Calculating enthalpy

Question

The ΔH for the oxidation of S in the gas phase to SO3 is -204 kJ/mol and for the oxidation of SO2 to SO3 is 89.5 kJ/mol. Find the enthalpy of formation of SO2 under these conditions.

Answer 1

Hess's Law

Given:

(1). S(g) + 3/2 O₂(g) ==> SO₃(g) ... ∆H = -204 kJ/mol

(2). SO₂(g) + 1/2 O₂(g) ==> SO₃(g) ... ∆H = 89.5 kJ/mol

Asked for:

S(g) + O₂(g) ==> SO₂(g) ... ∆H = ?

Procedure:

reverse (2): SO₃(g) ==> SO₂(g) + 1/2 O₂(g) ... ∆H = -89.5 kJ/mol

copy (1): S(g) + 3/2 O₂(g) ==> SO₃(g) ... ∆H = -204 kJ/mol

Add the two equations:

SO₃(g) + S(g) + 3/2 O₂(g) ==> SO₂(g) + 1/2 O₂(g) + SO₃(g)

Combine and/or cancel like terms to end up with...

S(g) + O₂(g) ==> SO₂(g)

∆H = -89.5 kJ/mol + (-204 kJ/mol)

∆H = -293.5 kJ/mol