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Teaching notes This experiment is an interesting introduction to the electrolysis of brine. It is probably best not used as the first electrolysis that students encounter. They would really struggle to explain for themselves what is going on. It could be followed by the electrolysis of salt solution in industry. Students should be able to notice bubbles of gas at each electrode. At the positive electrode, the indicator turns red initially, and is then bleached to colourless. This indicates the presence of chlorine. At the negative electrode the indicator turns purple. The remainder of the solution stays green. The product at the negative electrode is hydrogen. This can be difficult for students to understand. Some of the water will ionise, that is, turn to hydrogen (H+) and hydroxide (OH-) ions. When the sodium chloride is dissolved in water, the ions forming the ionic solid separate out. This means that there are actually 4 ions present in the solution: H+, OH-, Na+ and Cl-. The negative ions are attracted to the positive electrode. The chloride ions are discharged (giving chlorine) in preference to the hydroxide ions. These are left behind in solution. At the negative electrode, the hydrogen ions are discharged (producing hydrogen gas) in preference to the sodium ions. These are also left behind in solution. Thus sodium hydroxide solution remains. This is the cause of the purple colour of the indicator at the negative electrode. In time, the green colour of the indicator in the middle would change too, as the ions diffuse through the resulting solution. Equations: 2H+ + 2e– → H [negative electrode, cathode] 2 2Cl– → Cl + 2e– [positive electrode, anode] 2 H O → H+ + OH– 2 Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/oxidation-and-reduction/ colourful-electrolysis,54,EX.html Health & Safety checked, November 2006 Updated 29 Oct 2008 163PDF Image | Colourful electrolysis 54
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