Given that ?H o for the thermochemical equation 2 SO 2 (g) + O 2 (g) ? 2SO 3 (g)

Question

Given that ?Ho 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 ?Ho.
If you multiply a reaction through by a factor, you must multiply ?Ho by that factor also.

Explanation / Answer

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.

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