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CAIE A Level Chemistry 9701

Chemical energetics questions

Exam-style questions on Chemical energetics, ordered from foundation to challenge. Pick one, write your answer the way you would in the exam, and it comes back marked against the mark scheme, point by point.

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Written by Markquill to follow the Cambridge International syllabus. Markquill is independent and is not affiliated with or endorsed by Cambridge International.

  1. Compare5 marksStandardCompare the lattice energies of magnesium oxide, MgO, and calcium oxide, CaO. In your answer, explain how the properties of the ions influence the magnitude of the lattice energy.
  2. Compare5 marksStandardThe table shows the enthalpies of hydration and ionic radii for three ions. Ion Ionic radius / pm Δ H^°hyd / kJ mol^-1 Na^+ 102 –406 Mg^2+ 72 –1920 Al^3+ 53 –4660 Compare the enthalpies of hydration of these three…
  3. Explain4 marksStandardThe lattice energy of magnesium oxide is -3791 kJ mol^-1 and that of sodium chloride is -787 kJ mol^-1. Define the term lattice energy and explain, in qualitative terms, why the lattice energy of magnesium oxide is…
  4. Compare5 marksStandardThe standard enthalpy change of formation of carbon dioxide, CO2(g), is represented by Δ Hf^°. The standard enthalpy change of combustion of carbon (graphite), C(s), is represented by Δ Hc^°. Compare the two enthalpy…
  5. Explain4 marksStandardThe standard enthalpy change of formation of liquid butan-1-ol, CH3CH2CH2CH2OH(l), is not measured directly by reacting its elements together. A student plans to determine it from the standard enthalpy changes of…
  6. Explain4 marksStandardThe table below shows some enthalpy data for sodium chloride and magnesium chloride. Compound Lattice enthalpy / kJ mol⁻¹ Hydration enthalpy of cation / kJ mol⁻¹ Hydration enthalpy of chloride ion / kJ mol⁻¹ NaCl -770…
  7. Explain4 marksStandardWhen Na2SO4 dissolves in water, the enthalpy change of solution, Δ H extsol, is -2.4\,kJ\,mol^-1. The lattice energy of Na2SO4 is +215\,kJ\,mol^-1. A student writes an energy cycle linking Δ H extsol, lattice energy…
  8. Suggest4 marksStandardThe first electron affinity of chlorine is -349 kJ mol^-1 and that of oxygen is -141 kJ mol^-1. Suggest, in terms of atomic structure and the factors affecting electron affinity, why the first electron affinity of…
  9. Compare5 marksStandardEntropy, S, is defined as the number of possible arrangements of particles and their energy in a system. Compare the entropy change, Δ S, for the following two processes. Predict the sign of Δ S for each process and…
  10. Compare5 marksStandardThe standard enthalpy change for the hydration of ethene, C2H4(g) + H2O(l) → C2H5OH(l) , can be determined using Hess's law in two ways. Way 1: Using standard enthalpies of formation of C2H4(g), H2O(l) and C2H5OH(l)…
  11. Describe5 marksStandardThe combustion of ethanol is exothermic. C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(l) Describe how the reaction pathway diagram for this reaction shows the enthalpy change and the activation energy of the reaction. Explain…
  12. Explain4 marksStandardA sealed flask contains gaseous water at 120 °C. The flask is cooled to 90 °C, and the water condenses to a liquid. In terms of the arrangements of the particles and their energy, explain why the entropy of the water…
  13. Explain4 marksStandardThe standard enthalpy change for the formation of ethane, C2H6(g), from its elements cannot be measured directly because the reaction is slow and incomplete. The standard enthalpy changes of combustion of carbon, C(s)…
  14. Explain4 marksStandardHydrogen burns in oxygen to form water as shown. 2H2(g) + O2(g) → 2H2O(l) The bond energies are: H–H: 436 kJ mol^-1 O=O: 498 kJ mol^-1 O–H: 463 kJ mol^-1 Explain why this reaction is exothermic. In your answer, refer…
  15. Suggest4 marksStandardLimestone is largely calcium carbonate, CaCO3. When heated strongly it decomposes into calcium oxide, CaO, and carbon dioxide, CO2. The standard enthalpy changes of formation of CaCO3(s), CaO(s) and CO2(g) are known…
  16. Compare5 marksStandardReaction 1: ΔH = +100 kJ mol⁻¹, ΔS = +200 J K⁻¹ mol⁻¹ Reaction 2: ΔH = −100 kJ mol⁻¹, ΔS = −200 J K⁻¹ mol⁻¹ Compare the feasibility of these two reactions at different temperatures. In your answer, use the Gibbs…
  17. Explain4 marksStandardConsider the following changes: (a) The dissolution of ammonium nitrate in water: NH4NO3(s) → NH4+(aq) + NO3-(aq) (b) The decomposition of ammonia: 2NH3(g) → N2(g) + 3H2(g) For each change, predict the sign of the…
  18. Explain4 marksStandardA mixture of H2(g) and Cl2(g) can be kept at room temperature for a long time without reacting. When a spark is passed through the mixture, a vigorous exothermic reaction occurs, forming HCl(g). Explain these…
  19. Describe5 marksStandardThe table shows the ionic radii of three Group I ions and the numerical values of their enthalpy changes of hydration, Δ Hhyd. Ion Ionic radius / pm Δ Hhyd / kJ mol^-1 Li^+ 76 -519 Na^+ 102 -406 K^+ 138 -322 Describe…
  20. Explain4 marksStandardThe decomposition of calcium carbonate is represented by the equation: CaCO3(s) → CaO(s) + CO2(g) For this reaction, Δ H = +178\ kJ mol^-1 and Δ S = +160\ J K^-1mol^-1. The reaction is not feasible at 298\ K but…
  21. Suggest4 marksStandardA student burned ethanol from a spirit burner to heat water in a copper calorimeter and determined the enthalpy change of combustion of ethanol as −980 kJ mol⁻¹. A data book gives the standard enthalpy change of…
  22. Suggest4 marksStandardThe table shows the enthalpy change of hydration of the Group 2 ions ceMg^2+, ceCa^2+, ceSr^2+ and ceBa^2+. Ion Δ Hhyd / kJ mol^-1 ceMg^2+ -1920 ceCa^2+ -1650 ceSr^2+ -1480 ceBa^2+ -1360 Suggest why the enthalpy change…
  23. Compare5 marksStandardThree enthalpy changes for copper(II) sulfate are: CuSO4(s) + aq → CuSO4(aq) Δ H = -64.0\ mathrmkJ\,mol^-1 CuSO4.5H2O(s) + aq → CuSO4(aq) Δ H = +12.5\ mathrmkJ\,mol^-1 CuSO4(s) + 5H2O(l) → CuSO4.5H2O(s) Δ H = -76.5\…
  24. Describe5 marksStandardA student has data for the standard enthalpy changes of combustion of carbon (graphite), hydrogen and ethane. She wants to use these values, together with Hess's law, to determine the standard enthalpy change of…
  25. Describe5 marksStandardThe enthalpy change of atomisation, Δ Hat, is one of the energy terms used in a Born-Haber cycle for an ionic compound. (a) Define the enthalpy change of atomisation, Δ Hat. (b) In a Born-Haber cycle for sodium…
  26. Compare5 marksStandardMethane burns in oxygen, CH4(g) + 2O2(g) → CO2(g) + 2H2O(g), releasing heat, but the mixture must first be ignited. Nitrogen and oxygen react together, N2(g) + O2(g) → 2NO(g), only after the mixture has been heated…
  27. Explain4 marksStandardThe Born-Haber cycle for an ionic compound is used to determine its lattice energy. The lattice energy is the energy change when one mole of an ionic compound is formed from its gaseous ions. In a Born-Haber cycle, the…
  28. Explain5 marksChallengen-Butane, C4H10, can be converted into its branched-chain isomer 2-methylpropane, C4H10, using a suitable catalyst. The enthalpy change of this isomerisation is difficult to measure directly because the reaction is…
  29. Describe7 marksChallengeThe standard enthalpy change of formation of ethanol, Δ Hf^°[C2H5OH(l)], cannot be measured directly by calorimetry. The following standard enthalpy changes of combustion, Δ Hc^°, are known: C(s) + O2(g) → CO2(g) Δ…
  30. Describe7 marksChallengeThe Born–Haber cycle for the formation of magnesium oxide, MgO, is shown using the following enthalpy changes (all values in kJ mol⁻¹): Enthalpy of formation of MgO: –602 Enthalpy of atomisation of Mg: +148 First…
  31. Suggest7 marksChallengeA company that makes synthetic diamonds needs to know the standard enthalpy change for the conversion of graphite into diamond. The direct conversion requires extremely high pressure and temperature, so its enthalpy…
  32. Compare7 marksChallengeThe first electron affinities of oxygen and sulfur are -141\ kJ mol^-1 and -200\ kJ mol^-1 respectively. Compare these two values in terms of the relative ease of adding an electron to a gaseous atom, and explain the…
  33. Describe6 marksChallengeThe first electron affinities of the Group 16 elements are all exothermic, but the values become less exothermic down the group. Describe and explain, in terms of the factors affecting electron affinity, why the first…
  34. Describe7 marksChallengeA student studies the entropy changes in three processes. Process 1: Sublimation of solid carbon dioxide at room temperature: CO2(s) → CO2(g) Process 2: Heating liquid water from 25°C to 80°C. Process 3: Decomposition…
  35. Describe6 marksChallengeMethanol, CH3OH(l), burns completely in oxygen to form CO2(g) and H2O(l). CH3OH(l) + 3/2 O2(g) → CO2(g) + 2H2O(l) The standard enthalpy change of this combustion can be determined from the standard enthalpies of…
  36. Explain5 marksChallengeA student estimates the standard enthalpy change of formation of ethanol, C2H5OH, from mean bond energies. She lists every covalent bond in one molecule of ethanol and every covalent bond in the elements in their…
  37. Suggest7 marksChallengeTwo experiments are carried out at 25.0 °C and 101 kPa to measure the enthalpy change of neutralisation. In each experiment, 50.0 cm³ of 1.00 mol dm⁻³ acid is mixed with 50.0 cm³ of 1.00 mol dm⁻³ sodium hydroxide in a…
  38. Explain5 marksChallengeThe first electron affinities of the elements in Group 17 and Group 16 are shown in the table. Element First electron affinity / kJ mol⁻¹ F –328 Cl –349 Br –325 I –295 O –141 S –200 Se –195 Te –190 Explain the trends…
  39. Describe6 marksChallengeEthanol burns in oxygen according to the equation C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(g) The combustion is strongly exothermic, but ethanol in an open beaker at room temperature does not ignite. (a) Describe the energy…
  40. Describe7 marksChallengeThe enthalpy change of hydration, Δ Hhyd, is defined as the enthalpy change when one mole of gaseous ions dissolves in water to form an infinitely dilute solution. (a) Describe how the charge and the radius of an ion…
  41. Describe7 marksChallengeThe reaction between hydrogen and iodine is shown. H2(g) + I2(g) → 2HI(g) Δ H = +26 kJ mol^-1 Some average bond energies are: H–H: 436 kJ mol^-1 I–I: 151 kJ mol^-1 H–I: 299 kJ mol^-1 Describe the energy changes that…
  42. Describe6 marksChallengeWhen NaCl dissolves in water, the enthalpy change of solution, Δ H extsol, is +3.9 kJ mol^-1. The enthalpy change of hydration of Na+ is -406 kJ mol^-1 and that of Cl- is -364 kJ mol^-1. The lattice energy of NaCl is…
  43. Describe7 marksChallengeA reaction: A(s) + B(g) → C(g), has Δ H = +50 kJ mol^-1 and Δ S = +120 J K^-1 mol^-1. Describe the effect of temperature on the feasibility of this reaction. In your answer, include the Gibbs equation, the signs of Δ H…