2022 DHS Y6 H2 Prelim Paper 3 QP

© DHS 2022 9729/03 [Turn over DUNMAN HIGH SCHOOL Preliminary Examination Year 6 H2 CHEMISTRY Paper 3 Free Response Questions Candidates answer on the Question Paper. Additional Materials: Data Booklet 9729/03 21 September 2022 2 hours READ THESE INSTRUCTIONS FIRST Write your centre number, index number, name and class at the top of this page. Write in dark blue or black pen. 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 questions. Section B Answer one question. A Data Booklet is provided. The use of an approved scientific calculator is expected, where appropriate. The number of marks is given in brackets [ ] at the end of each question or part question. Name: Centre/Index Number: Class: For Examiner’s Use Section A 1 20 2 20 3 20 Section B 4 / 5 20 Total 80 This document consists of 32 printed pages.

2 © DHS 2022 9729/03 Section A Answer all the questions from this section. 1 (a) Alkynes is a class of organic compounds with the general formula, CnH2n−2. (i) Describe what is meant by sp hybridisation with reference to one carbon atom in ethyne, C2H2. Draw the hybrid orbitals of the carbon atom. [2] (ii) Use relevant radius values from the Data Booklet to calculate the bond length of a single carbon-hydrogen bond. Show your working clearly. [1] (iii) Table 1.1 shows the carbon-hydrogen bond length in ethene and ethyne. Table 1.1 molecule carbon-hydrogen bond length/ nm ethene 0.109 ethyne 0.106 With reference to Table 1.1, state which carbon -hydrogen bond is stronger. Use the concept of hybridisation to explain the difference in bond length of the carbon-hydrogen bond between these two molecules. [2] (iv) Write a balanced equation for the complete combustion of propyne, C3H4. [1] (v) A sample of propyne was burned in excess oxygen. When the remaining gases were passed through aqueous sodium hydroxide, the gas volume was reduced by 0.450 dm3. Calculate the mass of propyne in the sample. Assume all gas volumes were measured at r.t.p. [2] ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………………………… …………………………………………………………………………………………

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4 © DHS 2022 9729/03 (b) The use of hydrogen gas with Lindlar’s catalyst is a selective method which reduces alkynes to form the cis-isomer of alkenes. As an example, but-2-yne, C4H6, can be reduced to give cis-but-2-ene only. H H H2(g) Lindlar's catalyst Suggest the structure of the alkene formed when compound A is reduced with the use of Lindlar’s catalyst. A [1] ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… (c) Compound B has the following structure. 1 2 B (i) State the isomeric relationship between B and your answer in (b). [1] (ii) A student made the following deductions about compound B: “Since carbon atoms labelled 1 and 2 are chiral and there are two carbon-carbon double bonds, compound B has 16 possible stereoisomers.” Explain where the student has gone wrong in his deductions. [3] (iii) Draw all the organic products that are formed when compound B is heated with acidified potassium manganate(VII). [1]

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6 © DHS 2022 9729/03 (d) Compound C is an isomer of compound B. C Draw the structure of the major product formed when compound C is reacted with excess HBr(g). Explain your answer. [2] ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… (e) Lithium aluminium hydride, LiA lH4, is a reducing agent commonly used in organic chemistry. (i) Assuming LiAlH4 as a source of hydride (H−) ions, suggest why the reduction of alkynes using LiAlH4 is likely not a suitable method. [1] (ii) LiAlH4 can be synthesised from aluminium chloride, which exists as a dimer, Al2Cl6, at room temperature. Draw a dot-and-cross diagram to illustrate the bonding present in Al2Cl6. [1] (iii) When heated, LiAlH4 decomposes to LiAl(s) as one of its products. LiAl(s) has a melting point of 718 C. It can conduct electricity when in solid and molten states. Suggest the structure of LiAl(s) and describe the bonding present. [2]

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