Energetics IIEdexcel A-Level Chemistry: Topic test

20 questions, 54 marks

Edexcel A-Level Chemistry

Energetics II topic test

Total 54 marks

Name

Class

Date

1
A student constructs a Born–Haber cycle for potassium chloride, KCl, and uses it to study the energy changes involved in forming the ionic solid from its elements.
(a)
Which equation represents the lattice energy of potassium chloride?
[1 mark]
AKCl(s) → K⁺(g) + Cl⁻(g)
BK⁺(g) + Cl⁻(g) → KCl(s)
CK(s) + ½Cl₂(g) → KCl(s)
DK⁺(aq) + Cl⁻(aq) → KCl(s)
(b)
Which change has an enthalpy change equal to the enthalpy change of atomisation of chlorine?
[1 mark]
ACl₂(g) → 2Cl(g)
BCl(g) + e⁻ → Cl⁻(g)
CCl(g) → Cl⁺(g) + e⁻
D½Cl₂(g) → Cl(g)
(c)
Define the term first electron affinity.
[2 marks]
Total for question 1: 4 marks
2
A chemist compares the polarising power of the cations Na⁺, Mg²⁺ and Al³⁺, whose ionic radii are 0.102 nm, 0.072 nm and 0.054 nm respectively. She also compares the halide ions F⁻, Cl⁻ and I⁻, with radii 0.133 nm, 0.181 nm and 0.220 nm.
(a)
Which cation has the greatest polarising power?
[1 mark]
AAl³⁺
BMg²⁺
CNa⁺
DAll three are equal because the ions have the same number of electrons
(b)
Which anion is the most polarisable?
[1 mark]
AF⁻
BCl⁻
CI⁻
DAll three are equal because each has a charge of 1−
(c)
Explain why a compound with considerable covalent character has an experimental lattice energy that is more exothermic than the theoretical value.
[2 marks]
Total for question 2: 4 marks
3
Data for sodium bromide, NaBr, in kJ mol⁻¹: standard enthalpy change of formation −361; enthalpy change of atomisation of sodium +107; enthalpy change of atomisation of bromine (per mole of Br atoms formed from Br₂(l)) +112; first ionisation energy of sodium +496; first electron affinity of bromine −325.
(a)
Use a Born–Haber cycle to calculate the lattice energy of sodium bromide.
[3 marks]
(b)
The lattice energy of calcium oxide, CaO, is −3401 kJ mol⁻¹, much more exothermic than that of sodium bromide. The ionic radii are Na⁺ 0.102 nm, Ca²⁺ 0.100 nm, Br⁻ 0.196 nm and O²⁻ 0.140 nm. Explain this difference.
[4 marks]
Total for question 3: 7 marks
4
Anhydrous calcium chloride, CaCl₂, is used in self-heating food packs because it releases energy when it dissolves in water. Data in kJ mol⁻¹: lattice energy of CaCl₂ (formation of the solid from gaseous ions) −2258; enthalpy change of hydration of Ca²⁺(g) −1616; enthalpy change of hydration of Cl⁻(g) −364.
(a)
Calculate the enthalpy change of solution of calcium chloride using an energy cycle, and use your answer to explain why the pack heats up.
[6 marks]
(b)
The enthalpy change of hydration of K⁺ is −322 kJ mol⁻¹ and the lattice energy of potassium chloride is −711 kJ mol⁻¹. The ionic radius of K⁺ is 0.138 nm and that of Ca²⁺ is 0.100 nm. Explain, in terms of ionic charge and ionic radius, why the enthalpy change of hydration of Ca²⁺ is much more exothermic than that of K⁺, and why the lattice energy of CaCl₂ is more exothermic than that of KCl.
[6 marks]
Total for question 4: 12 marks
5
Dry ice (solid carbon dioxide) is used to keep food cold during transport. At atmospheric pressure it sublimes at 195 K without forming a liquid.
(a)
Which row correctly gives the signs of ΔH and ΔS system for the sublimation of CO₂(s) to CO₂(g)?
[1 mark]
AΔH negative, ΔS system negative
BΔH negative, ΔS system positive
CΔH positive, ΔS system negative
DΔH positive, ΔS system positive
(b)
Which reaction has the most positive ΔS system?
[1 mark]
AN₂(g) + 3H₂(g) → 2NH₃(g)
BCaCO₃(s) → CaO(s) + CO₂(g)
C2H₂(g) + O₂(g) → 2H₂O(l)
DAg⁺(aq) + Cl⁻(aq) → AgCl(s)
(c)
Explain why the entropy of carbon dioxide increases when dry ice sublimes.
[2 marks]
Total for question 5: 4 marks
6
A sealed flask holds a mixture of hydrogen and oxygen at 298 K and 100 kPa. For H₂(g) + ½O₂(g) → H₂O(l), ΔH = −286 kJ mol⁻¹ and ΔS system = −163 J K⁻¹ mol⁻¹. No water forms in the flask, even after several months.
(a)
What is the value of ΔG for this reaction at 298 K?
[1 mark]
A−237 kJ mol⁻¹
B−335 kJ mol⁻¹
C−123 kJ mol⁻¹
D+4.83 × 10⁴ kJ mol⁻¹
(b)
Which statement about the equilibrium constant, K, for this reaction at 298 K is correct?
[1 mark]
AK is very small, so the reactant is favoured at equilibrium
BK is exactly 1
CK is very large, so the product is favoured at equilibrium
DK cannot be found from ΔG
(c)
Explain why no reaction is seen even though ΔG is negative.
[2 marks]
Total for question 6: 4 marks
7
Sodium hydrogencarbonate decomposes on heating: 2NaHCO₃(s) → Na₂CO₃(s) + H₂O(g) + CO₂(g). Standard entropies in J K⁻¹ mol⁻¹: NaHCO₃(s) 101.7; Na₂CO₃(s) 135.0; H₂O(g) 188.8; CO₂(g) 213.8.
(a)
Calculate ΔS system for the reaction as written.
[3 marks]
(b)
The enthalpy change for the reaction as written is +135.6 kJ mol⁻¹. Calculate ΔS surroundings and ΔS total at 298 K, and state whether the decomposition is feasible at this temperature.
[4 marks]
Total for question 7: 7 marks
8
Aluminium iodide, AlI₃, has a lattice with considerable covalent character. It can be made directly from its elements: 2Al(s) + 3I₂(s) → 2AlI₃(s), for which ΔH = −604 kJ mol⁻¹ and ΔS system = −87 J K⁻¹ mol⁻¹. Ionic radii: Na⁺ 0.102 nm, Al³⁺ 0.054 nm, I⁻ 0.220 nm.
(a)
Explain, in terms of polarisation, why aluminium iodide has considerable covalent character whereas sodium iodide is almost purely ionic, and how this affects the comparison of experimental and theoretical lattice energies for AlI₃.
[6 marks]
(b)
Calculate ΔG for the formation of AlI₃ at 298 K and ln K, using ΔG = −RT ln K (R = 8.31 J K⁻¹ mol⁻¹). Explain what your values show about the feasibility of the reaction and the position of equilibrium.
[6 marks]
Total for question 8: 12 marks

End of questions

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).