Unit 6: Electrolysis - Principles & Examples
Using electrical energy to drive non-spontaneous chemical reactions.
6.20 Principles of Electrolysis
Electrolysis is the process of using a direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. It is the opposite of a galvanic cell; it converts electrical energy into chemical energy.
An electrolytic cell consists of:
- An electrolyte: A molten ionic compound or an aqueous solution containing mobile ions.
- Two electrodes (usually inert, like graphite or platinum) submerged in the electrolyte.
- An external power supply (e.g., a battery).
How it Works:
- The power supply makes one electrode positive and the other negative.
- Positively charged ions (cations) in the electrolyte are attracted to the negative electrode.
- Negatively charged ions (anions) in the electrolyte are attracted to the positive electrode.
At the electrodes, redox reactions occur:
- At the cathode (negative electrode), cations gain electrons. Reduction occurs. (Mnemonic: Red Cat)
- At the anode (positive electrode), anions lose electrons. Oxidation occurs. (Mnemonic: An Ox)
Key Difference from Galvanic Cells: In electrolysis, the anode is positive and the cathode is negative. This is the opposite of the sign convention in a galvanic cell.
Solved Examples:
- What is electrolysis?
Solution: The decomposition of a substance (an electrolyte) by passing an electric current through it. - What type of energy conversion happens in an electrolytic cell?
Solution: Electrical energy is converted into chemical energy. - What is the name for the positive electrode in electrolysis?
Solution: The anode. - Which process, oxidation or reduction, occurs at the cathode?
Solution: Reduction. - What is an electrolyte?
Solution: A substance that contains mobile ions (either molten or in aqueous solution) and can conduct electricity. - To which electrode do cations migrate during electrolysis?
Solution: To the negative electrode (the cathode). - Why are inert electrodes like graphite often used?
Solution: To prevent the electrode itself from reacting, ensuring that only the ions in the electrolyte are involved in the redox processes. - What is the fundamental difference between an electrolytic cell and a galvanic cell?
Solution: A galvanic cell uses a spontaneous reaction to produce electricity, while an electrolytic cell uses electricity to force a non-spontaneous reaction to occur. - Do anions gain or lose electrons at the anode?
Solution: They lose electrons (they are oxidised). - Can solid sodium chloride be electrolysed? Why or why not?
Solution: No. In the solid state, its ions are held in a fixed lattice and are not mobile, so it cannot conduct electricity. It must be molten or dissolved in water.
6.21 Examples of Electrolysis
The products of electrolysis depend on what is in the electrolyte.
Electrolysis of Molten Salts:
This is the simplest case. The electrolyte contains only the cations and anions of the salt.
Example: Molten Lead(II) Bromide ($PbBr_2(l)$)
- At the Cathode (-): Lead ions are reduced.
$Pb^{2+}(l) + 2e^- \rightarrow Pb(l)$ - At the Anode (+): Bromide ions are oxidised.
$2Br^-(l) \rightarrow Br_2(g) + 2e^-$ - Overall: $PbBr_2(l) \rightarrow Pb(l) + Br_2(g)$
Electrolysis of Aqueous Solutions:
This is more complex because water can also be oxidised or reduced. There is a competition at each electrode.
Water's reactions:
- Reduction: $2H_2O(l) + 2e^- \rightarrow H_2(g) + 2OH^-(aq)$
- Oxidation: $2H_2O(l) \rightarrow O_2(g) + 4H^+(aq) + 4e^-$
Rules for predicting products in aqueous solution (for simple cases):
- At the Cathode (-): The ion of the less reactive metal is reduced. If the metal is more reactive than hydrogen (e.g., Na, K, Mg), then hydrogen gas is produced from the reduction of water instead.
- At the Anode (+): If the solution contains halide ions ($Cl^-, Br^-, I^-$), the halogen is produced. If not, then oxygen gas is produced from the oxidation of water (or hydroxide ions).
Example: Aqueous Copper(II) Sulphate ($CuSO_4(aq)$) with inert electrodes
- Ions present: $Cu^{2+}(aq)$, $SO_4^{2-}(aq)$, $H^+(aq)$, $OH^-(aq)$ (from water).
- At the Cathode (-): Copper is less reactive than hydrogen. Copper ions are reduced.
$Cu^{2+}(aq) + 2e^- \rightarrow Cu(s)$ (A layer of copper forms). - At the Anode (+): The solution does not contain halides. Oxygen is produced.
$2H_2O(l) \rightarrow O_2(g) + 4H^+(aq) + 4e^-$ (Bubbles are seen).
Solved Examples:
- What are the products of the electrolysis of molten sodium chloride?
Solution: Sodium metal at the cathode and chlorine gas at the anode. - What is produced at the cathode during the electrolysis of aqueous sodium chloride?
Solution: Hydrogen gas, because sodium is more reactive than hydrogen. - What is produced at the anode during the electrolysis of concentrated aqueous sodium chloride?
Solution: Chlorine gas, because a halide is present. - Write the half-equation for the reaction at the negative electrode during the electrolysis of molten aluminium oxide ($Al_2O_3$).
Solution: $Al^{3+}(l) + 3e^- \rightarrow Al(l)$. - During the electrolysis of aqueous copper(II) sulphate, what happens to the colour of the solution?
Solution: The blue colour fades, as the blue $Cu^{2+}(aq)$ ions are consumed. - What is produced at the anode during the electrolysis of dilute sulphuric acid?
Solution: Oxygen gas, as no halide is present. - What is produced at the cathode during the electrolysis of dilute sulphuric acid?
Solution: Hydrogen gas. - Why is a different product formed at the cathode when electrolysing aqueous $NaCl$ versus molten $NaCl$?
Solution: In the aqueous solution, water is present and is more easily reduced than the sodium ion, producing hydrogen gas. In the molten salt, only sodium ions are available for reduction. - A student electrolyses silver nitrate solution ($AgNO_3(aq)$). What is formed at the cathode?
Solution: Silver metal ($Ag$), because silver is less reactive than hydrogen. - What is the product at the anode during the electrolysis of aqueous potassium iodide ($KI(aq)$)?
Solution: Iodine ($I_2$), because a halide is present.
Knowledge Check (20 Questions)
Answer: Oxidation.
Answer: Negative.
Answer: Potassium metal and iodine.
Answer: Hydrogen gas.
Answer: Oxygen gas.
Answer: The electrolyte.
Answer: $2H_2O(l) \rightarrow O_2(g) + 4H^+(aq) + 4e^-$.
Answer: The positive electrode (the anode).
Answer: Silver metal.
Answer: It must contain mobile ions (be molten or in aqueous solution).
Answer: Positive.
Answer: $2H_2O(l) + 2e^- \rightarrow H_2(g) + 2OH^-(aq)$.
Answer: The extraction of aluminium metal.
Answer: Bromine ($Br_2$).
Answer: A cation.
Answer: Magnesium metal.
Answer: Electrical, chemical.
Answer: To lower its melting point, which reduces energy costs.
Answer: 2:1.
Answer: Hydrogen gas (because zinc is more reactive than hydrogen).