A-LevelChemistryChemical energetics9701/42 · Oct/Nov 2019 · Q46 Marks
The table shows some standard entropy data. substance | standard entropy, S° /J K⁻¹ mol⁻¹ PbO₂(s) | 77 PbO(s) | 69 O₂(g) | 205 Lead(IV) oxide, PbO₂, decomposes to lead(II) oxide, PbO, and oxygen when heated. 2PbO2(s) → 2PbO(s) + O2(g) ΔH° = +118 kJ mol-1
- (a)Use the data to calculate the value of ∆S° for this reaction. ∆S° =[2]
- (b)Use the value of ∆H° and your answer to (a) to calculate the temperature at which this reaction becomes feasible. T =[3]
- (c)Solid lead(II) oxide can be made by heating lead metal in air. Predict the sign of the standard entropy change of this reaction. Explain your answer.[1]
Mark scheme
Marking points from the official mark scheme (Oct/Nov 2019). The mark or mark type is shown in brackets.
- [2] M1: correct use of stoichiometry
- [2] M2: answer + 189
- [3] M1: States or uses correct form of Gibbs equation AG = ΔΗ – TAS
- [3] M2: appreciates / includes AG = 0 at temperature required
- [3] M3: uses 1000 correctly and answer +624(.339)
- [3] Award 3 marks for correct answer
- [1] negative and decrease in number / amount of gas molecules
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