Chemical energetics (A Level)

• Every quantity here is a direction. Lattice energy points down to the solid and is negative; ΔS is in joules while ΔH is in kilojoules; ΔG⦵ negative is feasible, but says nothing about how fast.
• Lattice energy: gaseous ions to solid, always negative. ΔS in joules, ΔH in kilojoules. ΔG⦵ negative is feasible, and feasible is not fast.

• 23.1.1 I can define — define and use the terms: (a) enthalpy change of atomisation, ΔH_at (b) lattice energy, ΔH_latt (the change from gas phase ions to solid lattice)
• 23.1.2 I can — (a) define and use the term first electron affinity, EA (b) explain the factors affecting the electron affinities of elements (c) describe and explain the trends in the electron affinities of the Group 16 and Group 17 elements
• 23.1.3 I can construct — construct and use Born-Haber cycles for ionic solids (limited to +1 and +2 cations, -1 and -2 anions)
• 23.1.4 I can — carry out calculations involving Born-Haber cycles
• 23.1.5 I can explain — explain, in qualitative terms, the effect of ionic charge and of ionic radius on the numerical magnitude of a lattice energy
• 23.2.1 I can define — define and use the term enthalpy change with reference to hydration, ΔH_hyd, and solution, ΔH_sol
• 23.2.2 I can construct — construct and use an energy cycle involving enthalpy change of solution, lattice energy and enthalpy change of hydration
• 23.2.3 I can — carry out calculations involving the energy cycles in 23.2.2
• 23.2.4 I can explain — explain, in qualitative terms, the effect of ionic charge and of ionic radius on the numerical magnitude of an enthalpy change of hydration
• 23.3.1 I can define — define the term entropy, S, as the number of possible arrangements of the particles and their energy in a given system
• 23.3.2 I can predict — predict and explain the sign of the entropy changes that occur: (a) during a change in state, e.g. melting, boiling and dissolving (and their reverse) (b) during a temperature change (c) during a reaction in which there is a change in the number of gaseous molecules
• 23.3.3 I can calculate — calculate the entropy change for a reaction, ΔS, given the standard entropies, S⦵, of the reactants and products, ΔS = ΣS⦵(products) − ΣS⦵(reactants) (use of ΔS_total = ΔS_surr + ΔS_sys is not required)
• 23.4.1 I can state — state and use the Gibbs equation ΔG⦵ = ΔH⦵ − TΔS⦵
• 23.4.2 I can — perform calculations using the equation ΔG⦵ = ΔH⦵ − TΔS⦵
• 23.4.3 I can state — state whether a reaction or process will be feasible by using the sign of ΔG⦵
• 23.4.4 I can predict — predict the effect of temperature change on the feasibility of a reaction, given standard enthalpy and entropy changes

Units and significant figures are marked. The syllabus states that failure to quote units, the inclusion of units in quantities defined as ratios, and answers given to an inappropriate number of significant figures are all liable to be penalised. Give a calculated answer to the same number of significant figures as the least precise data, or one more; keep full precision in the working and round only at the end. A fifth of the qualification is experimental: Papers 3 and 5 test AO3 only, and their questions may be set in contexts outside the syllabus content, so the practical work in this chapter is set out as procedure, recording and evaluation rather than as theory.

• “Lattice energy is the energy needed to break the lattice, so it is positive.” Correct In this syllabus lattice energy is the change from gas-phase ions to solid lattice. It is always negative. A positive lattice energy in an answer is wrong.
• “ΔG = −92 − 298 × (−199).” Correct ΔS is in J K⁻¹ mol⁻¹ and ΔH is in kJ mol⁻¹. Divide ΔS by 1000 before it goes into the Gibbs equation: −199 J K⁻¹ mol⁻¹ = −0.199 kJ K⁻¹ mol⁻¹.
• “ΔHat(Cl) = +242 kJ mol⁻¹.” Correct Atomisation is per mole of gaseous atoms: ½Cl₂(g) → Cl(g), ½ × 242 = +121 kJ mol⁻¹.
• “Electron affinity is exothermic.” Correct The first is, for the elements that form anions. The second is always endothermic and its arrow goes up in the cycle.
• “The lattice energy of MgO is smaller than that of NaCl, because −3841 is less than −784.” Correct Say more exothermic or larger in magnitude. The syllabus asks about the numerical magnitude.
• “ΔG⦵ is negative, so the reaction goes quickly.” Correct ΔG⦵ decides whether a reaction is feasible. How fast it goes depends on the activation energy. Diamond has a negative ΔG⦵ for turning into graphite and does not change.
• “Lattice energy is the energy needed to break up the lattice into gaseous ions, so it is positive.” Repair In this syllabus lattice energy is the enthalpy change when one mole of the solid is formed from its gaseous ions. It is always negative.
• “ΔHat(Cl) = +242 kJ mol⁻¹.” Repair Atomisation is per mole of gaseous atoms, ½Cl₂(g) → Cl(g): ½ × 242 = +121 kJ mol⁻¹. Use 242 only when the cycle needs two moles of chlorine atoms.
• “Electron affinity is always exothermic.” Repair The first electron affinity is exothermic for these elements; the second is always endothermic, because the electron is added to an ion that is already negative.
• “Fluorine has the most exothermic electron affinity in Group 17 because it is the smallest atom.” Repair Chlorine’s is more exothermic. Fluorine’s small, crowded 2p sub-shell repels the incoming electron and reduces the energy released.
• “For NaCl I added IE₁ + IE₂ of sodium.” or “For MgO I used IE₁ of magnesium.” Repair One ionisation energy per unit of positive charge: Na⁺ uses IE₁ only; Mg²⁺ uses IE₁ + IE₂.
• “In the MgO cycle, EA₂ = −798.” Repair EA₂ is +798 and its arrow goes up. The wrong sign moves the lattice energy by 1596 kJ mol⁻¹.
• “The lattice energy of MgO is smaller than that of NaCl, because −3841 < −784.” Repair It is more exothermic, of larger magnitude. The syllabus’s word is magnitude, so use it.
• “ΔHsol = ΔHlatt + ΣΔHhyd.” Repair The solid must first be broken into gaseous ions, the reverse of lattice formation: ΔHsol = ΣΔHhyd − ΔHlatt.
• “For MgCl₂, ΣΔHhyd = −1920 + (−374).” Repair Hydration enthalpy is per mole of ions, and the formula has two chloride ions: −1920 + 2(−374) = −2668.
• “Entropy is a measure of disorder.” Repair As a definition, give the syllabus’s wording: the number of possible arrangements of the particles and their energy in a given system.
• “The standard entropy of an element is zero, like its ΔHf.” Repair S⦵ of every substance is positive: O₂ is 205 J K⁻¹ mol⁻¹. Only ΔHf of an element is zero, by definition.
• “ΔG = −92 − 298 × (−199) = +59 210.” Repair Convert ΔS to kJ K⁻¹ mol⁻¹ first: −92 − 298 × (−0.199) = −32.7 kJ mol⁻¹. The T in the equation is in kelvin, never °C.
• “ΔG⦵ is negative, so the reaction happens quickly.” Repair A negative ΔG⦵ makes a reaction feasible. The rate depends on the activation energy, and a feasible reaction can be immeasurably slow.
• “Raising the temperature always makes a reaction more feasible.” Repair Only when ΔS⦵ is positive. When ΔS⦵ is negative, raising T makes −TΔS⦵ more positive and the reaction less feasible.

• Read the command word before you decide how much to write. This syllabus has twenty-two of them: analyse, calculate, compare, consider, contrast, deduce, define, demonstrate, describe, determine, discuss, evaluate, examine, explain, give, identify, justify, predict, show (that), sketch, state and suggest. Comment, estimate, name and outline are not among them: where a question wants something named it says identify, which the syllabus glosses as “name/select/recognise”. State and give want a fact and nothing more. Describe wants the points or the features. Explain wants the reasons and the relationships — a describe-level answer to an explain question is incomplete however well written it is. Deduce and determine want a conclusion reached from the information given, with the reasoning visible.
• Say “more exothermic” or “larger in magnitude”, never “larger”. −3841 is a smaller number than −784. The syllabus asks about the numerical magnitude, so compare magnitudes, and give the cause (charge, radius) and the mechanism (stronger electrostatic attraction between the ions, more energy released).
• Write the comparison in three parts: the cause (larger charge, smaller radius, higher charge density), the mechanism (stronger ion–dipole attraction to the water molecules) and the consequence (more energy released, ΔHhyd more exothermic). Use “more exothermic” or “greater magnitude”, as in 23.1.5.
• Interleave with the chapters that use this one. Chapter 24 links ΔG⦵ to cell potentials: when you reach it, re-answer “what does the sign of ΔG⦵ tell you, and what does it not?”. Chapter 27 uses lattice and hydration enthalpies for Group 2: re-answer “how do charge and radius set the magnitude of each?”. Recalling a topic inside a new context is worth more than another pass over this chapter on its own; at A Level, Paper 4 assumes the whole of the AS content, so nothing here is ever finished with.

• Cambridge International AS & A Level Chemistry 9701 has five components. Topic 23 is A Level content, so it is examined in Papers 4 and 5. A Level content: examined in Paper 4 (A Level structured, which also requires the AS content) and, as practical context, Paper 5. AS Level candidates take Papers 1, 2 and 3; A Level candidates take all five, either staged over two years (Papers 1–3 in year one, Papers 4 and 5 in year two) or together in one series. Examinations are available in the June and November series, and in March in India.
• Across both the AS Level and the A Level the assessment objectives are weighted AO1 40% (knowledge and understanding), AO2 40% (handling, applying and evaluating information) and AO3 20% (experimental skills and investigations). AS candidates are graded a–e; A Level candidates A*–E.
• There is no multiple-choice paper on A Level content. Paper 4 structured questions can ask you to define any of the enthalpy terms or entropy, to state the species at each level of a Born–Haber cycle, and to explain a trend in electron affinity, lattice energy or hydration enthalpy in terms of charge, radius and attraction.
• The numbers this topic produces are a lattice energy or electron affinity from a Born–Haber cycle, a hydration or solution enthalpy from the solution cycle, ΔS⦵ from standard entropies, ΔG⦵ at a stated temperature, and T = ΔH⦵/ΔS⦵. Ionisation energies and bond energies come from the Data section; the other enthalpies and every S⦵ are given in the question.
• The thermometric measurement of ΔHsol in an insulated cup: planning the quantities and the precision of each instrument, drawing and extrapolating a temperature–time graph, calculating ΔHsol to the right significant figures, and identifying the largest error and its improvement.
• Read the command word before you decide how much to write. This syllabus has twenty-two of them: analyse, calculate, compare, consider, contrast, deduce, define, demonstrate, describe, determine, discuss, evaluate, examine, explain, give, identify, justify, predict, show (that), sketch, state and suggest. Comment, estimate, name and outline are not among them: where a question wants something named it says identify, which the syllabus glosses as “name/select/recognise”. State and give want a fact and nothing more. Describe wants the points or the features. Explain wants the reasons and the relationships — a describe-level answer to an explain question is incomplete however well written it is. Deduce and determine want a conclusion reached from the information given, with the reasoning visible.