Chemical energetics

• Write the sign, divide by the right moles, and when in doubt draw the cycle and follow the arrows.
• Negative means exothermic. m is the solution. Formation: products minus reactants; combustion: reactants minus products.

• 5.1.1 I can understand — understand that chemical reactions are accompanied by enthalpy changes and these changes can be exothermic (ΔH is negative) or endothermic (ΔH is positive)
• 5.1.2 I can construct — construct and interpret a reaction pathway diagram, in terms of the enthalpy change of the reaction and of the activation energy
• 5.1.3 I can define — define and use the terms: (a) standard conditions (this syllabus assumes that these are 298 K and 101 kPa) shown by ⦵ (b) enthalpy change with particular reference to: reaction, ΔH_r; formation, ΔH_f; combustion, ΔH_c; neutralisation, ΔH_neut
• 5.1.4 I can understand — understand that energy transfers occur during chemical reactions because of the breaking and making of chemical bonds
• 5.1.5 I can use — use bond energies (ΔH positive, i.e. bond breaking) to calculate enthalpy change of reaction, ΔH_r
• 5.1.6 I can understand — understand that some bond energies are exact and some bond energies are averages
• 5.1.7 I can calculate — calculate enthalpy changes from appropriate experimental results, including the use of the relationships q = mcΔT and ΔH = −mcΔT/n
• 5.2.1 I can — apply Hess's law to construct simple energy cycles
• 5.2.2 I can — carry out calculations using cycles and relevant energy terms, including: (a) determining enthalpy changes that cannot be found by direct experiment (b) use of bond energy data

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.

• “The temperature went up by 6.5 °C, so ΔH = 54.3 kJ mol⁻¹.” Correct The sign is part of the answer, and m is the mass of the solution. A temperature rise means the reaction gave heat out, so ΔH = −54.3 kJ mol⁻¹. And in q = mcΔT the mass is that of the water or solution that was heated (100.0 g for 100 cm³), never the mass of the solid you added.
• “Standard conditions are 273 K and 100 kPa.” Correct This syllabus uses 298 K and 101 kPa, with every substance in its standard state. 273 K belongs to s.t.p. for gas volumes, which is a different idea.
• “ΔHf is the enthalpy change when one mole of a substance is formed from its elements.” Correct One mole of a compound, formed from its elements in their standard states, under standard conditions. Each of the three phrases is part of the definition.
• “Breaking bonds releases energy.” Correct Breaking a bond always requires energy. A bond energy is therefore always positive, and the negative sign of an exothermic ΔH comes from subtracting the larger total for the bonds formed.
• “For combustion data, ΔHr = ΣΔHc(products) − ΣΔHc(reactants), the same as for formation data.” Correct With combustion data it is reactants minus products. The arrows in a combustion cycle point down, away from the reaction line; in a formation cycle they point up, into it. Draw the cycle and the subtraction comes out the right way round.
• “Divide q by the moles of whichever reactant the question mentions first.” Correct n is the amount of the substance the enthalpy change refers to: the limiting reactant for a reaction, the water formed for neutralisation, the fuel burnt for combustion. A reactant in excess never sets n.
• “ΔH = +184 kJ mol⁻¹ because 184 kJ of energy is given out.” Repair Energy given out means an exothermic reaction, so ΔH is negative: −184 kJ mol⁻¹. The sign describes the reacting chemicals, which have lost enthalpy.
• “Bond breaking releases energy.” Repair Breaking a bond always requires energy; forming a bond releases it. A reaction is exothermic only because the bonds formed release more than the bonds broken absorbed.
• “Bond energies are negative for bond making.” Repair A bond energy is defined for bond breaking and is always positive. The negative contribution in a calculation comes from subtracting the bonds formed.
• “ΔHf of O₂ is −496 kJ mol⁻¹, from the O=O bond energy.” Repair ΔHf⦵ of any element in its standard state is zero. The bond energy is the energy to break O₂ into atoms, a different quantity.
• “ΔHc⦵ of methane is the enthalpy change for CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g).” Repair Under standard conditions water is a liquid; ΔHc⦵ refers to H₂O(l).
• “The standard enthalpy change of formation is when one mole of a substance is formed from its elements.” Repair One mole of a compound, from its elements in their standard states, under standard conditions.
• “The bond-energy answer and the experimental answer should be identical.” Repair Table 3(b) values are averages, and all bond energies refer to gaseous species; a real reaction involves specific bonds and may involve liquids or solids.
• “m in q = mcΔT is the mass of the solid added.” Repair m is the mass of the water or solution that is heated or cooled.
• “ΔT has to be converted from °C to K by adding 273.” Repair A temperature change is the same number in °C and in K. Only a temperature itself is converted.
• “q = 2090 J, so ΔH = −2090 J mol⁻¹.” Repair Divide by the moles of the limiting reactant and convert to kJ: ΔH = −q/n = −2090/0.0100 = −209 000 J mol⁻¹ = −209 kJ mol⁻¹.
• “n for ΔHneut is the moles of acid, whatever the acid.” Repair n is the moles of water formed. Each mole of H₂SO₄ neutralised forms two moles of water.
• “ΔHr = ΣΔHc(products) − ΣΔHc(reactants), like formation.” Repair For combustion data it is reactants minus products. Draw the cycle and follow the arrows.
• “Ea(reverse) = Ea(forward) + ΔH = 120 + (−50) = 70 kJ mol⁻¹.” Repair The reverse barrier is measured from the products level: Ea(reverse) = Ea(forward) − ΔH = 120 − (−50) = 170 kJ mol⁻¹. The reverse of an exothermic reaction has the larger barrier.
• “Heat loss is a random error.” Repair Heat loss always makes the measured ΔT too small, in the same direction every time, so it is systematic. It is corrected by extrapolating the cooling line back to the time of mixing.

• 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.
• Interleave with the chapters that use this one. Topic 7 uses the sign of ΔH to predict how temperature moves an equilibrium: re-answer “is the forward reaction exothermic?” there. Topic 8 redraws the pathway diagram with a catalysed hump: re-answer the reverse-barrier question. Topic 23 builds A Level energy cycles on the Hess’s law of section H: re-draw a formation cycle before you start it. 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 5 is AS Level content, so it is examined in Papers 1, 2 and 3. AS Level content: examined in Paper 1 (multiple choice), Paper 2 (AS structured) and, as practical context, Paper 3. Assumed knowledge for Papers 4 and 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.
• A multiple-choice item on this topic can turn on one word or one sign: which equation matches a named enthalpy change, which element has ΔHf⦵ = 0, which way round a Hess subtraction goes, or the reverse activation energy read from a pathway diagram. Structured questions ask you to define the named enthalpy changes and bond energy word for word, sketch and label a pathway diagram or an energy cycle, and explain why a bond-energy value differs from an experimental one.
• Bond-energy sums, calorimetry with \(q = mc\Delta T\) and \(\Delta H = -mc\Delta T/n\), and Hess cycles from formation, combustion or bond-energy data. The Data section gives you c = 4.18 J g⁻¹ K⁻¹, the standard conditions and every bond energy (table 3). It does not give enthalpy changes of formation, combustion or neutralisation: those are always supplied in the question.
• Topic 5 supplies the thermometric procedure the syllabus names for Paper 3: temperatures read to 0.5 °C in a plastic cup, a temperature–time graph extrapolated to the moment of mixing, an enthalpy change calculated, and possibly a Hess cycle to finish. A planning or evaluation question on it asks for the variables, the results table, the percentage error in ΔT and the heat-loss correction.
• 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.