2024 PU2 H1 Chem Paper 2 (ANS) M1

Class Adm No Candidate Name: This question paper consists of 18 printed pages and 2 blank pages. 2024 Preliminary Exams Pre-University 2 H1 CHEMISTRY 8873/02 Paper 2 Structured Questions 9 Sep 2024 2 hours Candidates answer on the Question paper. Additional materials: Data Booklet READ THESE INSTRUCTIONS FIRST Do not turn over this question paper until you are told to do so Write your name, class and admission number on all the work you hand in. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Section A Answer all the questions. Section B Answer one question. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. Question Section A Section B Total 1 2 3 4 5 6 7 Marks 11 18 16 6 9 20 20 80 H

2 Section A Answer all the questions in this section in the spaces provided. 1 Silicon compounds are used in the production of glass, ceramics, and refractory materials. Silicon dioxide is a major component of glass and is used in a wide range of applications, including construction, optics, and consumer products. For Examiners’ Use (a) A sample of silicon contains three isotopes, as shown in Table 1.1. Table 1.1 Isotope Percentage abundance / % 28Si 92.23 29Si 4.67 30Si 3.1 (i) Define the term relative atomic mass of silicon. ………………………………………………………………………………………………… [1] Weighted average mass of a silicon atom compared to 1/12 the mass of a 12C atom. (ii) Calculate the relative atomic mass of silicon from this sample, giving your answer in two decimal places. [1] Relative atomic mass of silicon = [(92.23 x 28) + (4.67 x 29) + (3.1 x 30)] / 100 = 28.11 (b) (i) State the full electronic configuration of silicon. Si : .…………………………………………………………………………………………… [1] 1s2 2s2 2p6 3s2 3p2 (ii) With reference to the Data Booklet, compare the values of second and third ionisation energies of silicon. Explain why these two energies are different and include an equation representing the third ionisation energy of silicon in your answer. …………………………………………………………………………………………………… ……………………………………………………………………………………………………

3 [Turn over ………………………………………………………………………………………………… [3] Si2+(g) → Si3+(g) + e- 2nd IE of Si: 1580 kJ mol-1 3rd IE of Si: 3230 kJ mol-1 The third IE of Si is larger than its second ionisation energy because the remaining electrons are closer to, and less shielded from the nucleus. (c) Describe the reaction, if any, of silicon dioxide with water. Include the approximate pH value of any resulting solution. ……………………………………………………………………………………………………… [1] SiO2 does not react with water and thus the pH remains at 7. (d) Silicon dioxide can react with carbon under high temperature to form silicon carbide, SiC. Silicon carbide has a high melting point of 2730 oC. It is very hard and can be used as a semiconductor material. (i) Suggest the type of structure and bonding of SiC. Structure : ………………………………………………………………………………………. Bonding : …………………………………………………………………………………… [1] Structure: giant covalent Bonding: strong covalent bonds between atoms (ii) Another method to produce silicon carbide uses silane, SiH4, and methane gas. SiH4 + CH4 → SiC + 4H2 Draw the ‘dot-and-cross’ diagram of silane. State and explain the shape and bond angle around the silicon atom in silane. …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… ………………………………………………………………………………………………… [3] Shape: Tetrahedral Bond angle: 109.5°

4 According to Valence Shell Electron Pair Repulsion (VSEPR) theory, SiH 4 has 4 bond pairs around silicon atom. Having a tetrahedral shape is to minimise bond pair bond pair repulsion. [Total: 11]

5 [Turn over 2 (a) Silver can exist in the form of nanoparticles. The properties of silver nanoparticles make them valuable in a wide range of fields, from healthcare to materials science. For Examiners’ Use (i) Define the term nanoparticle. …………………………………………………………………………………………………… ………………………………………………………………………………………………… [1] Nanoparticles are defined as particles with all three dimensions on the nanoscale of 1 to 100 nm. (ii) Suggest why silver nanoparticles are more effective catalysts compared to a single 1 cm3 block of silver metal. …………………………………………………………………………………………………… …………………………………………………………………………………………………… ………………………………………………………………………………………………… [2] Silver nanoparticles provide larger surface area to volume ratio compared to a block of silver metal, resulting in higher frequency of effective collisions and increases rate of reaction. (iii) Suggest how silver nanoparticles may have an effect on human health. …………………………………………………………………………………………………… ………………………………………………………………………………………………… [1] Silver nanoparticles may be ingested / inhaled / absorbed into the body through skin, leading to adverse effect on human health. (iv) Silver can be synthesised in the form of nanoparticles, while graphene can be synthesised as a nanomaterial. Describe the structure of graphene and use it to explain why graphene has a high tensile strength. …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… ………………………………………………………………………………………………… [2] Graphene is a single layer of carbon atoms covalently bonded together to form a giant covalent structure. Each C atom is attached to 3 other C atoms by a network of strong covalent bonds, resulting in high tensile strength.

6 (b) Silver bromide is a sparingly soluble salt that can dissolve in concentrated ammonia. When ammonia is added to silver bromide, a diamminesilver(I) complex ion, [Ag(NH3)2]+, is formed. AgBr(s) + 2NH3(aq) ⇌ [Ag(NH3)2]+(aq) + Br−(aq) Kc = 8.5 ×10-6 (i) Write the expression for the equilibrium constant for the above reaction, including its units. [2] Kc = [[𝐴𝑔(𝑁𝐻3)2]+][𝐵𝑟−] [𝑁𝐻3]2 no units (ii) In an experiment, solid silver bromide was added to aqueous ammonia. Given that the equilibrium concentration of NH 3 is 0.50 mol dm -3, calculate the equilibrium concentration of [Ag(NH3)2]+. [2] 8.5 x 10-6 = x2 / (0.50)2 x = 0.00146 mol dm-3 (iii) Explain the effect of adding excess concentrated ammonia on I the position of equilibrium II Kc I ………………………………………………………………………………………………..... ………………………………………………………………………………………………….... II ………………………………………………………………………………………………… ………………………………………………………………………………………………… [3] I When excess concentrated ammonia is added, by Le Chatelier’s Principle, position of equilibrium of reaction shifts to the right to decrease the concentration of ammonia. II Kc remains constant as it is only affected by temperature.

7 [Turn over (iv) Predict the sign for the enthalpy change when one mole of AgBr is dissolved in water. Explain your answer. …………………………………………………………………………………………………… ………………………………………………………………………………………………… [2] Positive. Since AgBr is sparingly soluble, energy released during the formation of ion - dipole interactions with water is insufficient to overcome the energy absorbed to break the strong electrostatic forces of attraction between Ag+ and Br- ions. (c) Table 2.1 contains the values of lattice energies