2023 NJC H2 Chem Prelim P3 (Ans)

1 NJC SH2 Preliminary Examination 9729/03/23 [Turn over NATIONAL JUNIOR COLLEGE SH2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME SUBJECT CLASS REGISTRATION NUMBER CHEMISTRY Paper 3 Free Response Candidates answer on Question Paper. Additional Materials: Data Booklet 9729/03 14 September 2023 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. You may use a soft pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. 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 /16 3 /24 Section B 4 /20 5 /20 Paper 3 Total /80 This document consists of 28 printed pages.

2 NJC SH2 Preliminary Examination 9729/03/23 Section A Answer all the questions in this section. 1 (a) Table 1.1 list physical properties of some Period 4 elements. Table 1.1 property K Ca Fe As Se relative atomic mass 39.1 40.1 55.8 74.9 79.0 atomic radius (metallic) / nm 0.197 0.126 melting point / K 1112 1808 density / g cm−3 1.54 7.86 1st I.E. / kJ mol−1 944 941 2nd I.E. / kJ mol−1 3070 1150 (i) Explain why the atomic radius of Fe is less than that of Ca. [2] (ii) Use relevant data from the table to explain why the density of Fe is significantly greater than that of Ca. (no calculations are required) [2] (iii) Suggest why the melting point of Fe is significantly higher than the melting point of Ca. [2] (iv) With reference to the electronic configuration of K and Ca, explain why the 2nd I.E. of Ca is lower than that of K. [2] (v) With reference to the electronic configurations of As and Se, explain why the 1st I.E. of Se is lower than that of As. [2] (i) Fe has higher nuclear charge than Ca as it has more protons. Although Fe has more inner-shell electrons than Ca, the additional 3d electrons are poor shielding electrons. Thus, the increase in nuclear charge outweighs the increase in shielding effect. Overall, the valence electrons in Fe experience a stronger nuclear attraction, resulting in smaller atomic radius than Ca. (ii) Density = mass/volume