Physical Chemistry
9 marks · HardThis question is about bond energies and equilibrium in a sealed container.
The reaction below shows the thermal decomposition of calcium carbonate, which is carried out in a sealed container:
Explain what is meant by a 'sealed container' in the context of this reversible reaction. [2]
(i) State whether the decomposition of calcium carbonate is an exothermic or endothermic process. Give a reason for your answer, including a statement about bond making or breaking. [2]
(ii) The following bond energies can be used to estimate the enthalpy change (ΔH) for the reaction:
| Bond | Bond energy / kJ mol⁻¹ |
|---|---|
| C=O (in CO2) | 805 |
| C–O (in CaCO3) | 745 |
| O–Ca (in CaCO3) | 350 |
The reaction is: Assume that 1 mole of calcium carbonate decomposes. Use the bond energies above to estimate the enthalpy change (ΔH) for the decomposition of calcium carbonate. Show your working. [3]
State and explain what would happen to the position of equilibrium if some carbon dioxide is removed from the sealed container once equilibrium is established. [2]
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) A sealed container means no substances (solids, liquids or gases) can enter or leave; therefore, both the forward and reverse reactions can occur and no gas escapes.
(b) Endothermic. The decomposition absorbs heat because bonds in CaCO3 must be broken, which requires energy.
(b) ΔH = Energy to break bonds in CaCO3 − Energy to form bonds in CaO and CO2 = (3 × 745 + 1 × 350) − (2 × 350 + 2 × 805) = [3 × 745=2235, 2235+350=2585] − [700+1610=2310]; ΔH = 2585 − 2310 = +275 kJ/mol (accept close answers depending on interpretation of number of bonds, method marks for clearly stated process) kJ/mol
(c) The position of equilibrium will shift to produce more carbon dioxide (to the right), in order to replace the CO2 that was removed, according to Le Chatelier's principle.
Common mistakes: mixing up bond making and breaking; incorrect calculation of enthalpy change; misunderstanding equilibrium shift
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Defines sealed container as one where no substances can enter or leave.
B2 (a) · 1 mark · States that both forward and reverse reactions can occur and gas cannot escape (both points needed for 2 marks).
M1 (b) · 1 mark · States the reaction is endothermic.
M2 (b) · 1 mark · Links endothermic nature to bond breaking requiring energy (or explains bond making/breaking correctly).
M3 (b) · 1 mark · Uses bond energies appropriately: calculates energy required to break and form bonds; shows clear working. (follow-through)
A1 (b) · 1 mark · Obtains correct value (close to +275 kJ/mol) with correct sign and correct process. (follow-through)
M4 (b) · 1 mark · States calculation as 'energy needed to break bonds minus energy released when new bonds formed' or equivalent.
B3 (c) · 1 mark · States position of equilibrium shifts to the right (towards products/more CO₂).
B4 (c) · 1 mark · Explanation: system shifts to replace CO₂ removed/from Le Chatelier's principle.
Worked solution
a) In a sealed container, no reactants or products can enter or leave the system. This means the carbon dioxide gas produced in the decomposition is trapped and cannot escape. Because the system is closed, both the forward reaction (decomposition) and the reverse reaction (recombination) can take place until equilibrium is reached.
b)(i) The decomposition of calcium carbonate is endothermic. In this reaction, energy is absorbed to break the bonds in the solid calcium carbonate to form calcium oxide and carbon dioxide. Bond breaking requires energy input (endothermic), while bond making releases energy (exothermic). Since more energy is used to break the bonds than is released making new ones, the reaction absorbs heat.
b)(ii) Calculation using bond energies:
Bonds to break in 1 mole of CaCO3 (assume 3 C–O and 1 Ca–O):
Total energy to break = 2235 + 350 = 2585 kJ
Bonds formed in CaO and CO2:
Total energy released = 700 + 1610 = 2310 kJ
Estimated enthalpy change:
Therefore, the reaction is endothermic (positive ΔH value).
c) If some carbon dioxide is removed from the sealed container after equilibrium is established, the equilibrium will shift to make more carbon dioxide (to the right/product side), according to Le Chatelier's principle. This is because the system counteracts the change by increasing CO2 concentration.