1. Electrolysis Of Brine
Noadswood Science, 2012

2. Friday, April 14, 2017
Electrolysis Of Brine
To understand the electrolysis of brine (salty water)

3. Ionic
Ionic substances form when a metal reacts with a non-metal – they contain charged particles called ions
For example, sodium chloride forms when sodium reacts with chlorine – it contains positively charged sodium ions and negatively charged chloride ions
Ionic substances can be broken down by electricity

4. Electrolysis
Electrolysis is the process by which ionic substances are decomposed (broken down) into simpler substances when an electric current is passed through them
For electrolysis to work, the ions must be free to move – ions are free to move when an ionic substance is dissolved in water or molten
For example, if electricity is passed through copper chloride solution, the copper chloride is broken down to form copper metal and chlorine gas…

5. Electrolysis
Positively charged ions move to the negative electrode during electrolysis – they receive electrons and are reduced
Negatively charged ions move to the positive electrode during electrolysis – they lose electrons and are oxidised
OILRIG – oxidation is loss, reduction is gain

6. Electrolysis Products
Ionic substances in solution break down into elements during electrolysis – different elements are released depending on the particular ionic substance…

7. Negative Electrode
At the negative electrode, positively charged ions gain electrons – this is reduction (ions have been reduced)
Metal ions and hydrogen ions are positively charged – whether you get the metal or hydrogen during electrolysis depends on the position of the metal in the reactivity series: -
The metal will be produced if it is less reactive than hydrogen
Hydrogen will be produced if the metal is more reactive than hydrogen
E.g. the electrolysis of copper chloride solution produces copper at the negative electrode, but the electrolysis of sodium chloride solution produces hydrogen

8. Element Given Off At +ve Electrode
Positive Electrode
At the positive electrode, negatively charged ions lose electrons
This is oxidation – the ions have been oxidised
-ve Ion In Solution
Element Given Off At +ve Electrode
Chloride, Cl-
Chlorine, Cl2
Bromide, Br-
Bromine, Br2
Iodide, I-
Iodine, I2
Sulfate, SO4-2
Oxygen, O2

9. Ionic Compounds & Electrolysis
Common ionic compounds and the elements released when their solutions are electrolysed: -
Ionic Substance In Solution
Element At –ve Electrode
Element At +ve Electrode
Copper chloride, CuCl2
Copper
Chlorine
Copper sulfate, CuSO4
Oxygen
Sodium chloride, NaCl
Hydrogen
Hydrochloric acid, HCl
Sulfuric acid, H2SO4

10. Brine
Brine is concentrated sodium chloride solution – if an electric current is passed through it, hydrogen gas forms at the negative electrode and chlorine gas forms at the positive electrode (a solution of sodium hydroxide forms)
Sodium metal is not deposited at the negative electrode as it is too reactive for this to happen – hydrogen is given off instead…

11. Experiment
Complete the brine electrolysis experiment – write the equation for the electrolysis of brine at the cathode (-ve) and anode (+ve)

12. Brine
Sodium chloride dissolved in water is called brine – electrolysis of brine gives hydrogen at the cathode and chlorine at the anode (sodium hydroxide remains dissolved in the solution)
The reactions at each electrode are ½ equations – the ½ equations are written so that the same number of electrons occur in each equation
2H+ + 2e- → H2 
(hydrogen gas at the –ve cathode)
2Cl- - 2e- → Cl2 
(chlorine gas at the +ve anode)

13. 2NaCl + 2H2O → 2Na+ + 2OH- + Cl2 + H2
Brine
Hydrogen ions gain electrons (reduction) to form hydrogen atoms – the hydrogen atoms combine to form molecules of hydrogen gas
Chloride ions lose electrons (oxidation) to form chlorine atoms – the chlorine atoms combine to form molecules of chlorine gas
2NaCl + 2H2O → 2Na+ + 2OH- + Cl2 + H2

14. Products
These three products - hydrogen, chlorine and sodium hydroxide all have important uses in the chemical industry…
Hydrogen: making ammonia; making margarine etc…
Chlorine: killing bacteria in drinking water; killing bacteria in swimming pools; making bleach; making disinfectants; making hydrochloric acid; making PVC; making CFC's etc…
Sodium hydroxide: making soap; making paper; making ceramics etc…