NJC H1 CHEM P2 answers

1 NATIONAL JUNIOR COLLEGE SH2 PRELIMINARY EXAMINATION Higher 1 CANDIDATE NAME SUBJECT CLASS REGISTRATION NUMBER CHEMISTRY Paper 2 Structured Questions Answer Section A on the Question Paper. Additional Materials: Data Booklet 8872/02 Friday 28 Aug 2015 2 hours READ THE INSTRUCTIONS FIRST Write your subject class, registration number and name 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 paper clips, highlighters, glue or correction fluid. Section A Answers all questions. Section B Answer two questions on separate answer paper. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use A1 A2 A3 A4 B5 B6 B7 Total This document consists of 13 printed pages and 0 blank page.

2 Section A Answer all questions in this section in the spaces provided. 1 Menthol is an important compound extracted from the peppermint plant. It has important uses in flavouring. The structure of a derivative of menthol is shown below. COOH OH CH3CH3 CH3 (a) Name two other functional groups, other than the carboxylic acid, that are present in the above structure. 2° alcohol, alkene [1] (b) State the type of reaction that occurs, if any, and d raw the structure of the organic products, when the above compound is added to the following reagents. Reagent and conditions Structure of organic product(s), if any (i) Acidified KMnO4, heat COOH O CH3CH3 CH3 HO O O Type of reaction: oxidation (ii) H2, Pt COOH OH CH3CH3 CH3 Type of reaction: reduction (iii) CH3CO2H, conc. H2SO4, reflux COOH O CH3CH3 CH3 C O CH3 Type of reaction: condensation [6] [Total: 7] For Examiner’s use

3 2 (a) Sodium, magnesium and phosphorus are all elements in Period 3 of the Periodic Table. What will you see when these elements are burned separately in excess oxygen? Give an equation, with state symbols, for each reaction. [4] 2Na(s) + ½ O2(g) → Na2O(s) Sodium burns with a bright yellow flame. 2Mg(s) + O2(g) → 2MgO(s) Magnesium burns with a brilliant white flame. P4(s) + 5O2(g) → P4O10(s) Phosphorus burns with a white flame. [1 mark each colour, any wrong equation minus ½ mark] (b) On the axes below, sketch a graph to illustrate the variation of ionic radius of the elements sodium to chlorine. Explain the shape of your sketch. Across each isoelectronic series, Na+ to Si4+, and P 3- to Cl-, there is increase in nuclear charge. In each series, there is increase in effective nuclear charge since there is increasing no of protons attracting similar no of electrons. Hence size of ionic radii decrease across e ach series. The anions are bigger than cations as there is increased shielding effect due to an additional shell of shielding electrons. [3] 8 AJC PRELIM 2014 8872/02/H1 [Turn over (b) Elements of the third period in the Periodic Table show various trends in their physical properties. (i) On the axes below, sketch a graph to illustrate the variation of atomic radius of the elements from sodium to chlorine. (ii) Explain the shape of your sketch. [3] [Total: 10] atomic radius Na Mg Al Si P S Cl For Examiner’s use ionic radius

4 0 500 1000 1500 2000 2500 3000 3500 A B C D E F G H (c) The graph below shows the second ioni sation energies of eight consecutive elements A to H in Period 3 and 4 of the Periodic Table. (i) Deduce which group does element F belongs to? Explain your answer. Group no: 0 Explanation: Element G has the highest 2nd IE since the second most loosely held electron is removed from an inner quantum shell (n=3) which is closer to the nucleus and experiences greater nuclear attraction than that for element H whose second most loosely held electron is removed from the outer quantum shell (n=4). Therefore element G belongs to grp I and hence element F belongs to grp O. [3] (ii) Give the formula of the compound formed between elements A and E. AE3 [Total: 11] 2nd IE/ kJ mol-1 For Examiner’s use

5 3 The table below gives some information of various organic compounds. (a) Explain the difference in the acid strength of propa noic acid, 2-chloropropanoic acid and methanoic acid. 2-chloropropanoic acid is a stronger acid than propanoic acid since the electronegative Cl can further disperse the negative charge on the conjugate base, making it more stable than the conjugate base of propanoic acid, favouring its dissociation to give more H+, i.e. stronger acid. Propanoic acid is a weaker acid than methanoic acid since the ethyl group is electron donating , intensifying the negative charge on the conjugate base, destabilising it, making its dissociation less favourable, i.e. weaker acid. [4] (b) Given that propanoic acid dimerises in non -polar solvents, draw the structure of this dimer. [2] CH3CH2C O O H H O CCH2CH3 O      hydrogen bond (c) Explain the observed trend for the boiling points of propanol, propanoic acid and methyl ethanoate. Propanoic acid forms stronger hydrogen bonds than propanol due to the additional C=O group in –COOH that withdraws electron density towards itself, making the H in propanoic acid more protonic. Hence, more energy is required to break the stronger hydrogen bonds between propanoic acid, resulting in the highest boiling point. Methyl ethanoate has permanent-dipole dipole interactions between its molecules which are weaker than hydrogen bonds, lesser energy required to break them, hence lowest boiling point. [3] [Total: 9] Compound Formula Mr Ka / mol dm3 Boiling point / °C Propanoic acid CH3CH2COOH 74 1.3 x 105 141 2-chloropropanoic acid CH3CHClCOOH 108.5 1.5 x 103 172 Methanoic acid HCOOH 46 1.8 x 104 101 Propanol CH3CH2CH2OH 60 1.0 x 1018 97 Methyl ethanoate CH3OCOCH3 74 - 57 For Examiner’s use

6 4 Our main source of energy is the combustion of fossil fuels, namely coal, oil and natural gas. They give out large amounts of energy when burned, but there are disadvantages to their use. Natural gas is a combustible mixture of hydrocarbon gases , mainly methane, ethane, propane and butane . Contaminants such as nitrogen and hydrogen sulfide are also present in small amounts. The chart below outlines the typical makeup of natural gas before it is refined. Typical Composition of Natural Gas (by volume) Methane CH4 70-90% Ethane C2H6 0-20% Propane C3H8 Butane C4H10 Carbon Dioxide CO2 0-8% Oxygen O2 0-0.2% Nitrogen N2 0-5% Hydrogen sulfide H2S 0-5% Rare gases Ar, He, Ne, Xe trace Natural gas is usually considered sour if the hydrogen sulfide content