O, and used this in Hess's Law.

However, the correct answer used the equation H2 + 1/2(O2) -> H2O. Thus, because I used an equation with different coefficients, I got the problem wrong.

In the future, how I am supposed to know which version to use? I thought we were to eliminate any fractional coefficients? If not, how do I know to use? Is it due to the issue of moles?

Thanks!

[David Nguyen 1K]

The enthalpy of a reaction is given by the amount of energy (J, kJ) released or absorbed per mole. While you can have 1/2 of a mole of some molecule, you cannot have 1/2 of a molecule, so fractional coefficients are allowed in Hess's Law. I believe the reason why the textbook wants the enthalpy calculation from that chemical equation because it prefers simpler coefficients. In 2H2 + O2 -> 2H2O, you have coefficients (other than 1) before 2 reactants and products, but with H2 + 1/2(O2) -> H2O, we have only 1.