Chemistry C6 - Electrolysis

What is Electrolysis

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Process that uses electricity to break down an ionic compound (when it is molten or dissolved in water) into its constituent elements

Why must an ionic compound be molten or dissolved for electrolysis to work?

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Ions must be free to move to carry charge

In solids, the ions are fixed in a lattice so the current can't flow

Name the ions present in molten lead bromide and state which electrode each moves to

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Lead ions ( pb2+ ) go to the cathode ( Negative )

Bromide ions go to the anode ( Br- ) ( Positive )

During electrolysis of aqueous sodium chloride what gases form at each electrode

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Cathode - Hydrogen Gas ( H2)

Anode - Chlorine gas ( Cl2 )

Explain why hydrogen forms instead of sodium at the cathode in aqueous sodium chloride

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Hydrogen irons from water are reduced more easily than sodium ions

Sodium is more reactive so stays in solution

What is meant by oxidation and reduction in terms of electrons

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Oxidation: Loss of electrons

Reduction: Gain of electrons

Describe a practical method to investigate products at each electrode during electrolysis of aqueous copper ( II ) chloride

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1. Set up a beaker of copper ( II ) chloride solution with two inert carbon electrodes

2. Connect to a d.c. Power supply using crocodile clips

3. Observe bubbling at both electrodes

4. Collect gas at anode and test with damp blue litmus = bleaches = chlorine

5. Copper forms on cathode surface

6. Clean electrodes and repeat for reliability

Explain why aluminium is extracted by electrolysis of aluminium oxide rather than by reduction with carbon

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Aluminium is more reactive than carbon, so can't be displaced by carbon

Electrolysis provides the energy to reduce Al + to aluminium metal

• Uses electricity: Electrical energy is supplied by a DC power source to drive the chemical reaction.

• Breaks down (decomposition): The process causes the compound to decompose, which is a non-spontaneous chemical change.

• Ionic compound (electrolyte): The substance being broken down must be an ionic compound, called an electrolyte.

• Molten or dissolved: The ions must be free to move, which means the ionic compound must be in a molten (liquid) state or dissolved in an aqueous solution.

• Forms elements: The final products are the elements from which the compound was made, produced at the electrodes (cathode and anode).

• Ions move to opposite electrodes: Positively charged ions move to the negative electrode (cathode) and negatively charged ions move to the positive electrode (anode). 

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