Question

Given that ?H^o for the thermochemical equation

2 SO₂(g) + O₂(g) ? 2SO₃(g)

is – 198 kJ, what is the enthalpy change for the decomposition of 1 mole of SO₃ to give O₂ and SO₂, all in the gas state?

Method: Write down the equation of the reaction required (decomposition of SO₃ to give O₂ and SO₂):
SO₃(g) ? SO₂(g) + ½O₂(g)
Compare this equation with the one for which the thermodynamic information is available, and decide what operations are necessary to make them identical.
If you reverse a reaction, you must change the sign of ?H^o.
If you multiply a reaction through by a factor, you must multiply ?H^o by that factor also.

Answer 1

Part a

for the thermochemical equation

2 SO2(g) + O2(g) ? 2SO3(g)

?Ho = - 198 kJ

Reverse the equation

2SO3(g) ?2 SO2(g) + O2(g)

?Ho = - (- 198) kJ = +198 kJ

For 1 mol decomposition of SO3, multiply by 0.5

SO3(g) ? SO2(g) + 0.5O2(g)

?Ho = 0.5 x 198 kJ = 99 kJ

Part b

From the thermodynamic data

Enthalpy change of Reaction

?Ho = sum of Enthalpy of formation of products - sum of Enthalpy of formation of reactants

?Ho = [?Hf(SO2) + 0.5?Hf(O2)] - [?Hf(SO3)]

= [2(-296.83) + 1(0)] - [2(-395.72)]

= 197.78 kJ

= 198 kJ

By comparing, ?Ho is same for the given data and for the thermodynamic data.