HCI H2 Chem Prelim P3

HWA CHONG INSTITUTION C2 PRELIMINARY EXAMINATION 9746 H2 CHEMISTRY Paper 3 Free Response 10 September 2008 2 hours Do not open this booklet until you are told to do so. READ THESE INSTRUCTIONS FIRST Write your name and class on all the work you hand in. Write in dark blue or black pen. You may use a soft pencil for any diagrams, graphs or rough working. Answer any four questions. Begin each question on a new piece of paper. A Data Booklet is provided. You may use a calculator. You are reminded of the need for good English and clear presentation in your answers. The number of marks is given in brackets [ ] at the end of each question or part question. At the end of the examination, fasten all your work securely together. Circle the question number that you have answered on the cover page provided. This question booklet consists of 8 printed pages.

2 Answer any four questions. 1 (a) (i) What do you understand by the term dynamic equilibrium? The ester, ethyl ethanoate, can be formed in the following reaction: CH3CO2H(l) + C2H5OH(l) = CH3CO2C2H5(l) + H2O(l) In an experiment, 0.5 mol of ethanoic acid and 1.0 mol of ethanol were shaken for a long time and allowed to reach dynamic equilibrium at room temperature. 0.42 mol of ethyl ethanoate was found to be present in the equilibrium mixture. (ii) Write an expression for the equilibrium constant, Kc, and calculate its value. (iii) Predict, with reasoning, the effect on the equilibrium when some aqueous sodium hydroxide is added to the equilibrium mixture. (iv) Give one other compound that can be used in place of CH 3CO2H to produce the same ester. [6] (b) Ethanoic acid is the main component in vinegar. When a 25.0 cm 3 sample of vinegar was titrated against 0.6 mol dm −3 aqueous potassium hydroxide using a pH meter, the following graph was obtained. (i) Using the information provided, show by calculations that the acid dissociation constant, Ka, of ethanoic acid has a numerical value of 1.84 × 10−5. Solution A was prepared by adding 10.0 cm 3 of the KOH solution to the 25.0 cm 3 vinegar sample. (ii) When a small amount of acid or base was added to solution A, its pH remained relatively constant. Explain qualitatively, with the aid of equations, why this is so. (iii) Calculate the pH of solution A. [8] 2.44 Volume of KOH / cm3 30.0 9746/03/HCI/C2 Prelim/2008

3 (c) Succinic acid, historically known as spirit of amber, plays an important biochemical role in the citric acid cycle. Its structure is shown below: HO2CCH2CH2CO2H succinic acid In no more than four steps, propose a reaction scheme to obtain succinic acid from ethanol. State the reagents and conditions required, and draw the structure of the compound obtained in each step of your reaction scheme. [4] Compound P is a structural isomer of succinic acid: (d) HO2CCHCHO CH2OH Compound P Suggest a simple chemical test that could be used to distinguish compound P from succinic acid. State the reagents and conditions used and the expected observation for each compound. [2] [Total: 20] 9746/03/HCI/C2 Prelim/2008

4 2 (a) (i) State the full electronic configuration of the Mn2+ ion. (ii) When acidified potassium manganate(VII ) is added to a solution of ethanedioic acid, H2C2O4, the reaction proceeds at a very slow rate. When a few crystals of manganese( II) sulphate are added to the mixture, the reaction speeds up and the colour changes from purple to red then finally to colourless. The red colouration is found to be due to the formation of an intermediate, Mn3+(aq). The reaction may be represented as: 2MnO4 –(aq) + 5C2O4 2–(aq) + 16H+(aq) → 2Mn2+(aq) + 10CO2(aq) + 8H2O(l) By considering relevant E, values from the Data Booklet and from the data below, show by means of balanced equations, how Mn 2+(aq) ions can act as a homogenous catalyst in the reaction. 2CO2 + 2e– = C2O4 2– E, = –0.49 V [5] (b) The red colouration described in (a) is due to an octahedral complex ion formed between Mn3+ and C2O4 2–. (i) Explain what you understand by the term complex ion. (ii) C2O4 2– is a bidentate ligand. Suggest the formula of the complex formed with Mn3+. (iii) Explain why many transition metal complexes are coloured whereas compounds of Group II metals are colourless. [7] (c) Potassium manganate(VII) is a useful oxidising agent, acidified solutions of which can be used to titrate reducing agents. (i) Describe in outline the practical details you would need to follow in order to use a standard solution of potassium manganate(VII ) to measure the percentage purity of a given sample of solid iron( II) ethanedioate, FeC 2O4. In your answer, you should include: • an equation for the titration reaction, • an outline of the sequence of steps to carry out the titration, • names of any other chemicals you would use. Do not include any details of the calculation. [3 mol of MnO4 – reacts with 5 mol of FeC2O4] (ii) When a 0.600 g sample of an impure iron( II) ethanedioate solid is dissolved in acid and titrated with 0.100 mol dm −3 KMnO 4, 20.50 cm 3 of oxidant solution is required to reach the end point. Calculate the percentage purity of the sample. [8] [Total: 20] 9746/03/HCI/C2 Prelim/2008

5 3 The elements, fluorine, chlorine, bromine and iodine are members of the halogen family. (a) The exceptional chemical reactivity of fluorine long defeated efforts to isolate it from its compounds. Henry Moisson, a French chemist, was the first to obtain fluorine by electrolysing KHF 2 dissolved in liquid HF using platinum-iridium alloy electrodes in a platinum vessel. (i) With reference to relevant data from the Data Booklet, explain why fluorine cannot be obtained by electrolysing aqueous sodium fluoride even if concentrated sodium fluoride is used. On the other hand, chlorine is made commercially by the electrolysis of concentrated sodium chloride (brine). The overall electrolytic process may be represented by the following equation: 2NaCl(concentrated) + 2H2O(l) 2NaOH(aq) + Cl⎯ ⎯ ⎯ ⎯⎯⎯ is electrolys → 2(g) + H2(g) (ii) Write the ion-electron half-equation for each of the electrode reactions in the electrolysis of brine using inert electrodes. (iii) Using the relevant half-equation in (a)(ii), calculate the current needed to produce 1 tonne of Cl2 per day. [1 tonne = 1 × 106 g] [8] (b) Chlorine and fluorine react exothermically to form an interhalogen compound, ClF3. (i) With the help of a diagram, describe and explain the shape of the C lF3 molecule. (ii) The interhalogen compounds are very strong oxidising agents. When gaseous C lF3 is added to water, a vigorous reaction occurs, giving three gases as the only products. The gaseous product mixture appears as white fumes. It not only turns moist blue litmus paper red but also bleaches it. In addition, it relights a glowing splint. Construct a balanced equation, including state symbols, for the reaction. [4] (c) Explain each of the following, writing balanced equations, including state symbols, for any chemical reaction that occur. (i) The boiling point of HF (19.5 °C) is higher than that of HI (–35.4 °C). (ii) HCl can be prepared by adding concentrated H 2SO4 to solid NaC l. However, the yield of H I is very low if concentrated H 2SO4 is added to