MediumChemistryHigh School (11–12)Chemical Thermodynamics › Entropy, Gibbs Energy and SpontaneityNumerical

Sulphur trioxide is made by 2SO₂(g) + O₂(g) → 2SO₃(g).

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Sulphur trioxide is made by $2\text{SO}_2(g) + \text{O}_2(g) \rightarrow 2\text{SO}_3(g)$ . The standard molar entropies are $248.2\ \text{J/(K mol)}$ for $\text{SO}_2$ , $205.1\ \text{J/(K mol)}$ for $\text{O}_2$ and $256.8\ \text{J/(K mol)}$ for $\text{SO}_3$ . Find $\Delta_r S^\circ$ . Does its sign match the change in moles of gas?

Need a reminder? The key ideaShow

A spontaneous change is one that happens on its own. Entropy measures how spread out matter and energy are; gases have far more than liquids, and liquids more than solids. $\Delta S = q_{rev}/T$ , so for melting or boiling $\Delta S = \Delta H/T$ . The second law: $\Delta S_{total} = \Delta S_{sys} + \Delta S_{surr} > 0$ for a spontaneous change, with $\Delta S_{surr} = -\Delta H/T$ . $\Delta_r S^\circ = \sum S^\circ(\text{products}) - \sum S^\circ(\text{reactants})$ .

The worked solution

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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.

← All Problems

← Predict the sign of Δ S for each change, and give the reason.Supercooled water freezes at 263 K. Take Δ H = -6.01 kJ/mol and Δ Ssystem = -22.0 J/(K mol). →

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Sulphur trioxide is made by 2SO₂(g) + O₂(g) → 2SO₃(g).

The worked solution

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