IB Chemistry HL Reactivity 1: What drives chemical reactions? questions

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Question 1

Paper 2Hard12 marks

Calcium oxide is produced by heating limestone, which contains calcium carbonate. The standard Gibbs energy change, ΔrG∘\Delta_\mathrm r G^\circ for the decomposition is shown in Figure 1. Assume that ΔrH∘\Delta_\mathrm r H^\circ and ΔrS∘\Delta_\mathrm r S^\circ remain constant over the temperature range shown.

7008009001000110012001300-40-30-20-10010203040506070T / KΔr G° / kJ mol-1
(a)

Write a balanced equation, including state symbols, for the thermal decomposition of calcium carbonate.

[1]
(b)

Use Figure 1 to estimate ΔrG∘\Delta_\mathrm r G^\circ at 1000 K1000\ \mathrm K

[1]
(c)

Use Figure 1 to determine ΔrH∘\Delta_\mathrm r H^\circ and ΔrS∘\Delta_\mathrm r S^\circ for the decomposition. Give signs and units, and show how you obtain both values.

[3]
(d)

Using your values from (c), determine the temperature at which ΔrG∘=0\Delta_\mathrm r G^\circ=0 State at which temperatures the forward decomposition is spontaneous under standard-state conditions.

[2]
(e)

At 1000 K1000\ \mathrm K a gas stream keeps the carbon dioxide partial pressure in the kiln at 0.10 bar0.10\ \mathrm{bar} For the reaction as written, take

Q=p(CO2)p∘,p∘=1.00 bar,Q=\frac{p(\mathrm{CO_2})}{p^\circ},\qquad p^\circ=1.00\ \mathrm{bar},

with pure solids omitted from QQ Under these conditions,

ΔrG=ΔrG∘+RTln⁡Q,\Delta_\mathrm r G=\Delta_\mathrm r G^\circ+RT\ln Q,

where R=8.31 J K−1 mol−1R=8.31\ \mathrm{J\,K^{-1}\,mol^{-1}}

Determine the carbon dioxide partial pressure at which ΔrG=0\Delta_\mathrm r G=0 at 1000 K1000\ \mathrm K Hence decide whether the forward decomposition is spontaneous at p(CO2)=0.10 barp(\mathrm{CO_2})=0.10\ \mathrm{bar} and assess the claim that a positive ΔrG∘\Delta_\mathrm r G^\circ at 1000 K1000\ \mathrm K means that the decomposition cannot be spontaneous.

[5]

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