So I am talking about complicated compounds and not simple reactions where you subtract heats of formation. I was trying to find the heat of reaction for an aryl substituted methyl aryl sulfide being oxidised to sulfone group. Now, I calculated BDE for S=O bond and subtracted O-O energy to find heat of reaction for oxidation to sulfoxide and then repeated the step for going to sulfone. To calculate that heat experimentally, I insulated my reaction vessel and bought it to reflux temperature of the solvent. I then added hydrogen peroxide and collected the evaporated solvent and measured its volume. I used its heat of vaporization as a proxy to calculate the heat of reaction and it came out to only 20% of the Heat of formation calculated from Hess's law
I would love to know where I went wrong and if my knowledge of Hess's law is wrong.