GYSS 2024 4E Chemistry Prelims Paper 2 QP Final

2 Section A Answer all questions. 1 Table 1.1 shows a list of substances. Table 1.1 ammonium carbonate barium chloride copper(II) sulfate potassium oxide silicon dioxide silver chloride sulfur dioxide zinc carbonate zinc sulfate Use the list of substances to answer the following questions. Each substance can be used once, more than once, or not at all. (a) Which substance(s) can be prepared by the reaction of a metal with a dilute acid? ………………………………………………………………………………………………. [1] (b) Which substance(s) will turn moist blue litmus paper red? ………………………………………………………………………………………………. [1] (c) Which substance(s) can be prepared by precipitation? ………………………………………………………………………………………………. [1] (d) Which substance(s) can react with both acids and alkalis to form gaseous products? ………………………………………………………………………………………………. [1] [Total: 4]

3 2 Separate beams of protons, neutrons and electrons are passed through an apparatus shown in Fig 2.1 . The apparatus contains a positively -charged plate and a negatively - charged plate, which sets up an electric field. Fig 2.1 Sub-atomic particles behave differently in the apparatus depending on their masses and charges. (a) (i) Draw an arrow to indicate the path of the particle which will be deflected the most by the electric field, and label the name of the particle. [1] (ii) Explain your answer. ………………………………………………………………………………………... ………………………………………………………………………………………... [1] (b) (i) In nuclear reactions, protons or neutrons are accelerated to high speeds and fired as ‘nuclear bullets’ at the nucleus of an atom of an element. Suggest why protons are less effective ‘nuclear bullets’ than neutrons. ………………………………………………………………………………………... ………………………………………………………………………………………... [1] (ii) When neutrons are fired at nucleus of atoms, the neutrons may become part of the nucleus of those atoms. Explain the effect of the presence of an extra neutron on the chemical properties of the new atoms formed. ………………………………………………………………………………………... ………………………………………………………………………………………... [1] [Total: 4] + – positively charged negatively charged beam of sub-atomic particles

4 3 Boron trifluoride, BF3 is often used as a versatile building block for other boron compounds. Fig 3.1 shows the bonding in a boron trifluoride molecule, showing only outer electrons. Fig 3.1 (a) Give a reason why the bonding in BF3 is typical and one reason why it is not typical of similarly bonded compounds. ………………………………………………………………………………………………. ………………………………………………………………………………………………. ………………………………………………………………………………………………. [2] (b) Boron trifluoride can react with caesium fluoride to form caesium tetrafluoroborate. BF3 + CsF → CsBF4 Caesium tetrafluoroborate consists of caesium cations and tetrafluoroborate anions. Tetrafluoroborate ion has a chemical formula BF4– and is formed when the fluoride ion from caesium fluoride donates a pair of electrons to the outermost electron shell of the central boron atom. Draw a ‘dot-and-cross’ diagram to show the arrangement of electrons in a tetrafluoroborate ion. Show outer electrons only. [2]

5 (c) Boron trifluoride and caesium t etrafluoroborate both contain boron and fluorine atoms but have vastly different physical properties. Boron trifluoride has a boiling point of −100.3 °C. Caesium tetrafluoroborate has a melting point of 555 °C. Explain, in terms of bonding and structure, why boron trifluoride and caesium tetrafluoroborate exist as different states at room temperature and pressure. ………………………………………………………………………………………………. ………………………………………………………………………………………………. ………………………………………………………………………………………………. ………………………………………………………………………………………………. ………………………………………………………………………………………………. ………………………………………………………………………………………………. [3] [Total: 7]

6 4 Titration experiments are commonly used to determine the concentration of chemical species in samples. (a) A student conducted a titration experiment to investigate the percentage purity of a sample. Substance Q is impure calcium carbonate. 50.00 cm3 of 0.200 mol/dm3 sulfuric acid was first added to a sample of Q. It is assumed that the impurities in Q do not react with acid. The excess sulfuric acid that did not react with Q was titrated with 0.320 mol/dm3 of dilute potassium hydroxide. The average volume of potassium hydroxide required for the titration was 25.00 cm3. Given that 0.835 g of substance Q was used in the experiment, calculate the percentage purity of Substance Q. percentage purity of Q = …………………% [4]

7 (b) The pH and electrical conductivity of the reaction mixture will change during titration experiments. In a separate experiment, dilute sodium hydroxide was slowly added from a burette to 24 cm3 of 0.100 mol/dm3 nitric acid. The graph in Fig 4.1 shows the pH changes during this titration experiment. Fig 4.1 The electrical conductivity of the reaction mixture was also measured. (i) The electrical conductivity of the reaction mixture increases from Q to R. Explain this observation. ……………………………………………………………………………………….. ……………………………………………………………………………………….. ……………………………………………………………………………………….. [1] (ii) The experiment is repeated using 24 cm3 of 0.100 mol/dm3 ethanoic acid. Given that the pH value of the ethanoic acid is 4, draw a graph on Fig 4.1 to show the results you would expect to obtain. [1] 14 12 10 8 6 4 2 0 0 10 20 30 40 50 pH volume of NaOH / cm3 Q R

8 (iii) Nitric acid and ethanoic acid used in the titration experiments have the same concentration. Explain why they have different pH values. ……………………………………………………………………………………….. ……………………………………………………………………………………….. ……………………………………………………………………………………….. ……………………………………………………………………………………….. [2] [Total: 8]

9 5 Some chemical reactions consist of multiple steps, with each step typically involving one or two reactants. One example of a multistep chemical reaction is the iodine clock reaction. The reaction uses peroxodisulfate ions, S2O82– to oxidise iodide ions to iodine in two separate steps. A small amount of iron(II) ions is also added to the reaction mixture during the reaction. In the first step, iron(II) ions react with peroxodisulfate ions to form iron(III) and sulfate ions. 2Fe2+ (aq) + S2O82– (aq) → 2Fe3+ (aq) + 2SO42– (aq) The iron(III) ions further react with iodide ions to form iodine. 2Fe3+ (aq) + 2I– (aq) → 2Fe2+ (aq) + I2 (aq) A brown solution is obtained at the end of the iodine clock reaction. (a) Explain the role of iron(II) ions in the iodine clock reaction. ………………………………………………………………………………………………. ………………………………………………………………………………………………. [1] (b) (i) Describe how iron(II) ions could be removed completely from aqueous iodine at the end of the experiment. ……………………………………………………………………………………….. ……………………………………………………………………………………….. ……………………………………………………………………………………….. [2] (ii) A few drops of potassium astatide was added to the solution after the Fe 2+ ions were removed at the end of the experiment. State the expected observations and explain your answer. ……………………………………………………………………………………….. ……………………………………………………………………………………….. ……………………………………………………………………………………….. [2] [Total: 5]

10 6 The chemical reactivity of four metals, W, X, Y and Z was investigated in a series of experiments. In the first experiment, small pieces of different metals were added to different solutions of metal ions in water. The results are summarised in Table 6.1. Table 6.1 W X Y Z W3+ reaction occurs reaction occ