NJC Prelim P2_Question_Paper

NJC SH2 Preliminary Examination 9729/02/17 [Turn over NATIONAL JUNIOR COLLEGE SH2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME SUBJECT CLASS REGISTRATION NUMBER CHEMISTRY Paper 2 Structured Questions Candidates answer on Question Paper. Additional Materials: Data Booklet 9729/02 Thursday 24 August 2017 2 hours READ THESE INSTRUCTIONS FIRST Write your subject class, registration number and name on all the work you hand in. Write in dark blue or black pen. You may use a soft pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid/tape. Answers all questions. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. The number of marks is given in brackets [ ] at the end of each question or part question. Appropriate significant figures and units are expected for final numerical answers. For Examiner’s Use 1 /14 2 /16 3 /11 4 /16 5 /18 Paper 2 Total /75 This document consists of 19 printed pages and 1 blank page.

2 NJC SH2 Preliminary Examination 9729/02/17 1 Nitrous oxide, N2O and nitrogen dioxide, NO2, are atmospheric pollutants. Nitrous oxide has an atmospheric residence time as long as 20 to 30 years but nitrogen dioxide has a residence time of only approximately 4 days. (a) Suggest how NO 2 is formed in a car engine and how it may be removed from car exhaust gases. [2] (b) (i) Draw the dot -and-cross diagrams of N 2O and NO 2, given that nitrogen is the central atom in both species. [2] (ii) Hence, explain for the difference in residence times between N2O and NO2. [1]

3 NJC SH2 Preliminary Examination 9729/02/17 [Turn over (c) In the gaseous state, NO2 can dimerise as follows. 2NO2(g) N2O4(g) The following data are for NO2(g) and N2O4(g) at 298 K. ΔHfo / kJ mol‒1 ΔSfo / J mol‒1K‒1 NO2(g) +33.2 +240 N2O4(g) +9.2 +304 (i) Calculate ΔHo and ΔSo for the above reaction. [2] (ii) Explain the signs of your calculated ΔHo and ΔSo. [2] (iii) The two gases in the above reaction can co-exist in an equilibrium. Calculate the temperature at which a dynamic equilibrium is established. [2]

4 NJC SH2 Preliminary Examination 9729/02/17 (iv) At 294 K, N2O4 liquefies. Given that the molar entropy change of vaporisation of N2O4 is +88 J mol ‒1K‒1, calculate the molar enthalpy change of vaporisation of N2O4. [1] (v) With reference to your answers in (c)(i) and (iv), calculate ΔH and ΔS for th e reaction below. 2NO2(g) → N2O4(l)