ΔrH =qp
n
Experimental determination of an enthalpy of reaction (ΔrH) involves measuring the heat absorbed or released at constant pressure (qp) for a specific amount of reaction (n).

Enthalpy changes depend on the state (The state of a system depends on the temperature, amount of substance, pressure, and for gases, the volume.) of reactants and products, not the pathway between them. Thus calculation of ΔrH for formation of SO3(g) from its elements can be based on either of the two pathways shown.
 
S(s) + 3/2 O2(g) SO3(g)
SO2(g)
Direct pathway:

Indirect pathway:

The enthalpy changes for the direct and indirect pathways are equivalent because the products and reactants of both pathways overall are the same.

(1) S(s) + O2(g)  SO2(g)
ΔH° = Δ1H°
(2) SO2(g) + ½O2(g) SO3(g)
ΔH° = Δ2H°
(3) S(s) + 3/2 O2(g) SO3(g)
Δ3H° =Δ1H° + Δ2H°

As the next set of questions shows, in most cases the reactions in the alternate pathway cannot be simply summed to give the desired reaction.
Dr Sheila Woodgate
© The University of Auckland 2002