Derive the relationship between `Delta H and Delta U `.

Explanation

Consider a reaction occuring at constant pressure P.
`A to B`
Let `H_(A)` be the enthalpy of reactant and `H_(B)` be the enthalpy of product. So internal energy and volume of product respectively. Therefore,
`H_(A) = U_(A) + pV_(A) and H_(B) = U_(B) + pV_(B)`
By putting these values in equation (ii), we get
`Delta H = (U_(B) + pV_(B)) - (U_(A) + pV_(A))`
or `Delta H = (U_(B) - U_(A)) + (pV-(B) - pV_(A))`
`Delta H = Delta U + p (V_(B) - V_(A))`
`Delta H = Delta U + p Delta V1`
Equation (iii) is required relationship between `Delta H and Delta U`.
For exothermic reactions, `Delta H` is negative.
For endothermic reactions, `Delta H` is positive.