Electrolysis

Introduction to Electrolysis

- electrolysis breaks down a substance using electricity

- ionic compounds can only be electrolysed when they are molten or dissolved in water, this is because their ions are then free to move and carry their charge to the electrodes

- in electrolysis, positive ions move to the cathode (negative electrode) while negative ions move to the anode (positive electrode)

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Changes at the Electrodes

- in electrolysis, the ions move towards the oppositely charged electrodes

- at the negative electrode (cathode) positive ions gain electrons, so they are reduced

- at the positive electrode (anode) negative ions lose electrons, so they are oxidised 

- when electrolysis happens in aqueous solution, the less reactive element, either hydrogen or the metal is usually produced at the cathode

- at the anode you get either: 

    oxygen gas given off from discharged hydroxide ions produced from water

    a halogen produced if the electrolyte is a solution of a halide

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Extraction of Aluminium

- aluminium oxide, from the ore bauxite is electrolysed in the extraction of aluminium metal

- the aluminium oxide is mixed with molten cryolite to lower its melting point, reducing the energy needed to extract the aluminium 

- aluminium forms at the cathode (negative electrode) and oxygen forms at the anode (positive electrode) 

- carbons anodes are replaced regularly as they gradually burn away as oxygen reacts with carbon anodes, forming carbon dioxide gas

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Electrolysis of Aqueous Solutions

- when you electrolyse sodium chloride solution (brine) you get three products - chlorine gas and hydrogen gas given off at the electrodes, plus sodium hydroxide solution (an alkali) left in solution

- hydrogen is produced at cathode (-) as H+ (aq) ions are discharged from solution in preference to Na+ (aq) ions

- chlorine is produced at the anode (+) as Cl- (aq) ions are discharges from solution in preference to OH0 (aq) ions

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