2 SO2 (g) + 1 O2 (g) → 2 SO3 (g)

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SO2            Mass: g  or Gas Volume: L
O2             Mass: g  or Gas Volume: L
SO3            Mass: g  or Gas Volume: L
Heat Released: kJ

Enthalpy of Reaction

[2ΔHf(SO3 (g))] - [2ΔHf(SO2 (g)) + 1ΔHf(O2 (g))]
[2(-395.72)] - [2(-296.83) + 1(0)] = -197.78 kJ
-197.78 kJ     (exothermic)

Entropy Change

[2ΔSf(SO3 (g))] - [2ΔSf(SO2 (g)) + 1ΔSf(O2 (g))]
[2(256.65)] - [2(248.11) + 1(205.03)] = -187.95 J/K
-187.95 J/K     (decrease in entropy)

Free Energy of Reaction (at 298.15 K)

From ΔGf° values:
[2ΔGf(SO3 (g))] - [2ΔGf(SO2 (g)) + 1ΔGf(O2 (g))]
[2(-371.08)] - [2(-300.19) + 1(0)] = -141.78 kJ
-141.78 kJ     (spontaneous)

From ΔG = ΔH - TΔS:
-141.74 kJ     (spontaneous)

Equilibrium Constant, K (at 298.15 K)

This process is favorable at 25°C.


Ebbing, Darrell D. General Chemistry 3rd ed.; Houghton Mifflin Company: Boston, MA, 1990; p 217.
Silberberg, Martin S. Chemistry: The molecular Nature of Matter and Change 4th ed.; McGraw-Hill: Boston, MA, 2006; p 127.
Atkins, Jones, and Laverman. Chemical Principles 6th ed.; W.H. Freeman and Company: New York, NY, 2013; p 428.

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