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’. …………………………….……………...……………………………………………………….