2025 H2 Chem Prelim P2 QP VJC

@VJC 2025 9729/02/PRELIM/2025 [Turn over CANDIDATE NAME CT GROUP VICTORIA JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 ……………………………………………….………….. …………………………….. CHEMISTRY 9729/02 Paper 2 Structured Questions 29 August 2025 2 hours Additional Materials: Data Booklet ________________________________________________________________________________________________________________ READ THESE INSTRUCTIONS FIRST Write your name and CT group on this cover page. 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 staples, paper clips, highlighters, glue or correction fluid. Answer all questions in the spaces provided on the Question Paper. 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 1 / 18 2 / 16 3 / 17 4 / 9 5 / 15 Total / 75 This document consists of 19 printed pages and 1 blank page

2 © VJC 2025 9729/02/PRELIM/2025 Answer all the questions in the spaces provided. 1 Alkynes are a class of organic compounds with the general formula CnH2n−2. (a) (i) With the aid of a labelled diagram, explain how the orbitals overlap to form the C≡C bond in ethyne, H–C≡C–H. ….………………………………………………………………………………………………….. ……………………………………………………………………………………………………... ……………………………………………………………………………………………………... ………………………………………………………………………………………………….. [3] (ii) Table 1.1 shows the carbon-hydrogen bond length in ethane, ethene and ethyne. Table 1.1 Molecule Carbon-hydrogen bond length / nm ethane 0.114 ethene 0.109 ethyne 0.106 Use the concept of hybridisation to explain the difference in bond length of the carbon - hydrogen bond between the molecules as shown in Table 1.1. ……………………………………………………………………………………………………... ……………………………………………………………………………………………………... ……………………………………………………………………………………………………... ………………………………………………………………………………………………….. [2]

3 © VJC 2025 9729/02/PRELIM/2025 [Turn over (b) Table 1.2 contains data that is relevant for this question. Table 1.2 Equation Ho / kJ mol–1 3C(s) + H2O(l) → CO(g) + C2H2(g) +401 2C(s) + O2(g) → 2CO(g) −221 2H2O(l) → 2H2(g) + O2(g) +572 (i) Write an equation to represent the standard enthalpy change of formation of C2H2(g). …………………………………………………………………………………………………. [1] (ii) Use data from Table 1.2 to calculate the standard enthalpy change of formation of C2H2(g). [2]

4 © VJC 2025 9729/02/PRELIM/2025 (c) (i) Using the bond energies in the Data Booklet , calculate the enthalpy change of combustion of ethyne shown in equation 1.1. equation 1.1 H–CC–H(g) + 5 2O=O(g) → 2O=C=O(g) + H–O–H(g) [2] (ii) The entropy change of combustion of ethyne is –2150 J K−1 mol−1 at 305 oC. Explain what is meant by the term entropy. ……………………………………………………………………………………………………... ………………………………………………………………………………………………….. [1] (iii) With reference to equation 1.1, explain why the entropy change of combustion of ethyne has a negative sign. …………………………………………………………………………………………………...... ………………………………………………………………………………………………….. [1]

5 © VJC 2025 9729/02/PRELIM/2025 [Turn over (d) Alkynes undergo similar reactions as alkenes. Ethyne can be reduced to ethane in a two - stage process using a transition metal as the heterogeneous catalyst as shown in Fig. 1.1. Fig. 1.1 The higher the activity of a catalyst, the more effective it is at catalysing the reaction. Fig.1.2 shows the relative activity of each catalyst against ΔHads, the enthalpy change of adsorption of hydrogen gas onto the catalyst surface in the reduction of alkyne. Fig. 1.2 (i) State the meaning of the term heterogenous catalyst. ……………………………………………………………………………………………………... …………………………………………………………………………………………………...... ………………………………………………………………………………………………….. [1] (ii) State which catalyst is the most effective in the reduction of alkyne. .…………………………………………………………………………………………………. [1] (iii) Use your knowledge of the mode of action of heterogenous catalysts , suggest an explanation for the trend observed in Fig.1.2. ……………………………………………………………………………………………………... ……………………………………………………………………………………………………... ……………………………………………………………………………………………………... ……………………………………………………………………………………………………... ………………………………………………………………………………………………….. [2] Hads / kJ mol–1 –180 –160 –140 –120 –100 –80 –60 –40 –20 0

6 © VJC 2025 9729/02/PRELIM/2025 (iv) In 1952, Herbert Lindlar found that adding a thin layer of impurity, such as lead(II) oxide, to palladium catalyst reduced its activity, allowing the reaction to stop at the alkene stage rather than reducing to the alkane as shown in Fig. 1.3. Fig. 1.3 This is also known as “poisoning” the catalyst. By considering the shape of the molecules in Fig.1.3, suggest how the addition of lead(II) oxide “poisons” the palladium catalyst. ……………………………………………………………………………………………………... ……………………………………………………………………………………………………... ……………………………………………………………………………………………………... ………………………………………………………………………………………………..... [2] [Total: 18]

7 © VJC 2025 9729/02/PRELIM/2025 [Turn over 2 An aquatic system thrives on a delicate balance based on key chemical processes. (a) Ammonia is the primary component of fish waste. When the concentration of ammonia in an aquatic system is too high, aquatic life is adversely affected. Ammonia can be removed with oxygen, in the presence of nitrifying bacteria, to form nitrite, NO2–, and water. (i) Write a balanced equation for the reaction of ammonia and oxygen. ………………………………………………………………………………………………….. [1] (ii) Draw the dot-and-cross diagram of nitrite ion, NO2–. [1] (iii) Some NO3– ions may also be formed from ammonia by the action of nitrifying bacteria. Given that the shape of NO 3– ion is trigonal planar , use VSEPR theory to explain the difference in the bond angles between NO2− and NO3– ion. ……………………………………………………………………………………………………... ……………………………………………………………………………………………………... ……………………………………………………………………………………………………... ………………………………………………………………………………………………….. [2] (iv) The ammonia levels in a 50 dm 3 freshwater aquarium tank was investigated. It is found that the concentration of dissolved ammonia and oxygen in the tank were 0.020 mol dm−3 and 0.030 mol dm−3 respectively. Upon adding nitrifying bacteria, 50% of ammonia was converted to NO2– ions after one hour. Use your answer to (a)(i) and the information given, d etermine the mass of NO2– ions formed. [2]

8 © VJC 2025 9729/02/PRELIM/2025 (b) Calcium and magnesium ions can be used to estimate the total dissolved solids (TDS) in an aquatic system. Different aquatic species require different TDS levels for optimal health and survival. (i) Explain why calcium has a lower first ionisation energy than magnesium. ……………………………………………………………………………………………………. ……………………………………………………………………………………………………. ……………………………………………………………………………………………………. ………………………………………………………………………………………………... [2] (ii) State two reasons why magnesium and ca