Question 1

Paper 2Medium7 marks

Calcium carbonate decomposes on heating according to the reaction

\ceCaCO3(s)−>CaO(s)+CO2(g).\ce{CaCO3(s) -> CaO(s) + CO2(g)}.

The standard entropy values and standard enthalpy change for this reaction are given below. Figure 1 shows how ΔrG∘\Delta_\mathrm r G^\circ varies with temperature.

Substance or quantityValue
S∘(\ceCaCO3(s))S^\circ(\ce{CaCO3(s)})92.9 J K−1 mol−192.9\ \mathrm{J\,K^{-1}\,mol^{-1}}
S∘(\ceCaO(s))S^\circ(\ce{CaO(s)})39.8 J K−1 mol−139.8\ \mathrm{J\,K^{-1}\,mol^{-1}}
S∘(\ceCO2(g))S^\circ(\ce{CO2(g)})213.7 J K−1 mol−1213.7\ \mathrm{J\,K^{-1}\,mol^{-1}}
ΔrH∘\Delta_\mathrm r H^\circ+178.3 kJ mol−1+178.3\ \mathrm{kJ\,mol^{-1}}
8009001000110012001300-40-30-20-100102030405060Temperature, T / KΔr G^° / kJ mol-1
(a)

Calculate ΔrS∘\Delta_\mathrm r S^\circ for the reaction. Include the sign and units.

[2]
(b)

Using Figure 1, determine the approximate temperature at which ΔrG∘\Delta_\mathrm r G^\circ changes sign. State whether the forward reaction is thermodynamically favoured under standard-state conditions at 1000 K1000\ \mathrm K

[2]
(c)

Using your answer to (a), calculate ΔrG∘\Delta_\mathrm r G^\circ at 1200 K1200\ \mathrm K State whether the forward reaction is thermodynamically favoured under standard-state conditions at this temperature.

[3]

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