TJC H2 CHEM P3 QN Prelim

1 9647/03/TJC Prelim 2014 [Turn Over CHEMISTRY (H2) 9647/03 Paper 3 Free Response Tuesday 16th September 2014 2 hours Candidates answer on separate paper. Additional materials: Answer paper Data Booklet READ THESE INSTRUCTIONS FIRST Write your name , Civics Group and Index number in the spaces provided on the cover page and on all the work you hand in. Write in dark blue or black pen on both sides of the paper. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer any four questions. A Data Booklet is provided. You are reminded of the need for good English and clear presentation in your answers. The number of marks is g iven in brackets [ ] at the end of each question or part question. At the end of the examination, fasten all your work securely together. This question paper consists of 15 printed pages.

2 9647/03/TJC Prelim 2014 [Turn Over Answer any four questions. 1 (a) The table below shows t he melting point s of aluminium fluoride, aluminium chloride an d silicon tetrachloride. Compound Melting point/ °C Aluminium fluoride 1290 Aluminium chloride 190 Silicon tetrachloride -90 (i) Explain the difference in their melting points in terms of their structure and bonding. (ii) With the aid of equations, explain the reactions that take place when silicon tetrachloride and aluminium chloride are mixed with excess water. [7] (b) Silver chloride is sparingly soluble in water. AgCl(s) ⇌ Ag+(aq) + Cl– (aq) (i) Briefly describe any difference in the solubility of silver chloride in water and in aqueous aluminium chloride. (ii) Given that the solubility product of silver chloride is 1 x 10 -10 mol2 dm-6, calculate the maximum [Ag+(aq)] before a precipitate starts to form in 0.0500 mol dm -3 of aqueous aluminium chloride. Silver ions form complexes with ammonia and with amines. Ag+(aq) + 2RNH2(aq) ⇌ [Ag(RNH2)2]+(aq) (iii) Write a KC expression for this reaction and state its units. (iv) KC has a numerical value of 1.7 x 10 7 when R = H. Using your expression for K C, calculate the [NH3(aq)] needed to change the [Ag+(aq)] in a 0.010 mol dm–3 solution of silver nitrate to the value that you calculated in (b)(ii). (v) Explain whether you would expect the K C for the reaction where R = C 2H5 to be greater or less than that for the reaction where R = H. [8]

3 9647/03/TJC Prelim 2014 [Turn Over (c) Compound A is a chloroalkane with the molec ular formula C 4H9Cl. Compound A rotates plane of polarised light. When compound A is reacted with aqueous sodium hydroxide, an optically inactive product is obtained. (i) Deduce the identity of compound A. (ii) Deduce the type of reaction that h as taken place when compound A is reacted with aqueous sodium hydroxide. (iii) With the aid of Data Booklet, explain how the rate of this reaction changes, if any, when the chloroalkane is replaced by an iodoalkane. [5] [Total: 20]

4 9647/03/TJC Prelim 2014 [Turn Over 2 This question is about magnesium and its applications. (a) Magnesium does not occur naturally in its elemental form. One method to produce elemental magnesium involves the thermal reduction of magnesium oxide. (i) Predict, with reasoning, whether the melting point of magnesium oxide is higher or lower than that of the Group II oxides further down the group. Magnesium oxide can be formed the decomposition of magnesium carbonate on heating. (ii) Write an equation for the decomposition of magnesium carbonate. (iii) State and suggest an explanation for the trend observed in the the rmal stability of the Group II carbonates. [5] Grignard reagent s are useful organometallic reagent s in organic synthesis, as it provides a convenient source of alkyl nucleophile (R –) that can attack an electron deficient carbon in organic functional groups such as carbonyls. One example of a Grignard reagent is phenylmagnesium bromide, which can be prepared by the reaction of magnesium metal (in excess) with bromobenzene using dry ether as the solvent: Br Mg ether MgBr

5 9647/03/TJC Prelim 2014 [Turn Over (b) The formation of Grignard reagents from aromatic halides takes place on the surface of the magnesium metal. The mechanism of this reaction is thought to involve two steps:  There is a transfer of a single electron from the magnesium surface to the halogen, followed by homolytic fission of the C 6H5–Br bond to give C 6H5 , Mg + and Br –. Br – immediately combines with the electron deficient Mg + to give a surface -bound radical, MgBr.  The two radicals then react together to give C6H5MgBr. .MgBr. .MgBr. Br MgBr Mg . . . . Step 1: Step 2: (i) Explain what is meant by the term free radical. (ii) Suggest which step in the mechanism above is the rate-determining step. Explain your reasoning. [3]

6 9647/03/TJC Prelim 2014 [Turn Over (c) 3-nitrobenzoic acid is an important precursor in pharmaceutical and organic synthesis. It can be synthesised from phenylmagnesium bromide via the following reaction scheme: Step I O2N CH2OHMgBr COOH Step III COOH Step II O H H (i) Suggest reagents and conditions for Steps II and III. (ii) Describe the mechanism for the reaction in Step III. (iii) Step I is an example of nucleophilic addition, which proceeds via two stages. The reaction kinetics of the reaction was investigated, and the following data was obtained. MgBr O H H C O H H MgBrStage 1: H2O Mg(OH)BrCStage 2: CH2OHO H H MgBr

7 9647/03/TJC Prelim 2014 [Turn Over Experiment Initial [C6H5MgBr] / mol dm-3 Initial [HCHO] / mol dm-3 Initial rate of reaction / mol dm-3 min-1 1 0.30 0.40 0.0240 2 0.10 0.40 0.0080 3 0.20 0.60 0.0240 Use these data to deduce the order of reaction with respect to each of the reagents, showing how you arrive at your answers. Hence, write the rate equation for the above reaction. (iv) The variation of the rate constant, k, for Step I with temperature, T, is given by the following equation: ln 1 2 k k = − R Ea        12 11 TT where Ea denotes the activation energy in J mol −1, R is the gas constant, and T is the temperature in Kelvin. Given that the rate constant and activation energy of Step I is 9.16  10−3 mol−1 dm3 s−1 and +102 kJ mol −1 at 25 oC respectively, show, using calculations, the effect of temperature change on the rate of the reaction when the temperature is raised by 5 oC. (v) Draw a labelled energ y profile diagram for Step I, given that the reaction is exothermic. [12] [Total: 20]

8 9647/03/TJC Prelim 2014 [Turn Over 3 Sodium propanoate, represented by the food labeling E281 in Europe, is used primarily as a mould inhibitor