SAJC H2 CHEM P3

2 Answer any four questio ns 1 (a) Acidified potassium manganate, KMnO4, is a strong oxidising agent which can oxidise both X2+ and C2O4 2- in XC2O4. One of the products of this reaction is CO2. 15.00 cm3 of 0.1 mol dm-3 acidified KMnO4 was found to completely oxidise 25.0 cm3 of 0.1 mol dm-3 XC2O4 solution. (i) Write a balanced equation for the reaction between MnO4 - and C2O4 2- only. Hence, determine the volume of KMnO4 needed to oxidise C2O4 2- only. (ii) Using your answer in (a)(i), find the oxidation state of X in the product. [5] (b) The following table lists the boiling points of some organic compounds. Compound Formula M r Boiling point / °C A CH3CH2CH2CH2OH 74 118 B CH3CH2CH2CH2CH3 72 36 C 191 130 By reference to the type and extent of relevant intermolecular forces, explain the differences in boiling points between A and B and between A and C. [4] (c) The elements Y and Z can be one of the following: Na, Mg, A l, Si and P. Both the chloride and oxide of Y can react with aqueous sodium hydroxide. The oxide of Y can also react with aqueous hydrochloric acid. Element Z has a chloride and an oxide and both react vigorously with water to form a solution of pH = 2. Identify the elements Y and Z. Explain the above observations with the help of relevant balanced equations. [5] [Turn over

3 1 (d) Suggest a simple chemical test to distinguish between each pair of compounds. State the reagents and conditions needed and give the expected observations for each compound. (i) CH3CH2CCH2OH O and CH3CCH2CH2OH O P Q (ii) and R S [4] (e) Suggest a synthesis to convert CH 2=CHCH2Br into CH2=CHCHO. Include all reagents and conditions and draw all intermediates formed. [2] [Total：20]

4 2 2 (a) In one compartment of a voltaic cell, a graphite rod dips into an acidic solution of K2Cr2O7 and Cr(NO3)3. In the other compartment, a tin bar dips into a Sn(NO3) solution. A KNO3 salt bridge joins the two compartments. (i) Draw an apparatus diagram to show the set up of the cell. In your diagram, include the direction which electrons flow when the two compartments are connected by a wire. (ii) Give one use of the KNO 3 salt bridge in the set up. (iii) Write an equation for the overall cell reaction and calculate the overall cell potential under standard conditions. (iv) If a current of 0.2 A is drawn from the cell for 2.5 hours, what is the change in mass of the tin bar? [8] (b) (i) When iodide ions ( I-) and peroxodisulfate ions (S2O8 2-) are mixed, the reaction proceeds at a very slow rate. However, when aqueous CoCl3 is added, the reaction speeds up, producing SO4 2- and I2. By considering relevant E values from the data booklet, show by means of balanced equations, how CoCl3 carries out its role in this reaction. (ii) Adding excess dilute ammonia to an aqueous solution of copper(II) sulfate, causes the formation of a deep blue solution. When a small amount of aqueous hexadentate edta (edta 4-) is added, the deep blue solution changes to a paler blue. Explain the colour changes by means of balanced equations. Hence, arrange the ligands involved in order of increasing strength. [5] [Turn over

5 2 (c) Ibuprofen is an anti-inflammatory drug which is only available under doctor’s prescription in Singapore. It can be synthesized in the following manner. CH3 OH H3C CH3 CH3 CO2H H3C CH3 A Ibuprofen (i) Suggest a synthesis to convert A into Ibuprofen. Include all reagents and conditions and draw all intermediates formed. (ii) Describe with the aid of a diagram, the type of isomerism present in Ibuprofen. (iii) Explain if A or Ibuprofen has a higher pK a value. [7] [Total：20] [Turn over

6 3 (a) The values of pV/RT are plotted against p for one mole of the following gases at the specified temperature:  One mole of CH4 at 300K  One mole of CH4 at 500K  One mole of NH3 at 300K Given that graph A is obtained for 1 mole of CH4 at 300K, identify and explain what Graphs B and C represent. [4] Graph BpV/RT Graph A: 1 mole of CH4 at 300K Graph C Ideal gas p

7 3 (b) Methane can react with steam to give carbon monoxide and hydrogen gas in the presence of a nickel catalyst. CH4(g) + H2O(g) CO(g) + 3H2(g) When a 3:1 mole ratio of methane and steam at a total initial pressure of 4 atm is passed over nickel catalyst at 450 oC, the partial pressure of carbon monoxide at equilibrium is found to be 0.66 atm. (i) Write an expression for Kp and calculate its value, giving its units. (ii) Predict and explain the effect of increasing pressure on the equilibrium position and the value of Kp. (iii) The graph below shows how the fraction of CO produced varies with temperature. Fraction of CO in equilibrium Temperature/K Deduce whether the reaction of methane with steam is an exothermic or an endothermic process. [7]

8 3 (c) Benzene can be used as a starting reagent for many chemical compounds. (i) Name the type of reaction in Step I and outline the mechanism. (ii) List the reagents and conditions required for Steps II to III. (iii) Draw the structures of P and Q. (iv) State and explain which of the following compounds, R or S, will have a higher pKb value. C Br NH2 NO2 R C H NH2 NO2 S HH [9] [Total：20] [Turn over

9 4 (a) The major acidic component of soured milk is lactic acid, CH3CH(OH)COOH. When 10.0 cm3 of a solution of 0.08 mol dm-3 of the lactic acid was titrated against sodium hydroxide solution, the following titration curve was obtained. (i) The pH of lactic solution before adding any NaOH(aq) is 2.5. Deduce the acid dissociation constant, K a, of lactic acid. (ii) Calculate the pH of the solution at point A. (iii) Explain, with the aid of an equation, what happens when a small amount of acid is added at point A. (iv) Calculate the pH at the equivalence point and state a suitable type of indicator for this titration. [7] A 8.0 16.0 Volume of NaOH added/cm3 pH

10 4 (b) Barium is a group II element that is commonly used in glassmaking and fireworks to give a green colouration. Common compounds of barium include barium nitrate and barium chloride. (i) Write a balanced equation for the thermal decomposition of barium nitrate. (ii) How does the temperature required to decompose the nitrates of Group II elements vary down the group? Explain this trend. (iii) How would you expect the lattice energy of barium nitrate to differ from barium chloride? Explain. [5] (c) Stereoisomer A, with molecular formula C 6H11Cl, reacts with hot acidified potassium manganate(VII) solution to form a mixture of B and D. B is an unsymmetrical molecule which forms a yellow precipitate with hot alkaline iodine solution. Cold alkaline hydrogen cyanide is added to B and the product formed is reduced to form compound C. Compound D forms a white precipitate when