catholic high school chemistry paper 2 sect b prelim 2023

[Turn Over Name: Index Number: Class: CATHOLIC HIGH SCHOOL Preliminary Examination Secondary 4 (O-Level Programme) CHEMISTRY 6092/02 Paper 2 22 August 2023 1 hour 45 minutes Candidates answer on the Question Paper. No Additional Materials are required. READ THESE INSTRUCTIONS FIRST Write your name, index number and class 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 B Answer all three questions. The last question is in the form either/or. Answer all questions in the spaces provided. The number of marks is given in brackets [ ] at the end of each question or part question. A copy of the Periodic Table is printed on page 12. The use of an approved scientific calculator is expected, where appropriate. For examiner’s use only: B9 / 12 B10 / 8 B11 EITHER / OR / 10 Total / 30 This document consists of 12 printed pages. B

2 Section B Answer all three questions in this section. The last question is in the form of an either/or and only one of the alternatives should be attempted. B9 The pH scale The pH scale was developed in 1909 by Søren Sørensen to express the concentration of hydrogen ions. The scale is important for a wide range of areas in everyday life, such as treating soil, reducing tooth decay, medicine, and monitoring the acidity of water. The term ‘pH’ means ‘potential of hydrogen’, and the scale is the negative base 10 logarithm of the concentration of hydrogen ions in a solution. pH = - log10 [H+] where [H+] represents the concentration of hydrogen ions in a solution The concentration of hydrogen ions, which are also known as ‘ protons’, in a solution determines how acidic or basic the solution is. Since this concentration can vary drastically, a logarithmic scale is used. And since the scale is negative, the smaller the pH value, the higher the concentration of the ‘protons’. The Universal Indicator A Universal Indicator is a mixture of different types of indicators that displays several colour changes at various pH levels across the pH scale. It is used to detect the acidic or basic nature of a substance or a solution. It can be in the form of a paper strip or a solution. The main components of a Univers al Indicator, in the form of a solution, are thymol blue, methyl red, bromothymol blue and phenolphthalein. Table 9.1 indicator colour in lower pH range pH range in which it changes colour colour in upper pH range thymol blue red 1.2 to 2.8 yellow yellow 8.0 to 9.6 blue methyl red red 4.2 to 6.3 yellow bromothymol blue yellow 6.0 to 7.6 blue phenolphthalein colourless 8.0 to 10.5 pink

3 [Turn Over (a) Using ideas about sub-atomic particles, explain why a hydrogen ion is also known as a ‘proton’. …………………………….……………...………………………………………………………. ……………………………..……………...…………………………………………………... [1] (b) Table 9.2 shows the pH value of a solution given the concentration of hydrogen ions. Table 9.2 pH value concentration of hydrogen ions in solution in mol/dm3 1.3 0.0500 1.6 0.0250 1.9 0.0125 2.0 0.0100 (i) Determine the pH of a sample of 0.0250 mol/dm3 of sulfuric acid. [2] (ii) A student claims that the pH value of a solution is inversely proportional to the concentration of hydrogen ions in a solution. By means of a calculation, determine if the student’s claim is true or false. [3]

4 (c) Secondary colours such as purple, orange and green can be made by mixing primary colours which are red, blue and yellow. Red and blue make purple, red and yellow make orange, and yellow and blue make green. (i) Using relevant information, explain why a solution of pH 3 turns the Universal Indicator orange. ……………………….…………………...………………………………………………… ……………………….…………………...………………………………………………… ……………………….…………………...………………………………………………… ……………………….…………………...…………………………………………..… [2] (ii) A student performed a titration experiment using dilute hydrochloric acid and aqueous sodium hydroxide. The graph in Fig. 9.1 shows how the pH changes during his titration experiment. The graph shows an equivalence point which occurs at pH 7. The equivalence point is the point in a titration experiment at which the volume of titrant added from a burette is just enough to completely neutralise the solution in the conical flask. Fig. 9.1 0 10 20 30 40 50 60 volume of NaOH added / cm3 pH 14 12 10 8 6 4 2 0 equivalence point there is a definite ‘step’ in the pH change

5 [Turn Over An indicator is suitable to be used in a titration experiment if it fulfils the criteria below. These include: • has a narrow pH range in which it changes colour • has a distinctive colour at lower pH and a different colour at higher pH • changes colour only once during the titration • pH range of indicator in which indicator changes colour falls within the definite ‘step’ of the reaction Explain whether the Universal Indicator fulfils each of these criteria for the titration experiment above. ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ….………………………………………………..……………………………………… [4] [Total: 12]

6 B10 (a) Transition elements have variable oxidation states. State two other properties typical of transition elements. …………………………….……………...…………………………………………………..……. …………………………….……………...…………………………………………..……………. ……………………………..……………...……………………………………………..……... [2] (b) Figs. 10.1 to 10.3 show the ‘dot-and-cross’ diagrams of the compounds formed when an unidentified element J (not its actual chemical symbol) bonds with various elements. Fig. 10.1 Fig. 10.2 Fig. 10.3 J Cl Cl Cl J J F F F F H H H J x • x • x • x • x • x • x x x x x x x x x x x • x • x • x • •• •• •• •• •• •• •• •• •• •• •• •• •• •• •• •• •• •• •• •• ••

7 [Turn Over (i) Determine the oxidation state of J in the various compounds. chemical formula oxidation state of J J2F4 JCl3 JH3 [2] (ii) A student claim s that since element J has variable oxidation states, J must be a transition element. Give two pieces of evidence from Figs. 10.1 to 10.3 and your answer in (b)(i) which suggest that the student’s claim is false. Explain your reasoning. evidence…………………………………………………………………………………… ……………………………………………………………………………………………… reasoning.……………………………………………………………………………….… ……………………………………………………………………………………………… evidence…………………………………………………………………………………… ……………………………………………………………………………………………… reasoning.……………………………………………………………………………….… ………………………………………………………………………………………...… [4] [Total: 8]

8 EITHER B11 Haematite, limestone and coke are heated together in a blast furnace in the manufacture of iron. (a) Explain why each of the substances below are needed in a blast furnace. limestone ………………………………………………………………………………………... …………………………….……………...………………………………………………… ……. coke………………………………………………………………………………………… ……. …………………………….……………...………………………………………………… … [2] (b) Scrap high carbon steel can be recycled to make pure iron. T