2022 DHS Y6 H2 Prelim Paper 3_Suggested Solutions

© DHS 2022 9729/03 [Turn over Suggested Solutions 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. For Examiner’s Use Section A 1 20 2 20 3 20 Section B 4 / 5 20 Total 80 This document consists of 23 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] One 2s and one 2p atomic orbitals are mixed to form two 2sp hybrid orbitals that are degenerate / of equal energy. internuclear axis (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] From the Data Booklet, single covalent radius of hydrogen = 0.037 nm single covalent radius of carbon = 0.077 nm Bond length = sum of covalent radii = 0.037 nm + 0.077 nm = 0.114 nm (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] The carbon-hydrogen bond of ethyne is stronger. The sp hybridised carbon atom in ethyne has a higher percentage s character than the sp 2 hybridised carbon atom in ethene. Hence, the extent of orbital overlap between the sp hybridised carbon atom and H atom is greater, resulting in a shorter bond length. (iv) Write a balanced equation for the complete combustion of propyne, C3H4. [1] C3H4 + 4O2 → 3CO2 + 2H2O (v) A sample of propyne was burned in excess oxygen. When the remaining gases were passed through aqueous sodium hydroxid