Electrolysis and fuel cells
Using electricity to break down ionic compounds, predicting the products at each electrode, electroplating and hydrogen fuel cells.
📌 Key points
- Electrolysis is the decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current.
- The cathode is the negative electrode: positive ions gain electrons there (reduction). The anode is the positive electrode: negative ions lose electrons there (oxidation).
- A molten binary compound gives the metal at the cathode and the non-metal at the anode, e.g. molten lead(II) bromide gives lead and bromine.
- In aqueous solutions, hydrogen forms at the cathode unless the metal is less reactive than hydrogen (e.g. copper). Oxygen forms at the anode unless the solution contains a concentrated halide (then the halogen forms).
- Concentrated aqueous sodium chloride gives hydrogen at the cathode and chlorine at the anode, leaving sodium hydroxide solution. Dilute sulfuric acid gives hydrogen at the cathode and oxygen at the anode.
- Aqueous copper(II) sulfate with carbon electrodes gives copper at the cathode and oxygen at the anode, and the blue colour fades. With copper electrodes, the anode dissolves and copper is deposited on the cathode, so the colour stays the same; this is used to purify copper.
- Electroplating: the object is the cathode, the plating metal is the anode, and the electrolyte contains ions of the plating metal. It improves appearance and resistance to corrosion.
- Ionic half-equations show electron transfer, e.g. Cu2+ + 2e− → Cu at the cathode and 2Cl− → Cl2 + 2e− at the anode.
- A hydrogen–oxygen fuel cell uses hydrogen and oxygen to produce electricity, and the only product is water. Hydrogen is difficult to store and is often made from fossil fuels.
📖 Key terms
- Electrolysis
- Decomposition of an ionic compound, molten or in aqueous solution, by an electric current.
- Electrolyte
- A molten or aqueous ionic compound that conducts electricity and is decomposed by it.
- Cathode
- The negative electrode, where reduction happens.
- Anode
- The positive electrode, where oxidation happens.
- Inert electrode
- An electrode that does not react during electrolysis, e.g. carbon (graphite) or platinum.
- Oxidation
- Loss of electrons.
- Reduction
- Gain of electrons.
✏️ Worked example
Predict the products at each electrode when concentrated aqueous sodium chloride is electrolysed with carbon electrodes.
- Ions present: Na+ and Cl− from the salt, H+ and OH− from water.
- Cathode: sodium is more reactive than hydrogen, so H+ ions are discharged: 2H+ + 2e− → H2.
- Anode: the chloride is concentrated, so Cl− ions are discharged: 2Cl− → Cl2 + 2e−.
- Na+ and OH− ions remain in solution.
- Answer: hydrogen at the cathode, chlorine at the anode, sodium hydroxide left in solution.
Remember OIL RIG (Oxidation Is Loss, Reduction Is Gain of electrons) and PANIC (Positive Anode, Negative Is Cathode).
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