Question
Show that the change in enthalpy at constant pressure for a reaction involving gases is given by the expression.
Solution
Let us consider a reaction involving gases. Let the process be isothermal and carried out at constant pressure (P). Let Vr be the total volume of the gaseous reactants and Vp be the total volume of the gaseous products, nr be the number of moles of the gaseous reactants and np be the number of moles of the gaseous products.
Using ideal gas equation;
PV = nRT,
we may write,
For reactions:
PVr = nRT ...(1)
(at constant T and P)
For products: PVp = npRT ...(2)
(at constant T and P)
Subtracting (1) from (2),
P(Vp - Vr) = npRT - nrRT
= (np - nr)RT
where ∆ng is the difference in the number of moles of gaseous products and reactants.
Using ideal gas equation;
PV = nRT,
we may write,
For reactions:
PVr = nRT ...(1)
(at constant T and P)
For products: PVp = npRT ...(2)
(at constant T and P)
Subtracting (1) from (2),
P(Vp - Vr) = npRT - nrRT
= (np - nr)RT
where ∆ng is the difference in the number of moles of gaseous products and reactants.
