Equilibria

• Constant, not equal — and only temperature changes K.
• Constant, not equal. Only temperature changes K. Proton donor, proton acceptor. The whole indicator range inside the vertical section.

• 7.1.1 I can — (a) understand what is meant by a reversible reaction (b) understand what is meant by dynamic equilibrium in terms of the rate of forward and reverse reactions being equal and the concentration of reactants and products remaining constant (c) understand the need for a closed system in order to establish dynamic equilibrium
• 7.1.2 I can define — define Le Chatelier's principle as: if a change is made to a system at dynamic equilibrium, the position of equilibrium moves to minimise this change
• 7.1.3 I can use — use Le Chatelier's principle to deduce qualitatively (from appropriate information) the effects of changes in temperature, concentration, pressure or presence of a catalyst on a system at equilibrium
• 7.1.4 I can deduce — deduce expressions for equilibrium constants in terms of concentrations, Kc
• 7.1.5 I can use — use the terms mole fraction and partial pressure
• 7.1.6 I can deduce — deduce expressions for equilibrium constants in terms of partial pressures, Kp (use of the relationship between Kp and Kc is not required)
• 7.1.7 I can use — use the Kc and Kp expressions to carry out calculations (such calculations will not require the solving of quadratic equations)
• 7.1.8 I can calculate — calculate the quantities present at equilibrium, given appropriate data
• 7.1.9 I can state — state whether changes in temperature, concentration or pressure or the presence of a catalyst affect the value of the equilibrium constant for a reaction
• 7.1.10 I can describe — describe and explain the conditions used in the Haber process and the Contact process, as examples of the importance of an understanding of dynamic equilibrium in the chemical industry and the application of Le Chatelier's principle
• 7.2.1 I can state — state the names and formulas of the common acids, limited to hydrochloric acid, HCl, sulfuric acid, H2SO4, nitric acid, HNO3 and ethanoic acid, CH3COOH
• 7.2.2 I can state — state the names and formulas of the common alkalis, limited to sodium hydroxide, NaOH, potassium hydroxide, KOH and ammonia, NH3
• 7.2.3 I can describe — describe the Brønsted–Lowry theory of acids and bases
• 7.2.4 I can describe — describe strong acids and strong bases as fully dissociated in aqueous solution and weak acids and weak bases as partially dissociated in aqueous solution
• 7.2.5 I can — appreciate that, under standard conditions, water has pH of 7, acid solutions pH of below 7 and alkaline solutions pH of above 7
• 7.2.6 I can explain — explain qualitatively the differences in behaviour between strong and weak acids including the reaction with a reactive metal and difference in pH values by use of a pH meter, universal indicator or conductivity
• 7.2.7 I can understand — understand that neutralisation reactions occur when H+(aq) and OH-(aq) form H2O(l)
• 7.2.8 I can understand — understand that salts are formed in neutralisation reactions
• 7.2.9 I can sketch — sketch the pH titration curves of titrations using combinations of strong and weak acids with strong and weak alkalis
• 7.2.10 I can — select suitable indicators for acid-alkali titrations, given appropriate data (pKa values will not be used)

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.

• “At equilibrium the concentrations of reactants and products are equal.” Correct They are constant, not equal. What is equal is the rate of the forward and reverse reactions. In the H₂ + I₂ example of this chapter the equilibrium mixture has 0.800 mol dm⁻³ of HI against 0.100 of each reactant.
• “Raising the pressure (or adding a reactant, or adding a catalyst) changes K.” Correct Only temperature changes K. Concentration and pressure move the position; the amounts readjust until the ratio in the expression equals the same K again. A catalyst changes neither the position nor K.
• “The equilibrium shifts to oppose the change.” Correct The syllabus wording is: if a change is made to a system at dynamic equilibrium, the position of equilibrium moves to minimise this change. The change is reduced, not cancelled.
• “Kc has units of mol dm⁻³.” Correct Work the units out from each expression. For H₂ + I₂ ⇌ 2HI there are none; for the ammonia equilibrium they are mol⁻² dm⁶.
• “A strong acid is a concentrated acid.” Correct Strong means fully dissociated; concentrated means many moles per dm³. 0.01 mol dm⁻³ HCl is a dilute strong acid.
• “Every titration is neutral, pH 7, at the equivalence point.” Correct Only strong acid – strong alkali. Weak acid – strong alkali is above 7 (8.7 in this chapter's example); strong acid – weak alkali is below 7 (5.3).
• “At equilibrium the concentrations of reactants and products are equal.” Repair They are constant; equal would be a coincidence. What is equal is the rate of the forward and reverse reactions.
• “At equilibrium the reaction has stopped.” Repair Both reactions continue at equal rates. That is what dynamic means.
• “A catalyst shifts the equilibrium to the right, so more product forms.” Repair It speeds up the forward and reverse reactions equally; the position and K are unchanged. Equilibrium is reached sooner.
• “Increasing the pressure increases Kc, because more product forms.” Repair Only temperature changes K. Pressure moves the position, and the concentrations readjust until the ratio equals the same K again.
• “Kc always has units of mol dm⁻³.” Repair Work them out from the expression each time. For H₂ + I₂ ⇌ 2HI there are none; for N₂ + 3H₂ ⇌ 2NH₃, mol⁻² dm⁶.
• “Kc = [H₂][I₂] / [HI]².” Repair Products on top: [HI]² / ([H₂][I₂]). The inverted expression is K for the reverse reaction, 1/Kc.
• “Partial pressure = mole fraction.” Repair Partial pressure = mole fraction × total pressure, in the total pressure's unit.
• “I put the moles straight into Kc because the volume always cancels.” Repair It cancels only when the total moles are equal on both sides. For PCl₅ ⇌ PCl₃ + Cl₂ it does not (calculator G1).
• “The Haber process uses a low temperature because the reaction is exothermic.” Repair A low temperature would give a high yield at a rate too slow to be economic; about 450 °C is the compromise between yield and rate.
• “The Contact process uses a high pressure to increase the yield.” Repair It uses 1–2 atm. Conversion is already about 98% there, so compression is not worth its cost.
• “A strong acid is a concentrated acid.” Repair Strong means fully dissociated; concentrated means many moles per dm³.
• “Ethanoic acid is weak because it has fewer hydrogen atoms.” Repair It has four; only the O–H hydrogen is donated, and only by a small fraction of the molecules at any moment.
• “A weak acid needs less alkali to neutralise it.” Repair The same amount. As H⁺ is removed the equilibrium moves right until all the acid has reacted.
• “An acid is a substance that contains hydrogen.” Repair An acid is a proton donor. Methane contains hydrogen and is not an acid; NH₄⁺ donates a proton and is one.
• “Every titration has pH 7 at equivalence.” Repair Only strong–strong. Weak acid – strong alkali is above 7; strong acid – weak alkali is below 7.
• “Any indicator that works for strong–strong works for the others.” Repair The whole range must lie within the vertical section, which is shorter when one partner is weak and absent when both are.

• 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 8 (reaction kinetics): re-answer “why does a catalyst not change K?”. Topic 12 (nitrogen and sulfur): re-answer “why is ammonia a base?” using the Brønsted–Lowry theory. Topic 25 (A Level equilibria): re-sketch the four curves before you learn to calculate the points on them. 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 7 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 a single distinction: position against value of K, strong against concentrated, the equivalence pH of a given pair, or which indicator range lies inside a vertical section. A structured question asks you to define Le Chatelier's principle or dynamic equilibrium, predict and explain a shift, explain an industrial condition as a compromise, describe the Brønsted–Lowry theory or sketch a titration curve.
• Calculating Kc or Kp from equilibrium amounts, with units derived from the expression; finding the amounts present at equilibrium from K (always linear or a perfect square, never a quadratic); mole fractions and partial pressures. Values of ΔH and K are supplied in the question; the Ar values you need are in the Periodic Table in the data section. Indicator ranges are supplied, too.
• Topic 7 is the theory behind the acid–alkali titration in Paper 3: burette readings to 0.05 cm³, concordant titres within 0.10 cm³, the indicator chosen from the curve. A Paper 5-style question may ask you to plan a pH-meter titration, draw the curve from readings and justify an indicator, or evaluate the error a wrong indicator introduces.
• 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.