6.3 Electrolysing brine

1. 6.3 Electrolysing brine • EXAM TIPS: • You may be asked to apply your understanding to industrial processes such as hair removal, plating metals etc

2. Objectives: I will understand • That brine can be electrolysed. • The uses of the products of the electrolysis of brine. Outcomes: I will demonstrate I understand at Grade C by • Recall the products of the electrolysis of brine. • List some uses of the products of the electrolysis of brine Outcomes: I will demonstrate I understand at Grade A by • Explain how brine can be electrolysed. • Generate half equations for the electrolysis of brine.

3. WHAT’S BRINE? Brine is a solution of sodium chloride water (salt water!)

4. Predict what happens... during the electrolysis of sodium chloride O I L R I G Write down which electrode the ions may move to, then look at reactivity

5. Predict what happens...

6. Use the diagram on p188and annotate!

7. Chlorine In the electrolysis of NaCl solution, the negative chloride ions (Cl-) are attracted to the positive electrode (anode). Here, the Cl- ionslose electrons to make chlorine atoms, which then form chlorine molecules (Cl2). oxidized • Are the Cl- ions oxidized or reduced? • How many electrons are lost by each Cl- ion? • How many Cl- ions join to make a Cl2 molecule? • What is the half-equation for this redox process? one two 2Cl-  Cl2 + 2e-(oxidation)

8. For all ionic compounds containing a metal that is more reactive than hydrogen, electrolysis will produce hydrogen rather than the metal. Why isn’t Sodium formed? In the electrolysis of sodium chloride solution, the Na+ions might be expected to form sodium at the negative electrode (cathode). Instead, hydrogen gas is produced here. This is because sodium chloride solution also contains H+ ions from the water: H2O (l)  H+ (aq) + OH- (aq). At the negative electrode, both the H+ ions and the Na+ions are attracted. However, the Na+are more stable than the H+ ions so the H+ ions gain electrons; and the Na+ ions stay in solution.

9. potassium sodium calcium magnesium aluminium (carbon) Product at the negative electrode Ionic compound zinc iron potassium chloride lead copper sulphate (hydrogen) copper sodium bromide silver silver nitrate gold zinc chloride platinum If an ionic compound contains a metal that is more reactive than hydrogen, electrolysis of a solution of the compound will produce hydrogen, not the metal. increasing reactivity Complete the table for these compounds hydrogen copper hydrogen silver hydrogen

10. Products of the Electrolysis of NaCl The electrolysis of sodium chloride solution produces three very useful products: • Chlorine used for killing bacteria in water, for bleach and making plastics like PVC. • Hydrogen used for making margarine and fertilizers, and for rocket fuel. • Sodium hydroxide used in many chemical reactions, such as making soap,neutralizing acids and making paper.

11. Which of the 3 products would be used for… H • Margarine • PVC • Bleach • Soap • Paper • Rayon fibres • Detergents • Purification of aluminium ore • Hydrochloric acid manufacture Cl NaOH Cl NaOH Cl NaOH NaOH Cl NaOH Cl H

12. Objectives: I will understand • That brine can be electrolysed. • The uses of the products of the electrolysis of brine. Outcomes: I will demonstrate I understand at Grade C by • Recall the products of the electrolysis of brine. • List some uses of the products of the electrolysis of brine Outcomes: I will demonstrate I understand at Grade A by • Explain how brine can be electrolysed. • Generate half equations for the electrolysis of brine.

13. Electrolysis of dilute sulfuric acid Water can be electrolysed to give hydrogen (for fuel cells) and oxygen. Water is covalent and so is a poor conductor of electricity. There are a few free H+ & OH- ions: H2O (l)  H+ (aq) + OH- (aq). The conductivity of water can be improved by adding dilute sulfuric acid. This releases more ions so that more current flows during electrolysis, which creates hydrogen and oxygen. Which product will form at each electrode? Oxygen gas at Anode (+), Hydrogen at cathode (-)

14. Predict the products & equations

15. 2H2O (l) 2H2(g) + O2 (g) Electrolysis of sulphuric acid At the negative (cathode) electrode: 2H+ + 2e- H2(reduction) At the positive (anode) electrode: 4OH- 2H2O+ O2 + 4e-(oxidation) What is the overall equation for the electrolysis of dilute sulfuric acid? Twice as much hydrogen forms as oxygen. In water, there are 2 hydrogenatoms for every oxygen atom, so the ratio by volume, of H2to O2, is 2:1.

16. Summary notes • Brine is a solution of sodium chloride in water. • When it is electrolysed, hydrogen is produced at the negative electrode from hydrogen ions in the water. • Chlorine is produced at the positive electrode from the chloride ions. This leaves sodium ions and hydroxide ions (from water) in the solution. • The half equations for the reactions at the electrodes are: • At the positive electrode: 2Cl- Cl2 + 2e- • At the negative electrode: 2H+ + 2e- H2 • Sodium hydroxide is a strong alkali and has many uses including making soap, making paper, making bleach, neutralising acids and controlling pH. • Chlorine is used to kill bacteria in drinking water and swimming pools, and to make bleach, disinfectants and plastics. • Hydrogen is used to make margarine and hydrochloric acid. • You may be asked to apply your knowledge of electrolysis to other industrial processes.

17. Check your understanding What are the four ions present in brine? Explain why sodium hydroxide is left in the solution? Give two uses for each of the products. CLICK AGAIN FOR THE ANSWERS • ANSWERS: • Sodium Na+, hydrogen H+, chloride Cl-, hydroxide OH- • Hydrogen ions and chloride ions are discharged, leaving sodium ions and hydroxide ions in the solution. • Hydrogen: making margarine, making hydrochloric acid • Chlorine: sterilising water, making bleach, making disinfectants, making plastics • Sodium hydroxide: making paper, soap, bleach, to control pH

18. Extra Notes