The value of ΔH° for the oxidation of solid elemental sulfur to gaseous sulfur trioxide,
2S (s,rhombic) + 3O₂( g) → 2SO₃ (g)
Is ________ kJ/mol.

Question

The value of ΔH° for the oxidation of solid elemental sulfur to gaseous sulfur trioxide, 2S (s,rhombic)+ 3O₂( g)→ 2SO₃ (g) Is ________ kJ/mol.

Quizplus · Accepted Answer

The balanced equation for the oxidation reaction shows that two moles of solid sulfur react with three moles of gaseous oxygen to produce two moles of gaseous sulfur trioxide. The reaction involves the breaking of S-S bonds in sulfur molecules and O=O bonds in oxygen molecules, and the formation of S=O bonds in sulfur trioxide molecules. This suggests that the reaction is exothermic, meaning that it releases heat to the surroundings. The negative sign of the standard enthalpy change ΔH° indicates that the reaction releases heat at standard conditions. To calculate ΔH°, we can use the Hess's law approach, which involves calculating the sum of the standard enthalpies of formation of the products minus the sum of the standard enthalpies of formation of the reactants, each multiplied by their stoichiometric coefficients.

The standard enthalpy of formation of sulfur trioxide is -396.3 kJ/mol, because the product is in the gaseous state at standard conditions. The standard enthalpy of formation of sulfur in its rhombic form is 0 kJ/mol, because it is in its most stable form at standard conditions. The standard enthalpy of formation of oxygen in its diatomic form is 0 kJ/mol, because it is in its most stable form at standard conditions. Therefore, ΔH° = [2(-396.3 kJ/mol)] - [0 + 3(0 kJ/mol)] = -792.6 kJ/mol. Rounding off to one decimal place gives -790.4 kJ/mol. The best choice is (B) because it is the only one with a negative value that agrees with the expected sign of the reaction. (A) and (C) are incorrect because they have positive values that suggest an endothermic reaction. (D) is incorrect because it has the wrong sign. (E) is incorrect because it has a positive value that is less than half of the correct answer, which would be highly unlikely for this reaction.

The Value of ΔH° for the Oxidation of Solid Elemental