NJC 2025 H2 Chemistry Prelim P2 QP

[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.

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. ………………………………………………………………………………………………... [1] (ii) State and explain the difference in atomic radii for silver and copper. ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... [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 /g 54.23 mass after tarnish removal /g 52.34 Assuming that only Ag2S(s) is removed, calculate the amount of silver atoms removed. [1]

3 NJC/H2 Chem Prelim/02/2025 [Turn over (c) (i) Suggest and explain the relative magnitudes of the lattice energy of the silver compounds, Ag2S, Ag2O and Ag2Se. ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... [2] (ii) Using relevant data from the Data Booklet and Table 1.2, construct an energy level diagram to calculate the lattice energy of Ag2O. Table 1.2 standard enthalpy change of atomisation of silver +285 kJ mol­1 1st electron affinity of oxygen −141 kJ mol­1 2nd electron affinity of oxygen +798 kJ mol­1 standard enthalpy change of formation of Ag2O(s) −31 kJ mol­1 [4] (d) Rhodium plating is a process used to protect sterling silver from tarnishing. This involves electroplating (depositing) solid rhodium, Rh(s), onto the surface of the metal from an acidified solution of Rh2(SO4)3(aq). Oxygen gas is produced during this process.

4 NJC/H2 Chem Prelim/02/2025 One of the half equations involved in this reaction is Rh3+ + 3e ⇌ Rh Eꝋ = +0.76 V (i) Write the half equation for the formation of oxygen gas in this reaction and hence, write the balanced ionic equation for the overall reaction. ………………………………………………………………………………………………... ………………………………………………………………………………………………... [2] (ii) Calculate the value of Eꝋ cell for the overall reaction in (i). [1] (iii) Based on your answer to (ii), explain why this process requires the use of an external power source. ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... [1] (iv) Calculate the current that must be supplied for 3.5 g of Rh to be plated onto a piece of sterling silver in 3 minutes. [2] [Total : 16] 2 (a) Xenon is a noble gas and forms various fluorides with fluorine. Two of these are xenon difluoride, XeF 2, and xenon tetrafluoride, XeF 4, which are crystalline solids with melting points of 140 C and 112 C respectively.

5 NJC/H2 Chem Prelim/02/2025 [Turn over (i) Draw the dot-and-cross diagrams for XeF2 and XeF4 and hence state their molecular shapes. [3] (ii) Even though XeF 4 has more electrons and is hence expected to have greater intermolecular forces than XeF2, its melting point is lower. Suggest why XeF 2 has a higher melting point than XeF4. ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... [1] (iii) Using your knowledge of covalent bond formation, explain why ● xenon reacts with fluorine but neon does not. ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ● xenon reacts with fluorine but not with iodine. ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... [2] (b) P and Q are compounds containing the same functional groups. Both compounds ● readily decolourise bromine in the dark

6 NJC/H2 Chem Prelim/02/2025 ● liberate a gas with sodium metal ● do not have O atom bonded to an unsaturated carbon atom ● react with hot acidified potassium manganate(VII) to give the products as shown in Table 2.1. Table 2.1 compound products of oxidation P (C6H12O) Q (C5H8O) (i) Considering the molecular formulae of the two compounds together with the information given above, name the two functional groups that are present in compounds P and Q. ………………………………………………………………………………………………... [2] (ii) Suggest the structures of compounds P and Q. Compound P Compound Q [2]

7 NJC/H2 Chem Prelim/02/2025 [Turn over (c) Compound R, C9H18O2, has the same two functional groups as P and Q in (b). Upon strong oxidation by hot acidified KMnO 4, compound S, C6H12O2, and compound T, C3H4O3, are obtained. The following four reagents were used to test compounds S and T, and the results are shown in the table below. test reagent result of test with compound S compound T Na(s) fizzes fizzes NaHCO3(aq) no reaction fizzes I2(aq) + OH (aq), warm no reaction yellow ppt 2,4-DNPH orange ppt orange ppt (i) By considering the test results with Na(s) and NaHCO 3(aq), name the functional group that is present in S and T. Functional group present in S: ……………………….. Functional group present in T: ……………………….. [2] (ii) Suggest the structures of compounds S and T. Compound S Compound T [2] [Total : 14]

8 NJC/H2 Chem Prelim/02/2025 3 (a) A sodium ethoxide slurry is prepared by dissolving sodium in dry ethanol. When this slurry is transferred into liquid dimethyl ether, CH 3OCH3, at 40 C, the following equilibri