RI H2 CHEM P3 Prelim

© Raffles Institution 2016 9647/03/S/16 [Turn Over 1 RAFFLES INSTITUTION 2016 YEAR 6 PRELIMINARY EXAMINATION Higher 2 CHEMISTRY 9647/03 Paper 3 Free Response 16 September 2016 2 hours Candidates answer on separate paper. Additional Materials: Answer Paper Data Booklet READ THESE INSTRUCTIONS FIRST Do not open this question booklet until you are told to do so. Write your name, class and index number 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. Begin each question on a fresh sheet of paper. A Data Booklet is provided. Do not write anything in it. You are reminded of the need for good English and clear presentation in your answers. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 12 printed pages.

© Raffles Institution 2016 9647/03/S/16 [Turn Over 2 Answer any four questions. Begin each question on a fresh sheet of paper. 1 (a) Gallium trichloride, GaC l3, can be formed by direct combination of the elements. It has a melting point of 78 C. Suggest the type of chemical bonding present in GaC l3 and explain why gallium has a high tendency to form chlorides of this nature. [2] (b) Gallium hydride, with empirical formula GaH3, can be prepared from GaCl3. A 0.55 g sample of gaseous gallium hydride occupies a volume of 68 cm 3 at a temperature of 243 K and a pressure of 1.12 x 105 Pa. Determine the Mr of gallium hydride and hence its molecular formula. [3] (c) GaH3 reacts with (CH 3)3N at 178 K to give compound A, which has the following composition by mass: H, 11.0%; C, 37.8%; N, 14.7%; Ga, 36.5%. A molecule of compound A has only one Ga atom. (i) Determine the molecular formula of A. [1] (ii) Draw a structure to show clearly  the shape of A around the gallium atom and  the types of bonds around the gallium atom. [2] (d) Lithium aluminium hydride, LiA lH4, and lithium gallium hydride , LiGaH4, are both donors of hydride, H−. Thus, they are capable of reducing a large number of organic molecules. Suggest a possible reason why LiAlH4 is a stronger reducing agent than LiGaH4. [1] (e) LiAlH4 can be used to reduce amides to amines as shown below: 2 1 1 2 DEET is the active ingredient in many insect repellents. The following reaction scheme involves the preparation and reaction of DEET. LiAlH4 in dry ether B DEET step 3 LiAlH4 in dry ether step 1 step 2

© Raffles Institution 2016 9647/03/S/16 [Turn Over 3 (i) Suggest the structures of compound B and DEET. [2] (ii) Suggest the organic reactant used in step 2. [1] (f) Venturamide A is a cyclic peptide with anti -malarial activity. It has the following structure. Suggest reagents and conditions for the basic hydrolysis of Venturamide A and draw the structures of the organic products. Assume that the 2 rings, and , are inert in the reaction. [3] (g) The identity of the side -chain and p Ka values associated with aspartic acid, asp , and alanine, ala, are as follows : amino acid aspartic acid, asp alanine, ala R −CH2COOH −CH3 amino acid aspartic acid, asp alanine, ala pKa1 2.10 2.35 pKa2 3.86 9.87 pKa3 9.82 (i) Draw the structures of the major species present in a solution of the asp -ala dipeptide at each of the following pH values at room temperature. pH = 1 pH = 3.86 pH = 12 [3] (ii) Draw the zwitterionic form of asp -ala and write an equation to show how it can remove small amounts of H+. [2] [Total: 20]

© Raffles Institution 2016 9647/03/S/16 [Turn Over 4 2 (a) Hydrazine, N2H4, is used as a rocket fuel. It combusts in oxygen according to the equation: N2H4(l) + O2(g)  N2(g) + 2H2O(l) Using the following data, calculate the enthalpy change of combustion of hydrazine. compound Hf / kJ molˉ1 N2H4(l) +51 H2O(l) –286 [2] (b) The standard electrode potential for the O2/H2O half-cell was measured. O2(g) + 4H+(aq) + 4eˉ 2H2O(l) E = +1.23 V Draw a labelled diagram to show how the standard electrode potential of the O2/H2O half -cell was measured. Include in your diagram the direction of electron flow. [3] (c) (i) Hydrazine is increasingly used in fuel cells, and these direct hydrazine fuel cells are a more practical and viable source of energy as compared to hydrogen fuel cells. Write balanced half -equations, with state symbols, for the reactions taking place at the anode and the cathode when the fuel cell, using aqueous sodium hydroxide as electrolyte, is in operation. [2] (ii) Given that Ecell for the reaction that takes place in the fuel cell is +1.56 V, use values from the Data Booklet to calculate the standard electrode potential for the N2/N2H4 half-cell. [1] (iii) Suggest an advantage of using hydrazine as a fuel over hydrogen gas in a fuel cell. [1] wire connecting a pair of graphite electrodes air (contains O2 gas) H2O N2 N2H4(l) NaOH anode cathode

© Raffles Institution 2016 9647/03/S/16 [Turn Over 5 (d) 2-chloronitrobenzene undergoes nucleophilic aromatic substitution when heated with aqueous NaOH at 100 oC. + NaOH reaction I+ NaCl Cl 2-chloronitrobenzene (i) Suggest what is unusual about reaction I. [1] The mechanism for reaction I involves a negatively charged anion intermediate. Three resonance structures of the anion intermediate are shown below: .. OH Cl OH Cl OH Cl 1 2 3 . . . . (ii) Explain why structure 3 is the most stable resonance structure. [1] Compounds Q and R can undergo nucleophilic aromatic substitution that is similar to reaction I in (d). FCl compound Q compound R (e) Despite the stronger C –F bond in R as compared to the C –Cl bond in Q, R is more reactive than Q. Suggest why this is so. [2] (f) The structure of 2,4-dinitrophenylhydrazine is given below. 1 2 Suggest, with reasons, how the basicity of the N atoms labelled N -1 and N-2 on 2,4-dinitrophenylhydrazine compare with each other. [1] Cl NaCl Cl Cl Cl Cl

© Raffles Institution 2016 9647/03/S/16 [Turn Over 6 (g) Nitronium tetrafluoroborate, NO 2+BF4ˉ, is a nitronium salt which can be used as a nitrating agent for aromatic compounds. C6H6 + NO2+BF4ˉ  C6H5NO2 + HF + BF3 p-anisidine may be prepared using the following route. (Assume that the –OCH3 group is unreactive.) (i) Name the type of reaction and describe the mechanism for the reaction in step 1 . Show relevant lone pairs and charges, and use curly arrows to indicate the movement of electrons. [3] (ii) Suggest reagents and conditions for step 2. [1] Fats and oils are sometimes oxidised to form aldehydes and ketones that cause a rancid smell in food. p-anisidine is widely used for detecting the presence of