(12)1, 2 When Fe is reacted with Fe 3+ (aq) ions, Fe 2+ (aq) are formed. Assuming the reaction goes to completion, how many moles of Fe and of Fe 3+ (aq) would result in a mixture containing equal numbers of moles of Fe 3+ (aq) and Fe2+ (aq) once the reaction had taken place? Moles of Fe 1 1 1 2
A B C D
Moles of Fe + (aq) 2 3 5 3
(12)1, 7 Two separate electrolyses were performed as follows, under the same conditions of temperature and pressure. 1. When molten copper (II) chloride was electrolysed for five minutes, 100 cm 3 of chlorine were collected from the anode. 2. When aqueous sulfuric acid was electrolysed for five minutes, 200 cm 3 of oxygen were collected from the anode. If the current used in electrolysis 1 was I was I , what was the current used in electrolysis 2? A. 0.5 I B. I C. 2 I D. 4 I
(12)3, 2a [7 marks] Chlorine and iodine differ in their relative reactivity as oxidising agents. Chlorine is a gas under standard conditions. Although iodine is not very soluble in water, it is freel y soluble in KI (aq), according to the following equilibrium. () () () A solution containing I3- (aq) reacts as if it were a solution of iodine in water. i.
ii. iii.
Draw a fully labelled diagram of the electrochemical cell you would set up in order to measure the relative oxidising powers of chlorine and iodine under standard conditions. Describe the measurements you would make, and explain their significance. Write a balanced equation for the reaction that would take place if the electrodes of the cell were connected together by an external circuit.
(12)3, 2b [3 marks] When solutions of I- (aq) and Fe 3+ (aq) are mixed, an equilibrium is set up. i. ii.
Write a balanced equation for this equilibrium. Use the Data Booklet to suggest how the position of equilibrium, involving I - and Fe3+, would change in the presence of CN - (aq) ions. Explain your answer.
(11)1, 8 A student electrolyses concentrated aqueous sodium chloride using carbon electrodes. She ensures that the solution is continually stirred. What happens as the electrolysis proceeds? A. H2 is produced at the anode. B. HCl is formed. C. NaClO is formed. D. The NaCl gets more concentrated.
(11)1, 9 Use of the Data Booklet is relevant to this question. In many areas, tap water becomes slightly acidic due to dissolved carbon dioxide. By considering the relevant E θ values, which metal will not be dissolved by tap water containing carbon dioxide? A. B. C. D.
Copper Iron Lead Zinc
(11)1, 10 The diagram shows the apparatus needed to measure E θ for the reaction below.
() ()
What are the identities of X and Y? X Fe Fe Pt Pt
A B C D
Y Fe (aq) + Fe (aq) + Fe + (aq) Fe + (aq) Fe + (aq) + Fe + (aq) +
(11)1, 17 In acid solution, dichromate(VI) ions oxidise hydrogen peroxide, H 2O2. In alkaline solution, hydrogen peroxide oxidises chromium(III) ions. Which conditions of oxidation produce oxygen gas? A. Both acid and alkaline oxidations B. Neither acid nor alkaline oxidation C. Only the acid oxidation D. Only the alkaline oxidation
(11)1, 32 Which conditions are necessary when an electrode potential is measured using a standard hydrogen electrode as the reference electrode? 1. The use of hydrogen gas at 101kPa (1 atm) 2. Measurement of the e.m.f. when the current delivered by the cell is effectively zero 3. A pH of 1.0 for the solution at the hydrogen electrode (11)2, 4d [5 marks] Lead forms two chlorides, PbCl 2 and PbCl4. When chlorine is passed into a saturated solution of PbCl 2 in NH4Cl(aq), a yellow salt, Y, is formed which has the following composition by mass: Cl, 46.7%; H, 1.76%; N, 6.14%; Pb, 45.4% Y is ionic with a formula mass of 456. One formula unit of Y contains one type of anion and one type of cation in the ratio 1:2.
i. ii. iii. iv.
Calculate the empirical formula of Y. Suggest the formulae of each of the ions present in Y. What is the oxidation state of Pb in Y. Suggest the shape of the lead-containing ion.
(11)2, 4e [2 marks] When Y is treated with cold concentrated H 2SO4, PbCl4 is formed as an oily liquid. By using appropriate values from the Data Booklet , predict what, if anything, will happen when a solution of PbCl 4 in hydrochloric acid is added to a solution containing V 3+(aq). (11)3, 5f [5 marks] i. ii.
Write an equation for the reaction between chlorine and hot NaOH(aq). An acidified solution of the salt KClOx (x = 1, 2 or 3) will oxidise Fe 2+(aq) to Fe3+(aq) quantitatively, the chlorine being reduced to Cl -(aq). When 0.150g of the salt KClO x was reacted with 0.500moldm -3 Fe2+(aq) in the presence of H+(aq), 11.3cm3 of Fe2+(aq) was needed for complete reaction. Calculate the value of x and construct an equation for the reaction between Fe 2+(aq) and acidified KClOx(aq).
(10)1, 17 Use of the Data Booklet is relevant to this question. If iron is heated separately with chlorine, bromine and iodine, what are the likely products?
A B C D
Chlorine FeCl2 FeCl3 FeCl3 FeCl3
Bromine FeBr 2 FeBr 2 FeBr 3 FeBr 3
Iodine FeI2 FeI2 FeI2 FeI3
(10)2, 1 We usually think of iron occurring in compounds in the +2 or +3 oxidation states. However, other oxidation states of iron are possible; for example, potassium ferrate(VI), K 2FeO4, contains iron in the +6 oxidation state. Solutions containing the FeO 42- ion are dark red in colour. The FeO 42- ion is a stronger oxidising agent than the MnO 4- ion. In acidic conditions, FeO 42- ions are reduced to Fe 3+ ions. A student was given a solution, FA1, containing potassium ferrate(VI). She was asked to find the concentration of FeO42- ions in FA1 by titrating it against a standard solution of sodium ethanedioate, Na2C2O4. Ethanedioate ions are oxidised as shown in the following equation.
She tested the 0.100moldm -3 solution of Na 2C2O4 provided. She found that 10 drops of FA1, when suitably acidified, required 32 drops of this Na 2C2O4 solution for complete reaction. She correctly decided that the concentration of this solution was too low and prepared a more concentrated solution of Na2C2O4, FA2, for titration against the FA1 solution. a) Construct a balanced equation for the reaction between FeO 42- ions and C2O42- ions. [1 mark] b) Explain why the 0.100moldm -3 solution of Na 2C2O4 first provided would have been unsuitable for the use in the student’s titration. [1 mark] c) Write a plan for the volumetric analysis of FA1. You may use the reagents and apparatus normally found in a school or college laboratory. In your plan you should give: details, including quantities, for the preparation of FA2 from solid anhydrous Na2C2O4; the essential details of the titration procedure. [7 marks]
d) In the student’s titration, she found that xcm3 of FA1 reacted with exactly ycm3 of -3 FA2 of concentration Mmoldm . Outline how you would use her results to determine the concentration of FA1. [2 marks] e) Identify one potential safety hazard in this experiment and state how you would minimise this risk. [1 mark] (10)3, 3 The direct oxidation of alcohols in a fuel cell represents potentially the most efficient method of obtaining useful energy from a renewable fuel. a) The first alcohol to be used successfully in a fuel cell was methanol. At the anode methanol and water undergo the following reaction.
The electrons pass around the external circuit to the cathode. The protons move through the conducting polymer electrolyte to the cathode, where they react with oxygen, producing water. [5 marks] i. ii. iii. iv.
Construct an equation for the cathode reaction. Hence construct the equation for the overall reacti on, showing your working. The cell is capable of producing an e.m.f. of 1.18V. By using suitable data from the Data Booklet , suggest a value for the E θ of the CO 2/CH3OH electrode reaction. Suggest a possible advantage of using the methanol fuel cell compared to a hydrogen (H2/O2) fuel cell.
b) Ethanol is also being investigated as a possible fuel in fuel cells. [4 marks] i. Write the equation for the complete combustion of 1 mol of ethanol. ii. The standard enthalpy change of combustion of ethanol, ΔH cθ , has a value of -1367kJmol-1, and the entropy change for the same process is -140Jmol-1K -1. Calculate the standard Gibbs free energy change, ΔGθ , for the combustion of ethanol at 298K. iii.
θ
ΔG and E
θ cell are
related by the following equation.
where ΔG is the standard Gibbs free energy in joules per mole, z is the number of moles of electrons transferred during the redox reaction and F is the Faraday Constant. Use the equation you have written in (i) to decide on a suitable value for z, and hence θ calculate the E cell for the ethanol/oxygen fuel cell. θ
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