Question

(c) Which quantity, ΔH or ΔU, is measured in heat combustion experiment? Why? (Consider experiment condition: under constant V or constant p). How to justify it using the 1st law of thermodynamics?

Answer 1

In heat combustion experiment, or 'change of Enthalpy' is measured as a unit of change in 'heat of reaction' at constant Pressure.The Enthalpy term is preferred over or change in 'Internal Energy'term at 'constant volume' as ,if we are to measure term for a combustion reaction( say combustion of Coal to carbon), then, from First law of Thermodynamics,

q=Heat of reaction

w=work done for combustion

So we have to measure both q and w for measuring of a reaction.

But we can measure Enthalpy of reaction by only measuring q,as at constant pressure-

So why we measure both q and w if we are with our results by only measuring q or Heat of energy.

Hope this helps!