Electrochemistry — AS & A Level Chemistry (9701) questions by topic
Redox balancing and electrode-potential predictions run through this topic: comparing oxidation-state increases across several reactions, defining standard electrode potential and standard conditions, and deducing the mole ratio of two sodium salts formed when chlorine dioxide reacts with sodium hydroxide.
Deduce, calculate, identify, state and draw are the leading command words across 33 questions from 2023 to 2025 papers, worth from 1 to 18 marks, where predicting outcomes from E-standard values is described as extremely difficult. Practising redox-equation balancing with instant marking catches an incorrect number of significant figures or a repeated calculation from an earlier part before either error carries through the rest of the response.
What examiners look for
- Examiners repeatedly note that quoting and using E-standard values to make correct predictions is found extremely difficult, particularly when an ion can be both oxidised and reduced.
- A common mistake was using the terms 'anode' and 'cathode', which are best avoided when describing electrochemical cells.
- Reports flag several candidates repeating an earlier calculation instead of carrying their previous answer forward.
- A recurring criticism is stating that warm alkaline I2(aq) with glucose gives a yellow precipitate, when in fact there is no reaction and the solution stays colourless.
- Strong scripts correctly balanced the manganate(VII) and iodide equation to give v = 16, and stated that acidified KMnO4 changes from purple to colourless while Fehling's reagent gives an orange or red precipitate.
May–June 2025, Paper 12
'Red lead' is a red pigment with the formula Pb3O4. Every formula unit of 'red lead' is the same. Each formula unit contains three lead ions and four oxide ions. Lead has two…
Answer this question and get it marked →Sodium bromide is warmed with concentrated sulfuric acid. Which row describes the change in the oxidation number of the sulfur and the role of the bromide ions in the reaction? A…
Answer this question and get it marked →May–June 2025, Paper 13
Chromium is present in compound X. - Two moles of compound X react with exactly 3 moles of silicon. - The only products of this reaction are 4 moles of chromium and 3 moles of a…
Answer this question and get it marked →May–June 2025, Paper 14
In this question, the average oxidation state of sulfur in S2O3^2- and sulfur in S2O4^2- should be used. In which reaction does the underlined element have the largest increase in…
Answer this question and get it marked →When K2MnO4 reacts with concentrated hydrochloric acid, the products include chlorine molecules and MnCl2. All of the manganese atoms are reduced to MnCl2. Both Mn and Cl change…
Answer this question and get it marked →May–June 2024, Paper 11
Sodium chromate(VI), Na2CrO4, is manufactured by heating chromite, FeCr2O4, with sodium carbonate in an oxidising atmosphere. Chromite contains Cr2O4^2- ions. 2FeCr2O4 + 4Na2CO3 +…
Answer this question and get it marked →Oxygen can be prepared by the reaction of potassium manganate(VII), KMnO4, hydrogen peroxide, H2O2, and sulfuric acid, H2SO4. Each H2O2 molecule loses two electrons in this…
Answer this question and get it marked →May–June 2024, Paper 12
Nitrogen dioxide reacts with water. 2NO2 + H2O - HNO2 + HNO3 Which statement about this reaction is correct? A Both products are formed because oxygen atoms gain electrons. B…
Answer this question and get it marked →Phosphorus reacts with concentrated sulfuric acid to produce phosphoric acid, sulfur dioxide and water. aH2SO4 + bP - cH3PO4 + dSO2 + eH2O a, b, c, d and e are all whole numbers.…
Answer this question and get it marked →May–June 2024, Paper 13
In which reaction does the oxidation number of chlorine change by the largest amount? A 2KClO3 - 2KCl + 3O2 B 2ClO^- - Cl^- + ClO2^- C Cl2 + H2O - HCl + HClO D 2NaClO2 + Cl2 -…
Answer this question and get it marked →W moles of HNO2 undergoes a disproportionation reaction to produce U moles of HNO3 and V moles of NO. - No other nitrogen containing product is produced. - Nitrogen is the only…
Answer this question and get it marked →May–June 2024, Paper 41
Define standard electrode potential, E-standard, including a description of standard conditions.
Answer this question and get it marked →May–June 2024, Paper 43
Define standard electrode potential, E-standard, including a description of standard conditions.
Answer this question and get it marked →October–November 2024, Paper 11
In alkaline solution, MnO₄⁻ ions oxidise SO₃²⁻ ions to SO₄²⁻ ions. The MnO₄⁻ ions are reduced to MnO₂. What is the ratio of the two ions in the balanced chemical equation for this…
Answer this question and get it marked →October–November 2024, Paper 12
Equations for some reactions of hydrogen peroxide are given. 1 2Fe²⁺ + H₂O₂ + 2H⁺ → 2Fe³⁺ + 2H₂O 2 2MnO₄⁻ + 5H₂O₂ + 6H⁺ → 2Mn²⁺ + 8H₂O + 5O₂ 3 2Fe³⁺ + H₂O₂ + 2OH⁻ → 2Fe²⁺ + O₂ +…
Answer this question and get it marked →The equation for the reaction of aqueous thiosulfate ions, S₂O₃²⁻, and aqueous dioxo-vanadium ions, VO₂⁺, is shown. 2S₂O₃²⁻ + xVO₂⁺ + yH⁺ → S₄O₆²⁻ + zVO²⁺ + 2H₂O Which row shows…
Answer this question and get it marked →October–November 2024, Paper 13
In alkaline solution, MnO₄⁻ ions oxidise SO₃²⁻ ions to SO₄²⁻ ions. The MnO₄⁻ ions are reduced to MnO₂. What is the ratio of the two ions in the balanced chemical equation for this…
Answer this question and get it marked →February–March 2023, Paper 12
Four equations representing reactions of nitrogen or one of its compounds are given. Which equation represents a disproportionation reaction? A 2HNO3 + CaCO3 → Ca(NO3)2 + CO2 +…
Answer this question and get it marked →The ore psilomelane may be considered to have the general formula Ba(Mn^(x+))(Mn^(y+))8O16(OH)4. In this general formula, x+ and y+ are the two different oxidation states of…
Answer this question and get it marked →Silicon reacts with a mixture of calcium oxide and magnesium oxide at 1200 °C. 2MgO + 2CaO + Si → 2Mg + Ca2SiO4 Which statement about this reaction is correct? A Calcium is…
Answer this question and get it marked →May–June 2023, Paper 11
Chlorine dioxide, ClO2, reacts with aqueous sodium hydroxide to produce water and a mixture of two sodium salts, NaClO2 and NaClO3. What is the mole ratio of NaClO2 to NaClO3 in…
Answer this question and get it marked →May–June 2023, Paper 12
LiAlH4 contains AlH4- ions in which aluminium has an oxidation state of +3. LiAlH4 reacts with water, as shown. LiAlH4 + 4H2O - 4H2 + LiOH + Al(OH)3 In this reaction, each of the…
Answer this question and get it marked →The vanadium salt, VOSO4, is soluble in water and reacts readily with powdered manganese in dilute sulfuric acid. The equation for the reaction is correct? Mn(s) + 2VOSO4(aq) +…
Answer this question and get it marked →May–June 2023, Paper 13
The ionic equation shows iodide ions reacting with manganate(VII) ions in acidic solution. uMnO4- + vH+ + wI- - xMn2+ + yH2O + zI2 The letters u, v, w, x, y and z all represent…
Answer this question and get it marked →Electronegativity differences can be used to help determine the oxidation number of an atom in different species. A number of rules are used which include: - The more…
Answer this question and get it marked →May–June 2023, Paper 42
Define standard cell potential, E°cell.
Answer this question and get it marked →October–November 2023, Paper 11
HOCl(aq) is the molecule that kills bacteria when chlorine is added to water. The reaction Cl2(g) + H2O(l) <= HOCl(aq) + H+(aq) + Cl-(aq) produces this molecule. Which statement…
Answer this question and get it marked →October–November 2023, Paper 12
Zinc reacts with concentrated nitric acid giving three products only: zinc nitrate, an oxide of nitrogen and water. 3.0 moles of zinc react with 8.0 moles of nitric acid; zinc…
Answer this question and get it marked →The equation for the reaction between aqueous copper ions and aqueous iodide ions is 2Cu2+(aq) + 4I-(aq) - 2CuI(s) + I2(aq). What is the change in oxidation state of copper? A: +2…
Answer this question and get it marked →October–November 2023, Paper 13
HOCl(aq) is the molecule that kills bacteria when chlorine is added to water. The following reaction produces this molecule. Cl2(g) + H2O(l) <= HOCl(aq) + H+(aq) + Cl-(aq) Which…
Answer this question and get it marked →October–November 2023, Paper 21
Potassium chlorate, KClO3, is widely used as an oxidising agent and to make O2(g). Define oxidising agent.
Answer this question and get it marked →October–November 2023, Paper 23
Potassium chlorate, KClO3, is widely used as an oxidising agent and to make O2(g). Define oxidising agent.
Answer this question and get it marked →October–November 2023, Paper 42
Some electrode potentials are shown in Table 3.1 (including V²⁺/V −1.20 V, V³⁺/V²⁺ −0.26 V, VO²⁺/V³⁺ +0.34 V, VO₂⁺/VO²⁺ +1.00 V, Fe²⁺/Fe −0.44 V, Fe³⁺/Fe −0.04 V, Fe³⁺/Fe²⁺ +0.77…
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