Chemical Effects of Electric Current

We know that metals conduct electricity, but many liquids do too. In this chapter you will build a simple tester with a cell, wires and a bulb or LED, and use it to test liquids such as lemon juice, vinegar, tap water, salt solution, sugar solution and vegetable oil. You will find that solutions of salts, acids and bases conduct electricity, while distilled water is a poor conductor; tap water conducts because of the salts dissolved in it. Since an LED glows even for a weak current, and a magnetic compass needle is deflected by a current, these are useful for detecting small currents. When an electric current passes through a conducting solution, it causes chemical reactions: bubbles of gas may form on the electrodes, metal may be deposited and the colour of the solution may change. These are the chemical effects of electric current. You will study electroplating, in which a thin layer of a metal such as copper, chromium, tin, zinc or silver is deposited on another metal object, and its uses.

Detailed Worksheet: Chemical Effects of Electric Current Section A - Definitions (10 marks) 1. What are good conductors and poor conductors of electricity? Give two examples of each among liquids. (2 marks) 2. Define electroplating. Name the effect of electric current on which it is based. (2 marks) 3. What are electrodes? To which terminal of the battery is the object to be electroplated connected? (2 marks) 4. List three observable changes that show that an electric current is causing a chemical reaction in a solution. (2 marks) 5. Why is an LED often used in place of a torch bulb in a tester for liquids? Name one other device that can detect a weak current. (2 marks) Section B - Calculations and Applications (15 marks) 6. Classify the following liquids as good or poor conductors of electricity: distilled water, tap water, lemon juice, vinegar, sugar solution, common salt solution, vegetable oil and kerosene. Give a reason for the behaviour of distilled water and tap water. (3 marks) 7. In a copper plating experiment, the copper plate had a mass of 50.0 g before the experiment and 48.8 g after it. The iron key being plated had a mass of 20.0 g at the start. Assuming all the copper removed from the plate is deposited on the key, find the final mass of the key. To which terminal was each connected? (3 marks) 8. When an electric current is passed through water made conducting with a few drops of dilute sulphuric acid, using carbon electrodes, bubbles form at both electrodes. Name the gas formed at the positive electrode and at the negative electrode. If 20 mL of gas is collected at the negative electrode, about how much gas would be collected at the positive electrode in the same time? (3 marks) 9. Boojho tested lemon juice with a tester containing a torch bulb, and the bulb did not glow. Explain whether this proves that lemon juice is a poor conductor. Describe two changes he could make to the tester to detect the current, and explain why a compass needle is deflected when current flows in a nearby wire. (3 marks) 10. Match each metal with its use in electroplating and give a reason for one of them: chromium, tin, zinc, silver. Uses: coating iron food cans, plating jewellery and ornaments, coating iron bridges and pipes, plating bathroom taps and car parts. (3 marks) Section C - Diagrams (10 marks) 11. Draw a neat labelled diagram of a tester for checking whether a liquid conducts electricity, using a battery, an LED, connecting wires and a beaker of the liquid. Show which lead of the LED is connected to the positive terminal. (4 marks) 12. Draw a labelled diagram of the set-up for electroplating copper on an iron object. Label the battery, switch, copper plate, iron object, copper sulphate solution and the terminals. (3 marks) 13. Draw a labelled diagram of the potato activity in which two copper wires connected to a cell are inserted into a cut potato, and mark where the greenish-blue spot appears. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Explain why distilled water is a poor conductor while tap water, sea water and rain water collected in a dirty container may conduct electricity. Using this, explain why we should never handle electrical appliances with wet hands or touch switches during a storm when standing in water. (5 marks) 15. Ravi wanted to copper-plate an iron spoon, but he connected the spoon to the positive terminal and the copper plate to the negative terminal. Predict what would happen. Explain the correct arrangement, why the colour of the copper sulphate solution does not fade during correct plating, and why a few drops of dilute sulphuric acid are added to the solution. (5 marks) 16. Chromium has a shiny appearance, does not corrode and resists scratches, but it is expensive and difficult to work with. Explain why objects are usually made of cheaper metals and only chromium-plated. Electroplating factories produce used solutions containing metal compounds. Explain why their disposal is a serious environmental concern and suggest one responsible practice. (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Draw all circuit diagrams neatly using a ruler, and never test the electricity supply from the mains or a generator in experiments; use only cells.