RI 01. Redox Reactions Tutorial (Questions)

-1- Raffles Institution Year 5 H2 Chemistry 2025 Tutorial 1 – Redox Reactions Self-Check Questions 1 Determine the oxidation number of the underlined element in the following species: (a) BaSO4 (c) TiO2+ (e) H2O2 (g) BrF (b) Na3VO4 (d) C2O42− (f) N2O4 (h) IF7 2 Identify if the following equations represent a redox or non-redox reaction. Determine the change in oxidation number for the relevant element to support your answer. (a) NH3(aq) + HCl(aq) → NH4Cl(aq) (b) 3Mg(s) + 2NH3(g) → Mg3N2(s) + 3H2(g) 3 A Level N2010/III/4(f) Ozone is usually made by passing oxygen gas through a tube between two highly charged electrical plates. 3O2(g) → 2O3(g) The reaction does not go to completion, so a mixture of the two gases results. The concentration of O3 in the mixture can be determined by its reaction with aqueous KI. O3 + 2KI + H2O → I2 + O2 + 2KOH The iodine formed can be estimated by its reaction with sodium thiosulfate. 2Na2S2O3 + I2 → Na2S4O6 + 2NaI When 500 cm 3 of an oxygen/ozone gaseous mixture at s.t.p. was passed into an excess of aqueous KI, and iodine titrated, 15.0 cm3 of 0.100 mol dm–3 Na2S2O3 was required to discharge the iodine colour. (a) Calculate the amount in moles of iodine produced. (b) Hence calculate the percentage of O3 in the gaseous mixture. 4 A Level N2009/III/3(c) When sodium is burned in air, a mixture of sodium oxide, Na2O, and sodium peroxide, Na2O2, is formed. The mixture reacts with water according to the following equations. Na2O + H2O → 2NaOH Na2O2 + 2H2O → 2NaOH + H2O2 The following information will allow you to calculate the relative amounts of the two oxides produced when sodium is burned. • The mixture obtained by burning a sample of sodium was dissolved in distilled water and made up to 100 cm3 to give solution H. • A 25.0 cm3 portion of solution H was titrated with 0.100 mol dm–3 HCl. 22.50 cm3 of acid was required to reach the end-point.

-2- NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l) • The H 2O2 content of solution H was found by titration of another 25.0 cm 3 portion with 0.0200 mol dm–3 KMnO4 in acidic medium. 10.0 cm3 of KMnO4 solution was required to reach the end-point. 2MnO4−(aq) + 5H2O2(aq) + 6H+(aq) → 2Mn2+(aq) + 5O2(g) + 8H2O(l) (a) Using the results of the HC l titration, calculate the total amount of NaOH in 100 cm 3 of solution H. (b) Using the results of the KMnO 4 titration, calculate the amount of H 2O2 in 100 cm3 of solution H. (c) Hence, calculate the amount of Na2O and Na2O2 formed during the burning of the sodium sample. Suggested solutions to the self -check questions are appended behind the practice questions. Please use these solutions to check through your working and your final answers. Consult your tutor if you have any questions. Practice Questions 5 Identify if the following equations represent a redox or non-redox reaction. Determine the change in oxidation number for the relevant element to support your answer. (a) 2KNO3(s) → 2KNO2(s) + O2(g) (b) 2CrO42−(aq) + 2H+(aq) → Cr2O72−(aq) + H2O(l) (c) Cu2O(s) + H2SO4(aq) → Cu(s) + CuSO4(aq) + H2O(l) (d) 2CH4(g) + 2NH3(g) + 3O2(g) → 2HCN(g) + 6H2O(g) [In (d), the oxidation numbers of hydrogen and nitrogen remain unchanged after reaction.] 6 Write balanced half -equations and the overall equation for each of the following reactions in acidic solution: (a) I−(aq) + H2SO4(aq) → I2(g) + H2S(g) (b) MnO42−(aq) → MnO2(s) + MnO4−(aq) (c) FeC2O4(aq) + Ce3+(aq) → Ce2+(aq) + Fe3+(aq) + CO2(g) [Note: FeC2O4 contains Fe2+ and C2O42ˉ. Determine the oxidation number of each element and find out which element(s) underwent oxidation or reduction.] 7 Write balanced half-equations and the overall equation for each of the following reactions in basic solution: (a) ClO−(aq) + Mn(OH)2(s) → Cl−(aq) + MnO2(s) (b) MnO4−(aq) + C2O42−(aq) → MnO2(s) + CO32−(aq) (c) ClO−(aq) → Cl−(aq) + ClO3−(aq)

-3- 8 RI 2014 Y5 CT Qn B1(c) Sodium metabisulfite, Na2S2O5, is often used as a food preservative. 100 g of preserved meat was boiled with hydrochloric acid. Na2S2O5 + 2HCl → 2NaCl + 2SO2 + H2O The sulfur dioxide gas produced reacted completely with 14.20 cm3 of 0.0100 mol dm−3 acidified potassium dichromate(VI). (a) Chromium(III) sulfate was formed in the reaction between sulfur dioxide and acidified potassium dichromate(VI). Write a balanced ionic equation for this reaction. (b) Calculate the concentration of sodium metabisulfite in the preserved meat in ppm (parts per million). [1 ppm = 1 g in 106 g of meat] 9 RI 2014 Y5 CT Qn A3 An aqueous solution containing 1 mole of C l2O7 oxidises 4 moles of H2O2 to form O2 and a chlorine–containing product. What is the chlorine–containing product of this reaction? A Cl– C ClO2– B ClO– D ClO3– 10 RI 2014 Y5 Promo Qn A2 An aqueous solution containing 0.010 mol of NaNO2 oxidises 0.005 mol of acidified (NH 4)2SO4 to form a single nitrogen- containing product. What is the nitrogen-containing product? A N2 C N2O B NO D NH2OH 11 Hydroxylamine, NH2OH, can be oxidised to nitrogen, N2, or nitrogen oxide, N2O, depending on the oxidising agent. (a) Write a half-equation for each of the oxidation reactions in an acidic medium. (b) In an experiment, 50 cm 3 of 0.050 mol dm −3 NH2OH was found to reduce 10 cm 3 of 0.50 mol dm−3 Fe3+ to Fe2+. Determine whether the NH 2OH is oxidised to N 2 or N2O by Fe3+ and write a balanced ionic equation for the reaction. 12 Solution FA1 contains 12.1 g dm−3 of sodium thiosulfate(VI) pentahydrate, Na2S2O3.5H2O. Solution FA2 contains 7.95 x 10–3 mol dm−3 of a compound MXO3. M is a univalent metal and X is the symbol of an unknown element. 25.0 cm3 of FA2 was mixed with ab out equal volumes of potassium iodide solution and dilute sulfuric acid. The iodine liberated required 24.40 cm3 of FA1 for reaction. (a) Calculate the number of moles of iodine liberated by 1 mole of XO3− when iodide ions are oxidised by XO3−. M+ does not react with iodide ions. (b) Using the answer obtained in (a), decide which of the following species could be the product obtained from the reduction of XO3−: XO2−, X+, X− or X2. (c) Hence, write a balanced equation for the reaction between XO 3− ions and iodide ions using the half-equation method.

-4- 13 RI Prelim 2018/4/1 (modified) Copper(II) ions react with iodide ions according to the following equation. 2Cu2+(aq) + 4I⁻(aq) → 2CuI(s) + I2 (aq) cream ppt brown solution The iodine produced in this reaction may be titrated against thiosulfate ions, S2O32ˉ, using starch solution as indicator. Thiosulfate ions react with iodine according to the following equation: 2S2O32–(aq) + I2(aq) → S4O62–(aq) + 2I–(aq) 41.00 cm3 of a solution containing CuSO 4 was placed in a 250 cm 3 volumetric flask. Deionised water was added to make a total volume of 250.0 cm 3 of the diluted solution. 25.0 cm 3 of this diluted solution was transferred to a conical flask. Excess K I was added, and the resultant solution titrated against 0.0500 mol dm –3 Na2S2O3, using starch solution as an indicator. It was found that 24.80 cm3 of Na2S2O3 was required for the dark blue colour to decolourise. Calculate the concentration of CuSO4 in the initial undiluted solution. 14 Solution X contains a mixture of a dibasic acid, ethanedioic acid (H2C2O4), and a salt, sodium ethanedioate (Na2C2O4). A student performed two titrations to determine the concentrations of ethanedioic acid and sodium ethanedioate in solution X. In the first titration, 25.0 cm3 of solution X required 14.75 cm3 of 0.100 mol dm–3 sodium hydroxide sol