NJC_2025_H2_Chemistry_Prelim_P2_Ans

[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 16 September 2025 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. Answers all questions. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use 1 /16 2 /14 3 /17 4 /16 5 /12 Paper 2 Total /75 Marks Weightings Paper 1 /30 15% Paper 2 /75 30% Overall Percentag e Paper 3 /80 35% Paper 4 /55 20% Grade This document consists of 19 printed pages and 1 blank page.

2 NJC/H2 Chem Prelim/02/2025 Answer all the questions in the spaces provided. 1 925 silver, also known as sterling silver, is an alloy that is commonly used to make jewellery. It consists of 92.5% silver and 7.5% other metals, such as copper, by mass. Over time, the alloy can form a tarnish of Ag2S(s) when it reacts with hydrogen sulfide, as represented by the following equation. 2Ag(s) + H2S(g)  Ag2S (s) + H2(g) (a) (i) Write the full electronic configuration for copper. 1s2 2s2 2p6 3s2 3p6 3d10 4s1 Note: Cu has a special configuration of 3d10 4s1 Common mistakes - Putting 4s1 before 3d10 - Configuration written as 1s2 2s2 2p6 3s2 3p6 3d10 4s1 [1] (ii) State and explain the difference in atomic radii for silver and copper. Ag has one more filled electronic shell than Cu. The distance of its valence electron is further away from nucleus and experience higher shielding effect . This outweighs the higher nuclear charge in Ag . Nuclear attraction for t he outermost electron in Ag is weaker. Thus, atomic radius of Ag is larger. Common mistakes - Students mention shielding effect but did not state the further distance as a factor as well. - Another common mistake is that having more electrons/more inner subshells does not always means a further distance of the valence electrons from the nucleus. - Students mention both nuclear charge and shielding effect being higher for Ag but did not mention that the higher shielding effect and further distance outweighs the higher nuclear charge. - Students also often miss out nuclear attraction for the valence electrons. - Outweigh and not offset/nullified/cancelled out. [2] (b) The Ag 2S tarnish on sterling silver can be removed until only sterling silver remains. A student weighs a tarnished sterling silver sample both before and after removing the Ag2S and records the data in Table 1.1. Table 1.1 mass before Tarnish Removal