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1 Name: Class: Redox revision questions Time: 50 minutes Marks: 50 Comments: Page of 7

2 Aqueous C 2 O ions react with MnO ions in acidic solution according to the equation 5 C 2 O + 2MnO + 6H + 2Mn CO 2 + 8H 2 O Under the same conditions Fe 2+ ions also react with MnO ions. How many moles of MnO ions are required to react exactly with one mole of Fe(C 2 O 4 ).2H 2 O? A 0.4 B 0.6 C 2.5 D 7.5 (Total mark) 2 A student carried out an experiment to find the mass of FeSO 4.7H 2 O in an impure sample, X. The student recorded the mass of X. This sample was dissolved in water and made up to 250 cm 3 of solution. The student found that, after an excess of acid had been added, 25.0 cm 3 of this solution reacted with 2.3 cm 3 of a mol dm 3 solution of K 2 Cr 2 O 7 (a) Use this information to calculate a value for the mass of FeSO 4.7H 2 O in the sample of X. (5) Page 2 of 7

3 (b) The student found that the calculated mass of FeSO 4.7H 2 O was greater than the actual mass of the sample that had been weighed out. The student realised that this could be due to the nature of the impurity. Suggest one property of an impurity that would cause the calculated mass of FeSO 4.7H 2 O in X to be greater than the actual mass of X. Explain your answer. (2) (Total 7 marks) 3 (a) The iron(ii) ions in well-water can be removed by oxidation using potassium manganate(vii) in alkaline solution. A mixture containing solid iron(iii) hydroxide and solid manganese(iv) oxide is formed. These solid products can be removed by filtration under reduced pressure. (i) Draw a diagram of the apparatus used for this filtration. Do not include the apparatus used to reduce the pressure. (2) Page 3 of 7

4 An equation representing the oxidation reaction is given below. 3Fe 2+ (aq) + KMnO 4 (aq) + 5OH (aq) + 2H 2 O(I) 3Fe(OH) 3 (s) + MnO 2 (s) + K + (aq) Calculate the mass, in grams, of KMnO 4 required to react with the iron(ii) ions in.00 dm 3 of well-water that has an iron(ii) concentration of mol dm 3. Give your answer to the appropriate precision. Show your working. (3) (iii) In practice, a slight excess of potassium manganate(vii) is used to treat the well-water. Although this treated water is safe to drink, this excess of potassium manganate(vii) is undesirable. Suggest one reason, other than colour, why the excess is undesirable. () (b) Suggest one reason why the colour of potassium manganate(vii) solution can be a source of error when using a volumetric (graduated) flask to prepare a standard solution. () (Total 7 marks) Page 4 of 7

5 4 This question is about Group 7 chemistry. (a) Sea water is a major source of iodine. The iodine extracted from sea water is impure. It is purified in a two-stage process. Stage l 2 + 2H 2 O + SO 2 2Hl + H 2 SO 4 Stage 2 2Hl + Cl 2 l 2 + 2HCl (i) State the initial oxidation state and the final oxidation state of sulfur in Stage. Oxidation state of S in SO 2... Oxidation state of S in H 2 SO 4... (2) State, in terms of electrons, what has happened to chlorine in Stage 2. () (b) When concentrated sulfuric acid is added to potassium iodide, iodine is formed in the following redox equations....ki +...H 2 SO 4...KHSO l 2 + S +...H 2 O (i) (iii) 8KI + 9H 2 SO 4 8KHSO 4 + 4l 2 + H 2 S + 4H 2 O Balance the equation for the reaction that forms sulfur. Deduce the half-equation for the formation of iodine from iodide ions. Deduce the half-equation for the formation of hydrogen sulfide from concentrated sulfuric acid. () () () (c) A yellow precipitate is formed when silver nitrate solution, acidified with dilute nitric acid, is added to an aqueous solution containing iodide ions. (i) Write the simplest ionic equation for the formation of the yellow precipitate. () Page 5 of 7

6 State what is observed when concentrated ammonia solution is added to this yellow precipitate. () (iii) State why the silver nitrate solution is acidified when testing for iodide ions. () (iv) Explain why dilute hydrochloric acid is not used to acidify the silver nitrate solution in this test for iodide ions. () (d) Chlorine is toxic to humans. This toxicity does not prevent the large-scale use of chlorine in water treatment. (i) Give one reason why water is treated with chlorine. () Explain why the toxicity of chlorine does not prevent this use. () (iii) Write an equation for the reaction of chlorine with cold water. () Page 6 of 7

7 (e) Give the formulas of the two different chlorine-containing compounds that are formed when chlorine reacts with cold, dilute, aqueous sodium hydroxide. Formula... Formula 2... () (Total 4 marks) 5 Oxidation and reduction can de defined in terms of electron transfer. (a) Define the term reduction in terms of electrons.... () (b) The oxide of nitrogen formed when copper reacts with nitric acid depends upon the concentration and the temperature of the acid. The reaction of copper with cold, dilute acid produces NO as indicated by the following equation. 3Cu + 8H + + 3Cu H 2 O + 2NO In warm, concentrated acid, NO 2 is formed. Oxidation states can be used to understand electron transfer in these reactions. (i) Give the oxidation states of nitrogen in, NO and NO Oxidation state in... Oxidation state in NO 2... Oxidation state in NO... Identify, as oxidation or reduction, the formation of NO 2 from ions in the presence of H + ions. Deduce the half-equation for the reaction. NO from... Half-equation.;... (iii) Deduce the half-equation for the formation of NO 2 from ions in the presence of H+ ions. Page 7 of 7

8 (iv) Deduce the overall equation for the reaction of copper with ions and H + ions to produce Cu 2+ ions, NO 2 and water. (8) (Total 9 marks) 6 (a) (b) In terms of electrons, what happens to an oxidising agent during a redox reaction?... Consider the following redox reaction. () SO 2 (aq) + 2H 2 O(l) + 2Ag + (aq) 2Ag(s) + (aq) + 4H + (aq) (i) Identify the oxidising agent and the reducing agent in this reaction. Oxidising agent... Reducing agent... Write a half-equation to show how sulphur dioxide is converted into sulphate ions in aqueous solution. (3) (c) Fe 2+ ions are oxidised to Fe 3+ ions by ions in acidic conditions. The ions are reduced to Cl ions. (i) Write a half-equation for the oxidation of Fe 2+ ions in this reaction. Deduce the oxidation state of chlorine in ions. (iii) Write a half-equation for the reduction of ions to Cl ions in acidic conditions. Page 8 of 7

9 (iv) Hence, write an overall equation for the reaction. (4) (d) Write an equation to show how sulphur is removed from impure iron obtained from the Blast Furnace. Identify the oxidising agent in this reaction. Equation... Oxidising agent... (2) (Total 0 marks) 7 The oxidation of ethanedioate (oxalate) ions by manganate(vii) ions can be represented by the half equations: C 2 O (aq) 2CO 2 (g) + 2e MnO (aq) + 8H + (aq) + 5e Mn 2+ (aq) + 4H 2 O(l) What volume (in cm 3 ) of 0.02 M KMnO 4 is required to oxidise completely a solution containing 0.02 mol of ethanedioate ions? A 25 B 40 C 250 D 400 (Total mark) Page 9 of 7

10 8 The following equation represents the oxidation of vanadium(iv) ions by manganate(vii) ions in acid solution. 5V 4+ + MnO 4 + 8H + 5V 5+ + Mn H 2 O What volume of mol dm 3 KMnO 4 solution is required to oxidise completely a solution containing 0.00 mol of vanadium(iv) ions? A 0 cm 3 B 25 cm 3 C 50 cm 3 D 00 cm 3 (Total mark) Page 0 of 7

11 Redox Mark schemes B [] 2 (a) moles of Cr 2 O 7 2 per titration = / 000 = (Cr 2 O H + + 6Fe 2+ 2Cr H 2 O + 6Fe 3+ ) Cr 2 O 2-7 :Fe 2+ = :6 If :6 ratio incorrect cannot score M2 or M3 moles of Fe 2+ = = Process mark for M 6 (also score M2) original moles in 250 cm 3 = = Process mark for M3 0 mass of FeSO 4.7H 2 O = = 5.33 (g) Mark for answer to M (allow 5.30 to 5.40) Answer must be to at least 3 sig figs Note that an answer of scores M, M4 and M5 (ratio : used) (b) (Impurity is a) reducing agent / reacts with dichromate / impurity is a version of FeSO 4 with fewer than 7 waters (not fully hydrated) Allow a reducing agent or compound that that converts Fe 3+ into Fe 2+ Such that for a given mass, the impurity would react with more dichromate than a similar mass of FeSO 4.7H 2 O OR for equal masses of the impurity and FeSO 4.7H 2 O, the impurity would react with more dichromate. Must compare mass of impurity with mass of FeSO 4.7H 2 O [7] 3 (a) (i) Flask with side arm Page of 7

12 Buchner funnel and horizontal filter paper Allow Hirsch funnel and horizontal filter paper. Do not allow standard Y-shaped funnel. If there is not a clear air-tight seal (labelled or drawn) between the funnel and the flask maximum mark. M r KMnO 4 = 58(.0) Mass = / 3 =.9 (g) Lose M2 if no working shown. Allow consequential mark on an incorrect M r for KMnO 4 (iii) Precision mark: three significant figures Allow if mass incorrect. (Unpleasant) taste Ignore smell. (b) Difficult to see meniscus / line on graduated flask Do not allow reference to over filling. [7] 4 (a) (i) M (+) 4 OR IV M2 (+) 6 OR VI 2 It / Chlorine has gained / accepted electron(s) OR Correctly balanced half-equation eg Cl 2 + 2e Credit or 2 electrons but not lone pair. The idea of reduction alone is not enough. 2Cl (b) (i) 6KI + 7H 2 SO 4 6KHSO 4 + 3I 2 + S + 4H 2 O Page 2 of 7

13 2I I 2 + 2e OR 8I 4I 2 + 8e Ignore charge on the electron unless incorrect. Or multiples. Credit the electrons being subtracted on the LHS. Ignore state symbols. (iii) H 2 SO 4 + 8H + + 8e H 2 S + 4H 2 O OR SO H + + 8e H 2 S + 4H 2 O Ignore charge on the electron unless incorrect. Or multiples. Credit the electrons being subtracted on the RHS. Ignore state symbols. (c) (i) Ag + + I AgI ONLY Ignore state symbols. Not multiples. The precipitate / solid / it does not dissolve / is insoluble / remains OR a white / cream / yellow solid / precipitate OR stays the same OR no (visible / observable) change OR no effect / no reaction Ignore nothing (happens). Ignore no observation. Page 3 of 7

14 (iii) The silver nitrate is acidified to react with / remove (an)ions that would interfere with the test Credit a correct reference to ions that give a false positive. prevent the formation of other silver precipitates / insoluble silver compounds that would interfere with the test Do not penalise an incorrect formula for an ion that is written in addition to the name. remove (other) ions that react with the silver nitrate If only the formula of the ion is given, it must be correct. (iv) react with / remove carbonate / hydroxide / sulfite (ions) Ignore sulfate. HCl would form a (white) precipitate / (white) solid (with silver nitrate and this would interfere with the test) It is not sufficient simply to state either that it will interfere or simply that the ions / compounds react to form AgCl (d) (i) Any one from Ignore to clean water. to sterilise / disinfect water Ignore water purification and germs. to destroy / kill microorganisms / bacteria / microbes / pathogens Credit remove bacteria etc / prevent algae. The (health) benefit outweighs the risk OR a clear statement that once it has done its job, little of it remains OR used in (very) dilute concentrations / small amounts / low doses Page 4 of 7

15 (b) (i) (+)5 or V or N 5+ (+)4 or IV or N 4+ (+)2 or II or N 2+ (iii) Cl 2 + H 2 O HClO + HCl OR Cl 2 + H 2 O 2H + + ClO + Cl OR 2Cl 2 + 2H 2 O 4HCl + O 2 Credit HOCl or ClOH Or multiples. Credit other ionic or mixed representations. Ignore state symbols. (e) In either order - Both required for one mark only Credit correct ionic formulae. NaClO (OR NaOCl) and NaCl Give credit for answers in equations unless contradicted. [4] 5 (a) Gain of electrons Reduction 4H + + NO 3 + 3e ( ) NO + 2H 2 O Page 5 of 7

16 (iii) 2H + + NO 3 + e ( ) NO 2 + H 2 O (iv) Cu + 4H NO 3 Cu H 2 O + 2NO 2 species balanced If electrons included, mark CE if these are not balanced [9] 6 (a) gains electrons () or accepts/takes electrons Allow an electron or just gains or reduction is gain of electrons, but NOT OILRIG even if stated Do not allow mention of electron pair(s) (b) (i) Oxidising agent: Ag + () (or Ag I) Reducing agent: SO 2 () (or S VI, not sulphur) 2 SO 2 + 2H 2 O SO 4 + 4H + + 2e () (or H 2 SO 4 + 2H + + 2e ) allow e i.e. no charge penalise E once only allow e on LHS (c) (i) Fe 2+ Fe 3+ + e () 5 () (or V or +5) 3 (iii) ClO 3 + 6H + + 6e Cl + 3H 2 O () (iv) ClO 3 + 6H + + 6Fe 2+ Cl + 3H 2 O +6Fe 3+ () Mark parts (i) to (iv) independently 4 Page 6 of 7

17 (d) Equation: Mg + S MgS () Oxidising agent: S () allow FeS + Mg MgS + Fe allow Ca Only award mark if first answer given unless no first answer then can allow 2 [0] D 7 8 D [] [] Page 7 of 7

(13) WMP/Jun14/CHEM2

(13) WMP/Jun14/CHEM2 13 Do not write outside the box 6 This question is about Group 7 chemistry. 6 (a) Sea water is a major source of iodine. The iodine extracted from sea water is impure. It is purified in a two-stage process.