YIJC 2023 H2 Chem Prelim Paper 3 Tcopy (1)

©YIJC [Turn over YISHUN INNOVA JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME CG INDEX NO CHEMISTRY Paper 3 Free Response Candidates answer on the Question Paper. Additional Materials: Data Booklet 9729/03 14 September 2023 2 hours READ THESE INSTRUCTIONS FIRST This document consists of XX printed pages For Examiner’s Use Section A 1 / 19 2 / 22 3 / 19 Section B 4 or 5 / 20 Penalty units significant figures Overall / 80 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 an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Answer all questions in the spaces provided on the Question Paper. If additional space is required, you should use the pages at the end of this booklet. The question number must be clearly shown. Section A Answer all the questions. Section B Answer one question. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. 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.

2 ©YIJC [Turn over Section A Answer all the questions in this section. 1 Alcohols are important organic compounds used in a wide variety of chemical reactions and frequently used as solvents. (a) In a fuel cell, t he direct oxidation of alcohols represents potentially the most efficient method of obtaining useful energy from a renewable fuel. For example, Direct Isopropanol Fuel Cell (DIFC) uses propan-2-ol in the fuel cell. The diagram below shows a DIFC. The Proton Exchange Membrane (PEM) allows the transfer of protons between the half-cells. The standard cell potential, Ecell o of DIFC is +1.07V. (i) By considering the given diagram , write the half -equations for the reactions occurring at electrodes A and B during discharge. Hence, write the overall equation for this reaction. [3] Electrode A: CH3CH(OH)CH3 → CH3COCH3 + 2H+ + 2e Electrode B: O2 + 4H+ + 4e → 2H2O Overall: 2CH3CH(OH)CH3 + O2 → 2 CH3COCH3 + 2H2O (ii) In certain DIFC, air containing oxygen gas could be supplied to the H 2O/O2 half- cell. Explain how the Ecell o would change, when pure oxygen, instead of air, is used for the O2/H2O half-cell. [2] With high concentration of O2, position of O 2/H2O equilibrium shifts to the right, to remove O2, resulting in a more positive E(O2/H2O) value. Hence, since Ecell = E(electrode B) – E(electrode A), Ecell becomes more positive. O2 H2O H+ H+ H+ Load electrode A electrode B PEM

3 ©YIJC [Turn over (iii) By considering the overall equation in (i), calculate G for the DIFC. [1] G = -nFEcell = -(4)(96500)(+1.07) = -413 020 J mol-1 = -413kJ mol-1 (b) The table below gives data on the boiling points of some alcohols. alcohol boiling point/ ºC ethanol 78 propan-1-ol 97 propane-1,2-diol 188 propane-1,3-diol 213 (i) In terms of structure and bonding, explain why the boiling point of ethanol is lower than the boiling point of propan-1-ol. [2] Both ethanol and propan -1-ol have simple molecular structures, with (intermolecular hydrogen bonds ) and instantaneous dipole -induced dipole (id -id) interaction. Propan-1-ol has a larger electron cloud size or is more polarisable and forms stronger intermolecular id -id interaction. More energy is required to break the stronger id-id in propan-1-ol, giving rise to higher boiling point. (ii) Suggest why propane-1,2-diol has a lower boiling point than propane -1,3-diol, despite both alcohols having same molecular mass. [1] Due to the proximity of 2 -OH groups in propane-1,2-diol, it can form intramolecular hydrogen bonds. Less energy is required to break the less extensive intermolecular hydrogen bonds in propane-1,2-diol giving rise to a lower boiling point. (c) Alcohols, such as ethanol, are soluble in water. Draw a labelled diagram to show the significant force of attraction between a water molecule and an ethanol molecule. Include the name of the attraction in your diagram. [2]

4 ©YIJC [Turn over (d) Alcohols can undergo substitution reactions in the presence of strong acids and sodium halides. For example, 1 -methylcyclohexanol reacts with hot concentrated H 2SO4 and NaBr to form 1-bromo-1-methylcyclohexane. When the reaction was investigated kinetically, it was found that its rate was independent of [Br –], but was first order with respect to [H +] and with respect to [1-methylcyclohexanol]. The mechanism for this reaction proceeds via 3 steps. Step 1 is a fast step whereby the alcohol is protonated as shown below. In step 2, a reactive carbocation is formed. In step 3, the product 1-bromo-1-methylcyclohexane is formed. Give equations for step 2 and 3 of this mechanism. Identify which of the steps is the rate- determining step. Show relevant lone pairs of electrons, charges and use curly arrows to indicate movement of electron pairs. [3]

5 ©YIJC [Turn over Step 2 is the rate-determining step. (e) The substitution reaction can be used as the first step in the following synthesis of compound Y from compound V, 3-hydroxy-4-methylpentanal. (i) Give the reagents and conditions for step 2 and 3 and draw the structure of X. [3] Step 1: ethanolic NaCN, heat Step 2: LiAlH4 in dry ether, room temp. or H2, Ni, heat or H2, Pt, room temp. X: (ii) W can be eliminated to give a mixture of three isomeric products with the molecular formula C6H10O. Draw the structures of the products formed. [2] [Total: 19] 2 (a) Limonene can be obtained via a series of organic reactions involving the Diels–Alder reaction. The Diels–Alder reaction is a cycloaddition mechanism involving a diene and a dienophile as shown below: diene dienophile

6 ©YIJC [Turn over The dienophile acts as the electrophile in the Diels–Alder reaction. Isoprene, a diene, can be used to synthesise Limonene and A using the cycloaddition mechanism. Reaction I: Isoprene Limonene Reaction II: Isoprene A (i) With reference to the cycloaddition mechanism, explain qualitatively why the rate of reaction II is faster than that for reaction I. [1] For reaction II: The electron withdrawing C=O group on the dienophile causes it to be more electron-deficient / decreases electron density of alkenes, thus making the dienophile a stronger electrophile. Hence, rxn is faster. (ii) D may also be synthesised from the Diels–Alder reaction. D Suggest the structural formula of reactants B and C from which D may be synth