Electrolysis
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Electrolysis definition
the decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current
What is the anode in an electrolytic cell?
positive electrode
What is the cathode in an electrolytic cell?
negative electrode
Electrolyte definition
The molten or aqueous substance that undergoes electrolysis
What are the products and observations during electrolysis of molten lead(II) bromide?
• Cathode: Lead metal forms (grey/silver molten droplets) • Anode: Bromine gas forms (brown/orange fumes)
What are the products and observations during electrolysis of concentrated aqueous sodium chloride (brine)?
• Cathode: Hydrogen gas (bubbles) • Anode: Chlorine gas (green gas with bleaching smell) • Solution left: Sodium hydroxide
What are the products and observations during electrolysis of dilute sulfuric acid with inert electrodes?
• Cathode: Hydrogen gas (bubbles) • Anode: Oxygen gas (bubbles) • Hydrogen is produced in twice the volume of oxygen
How is charge transferred during electrolysis in the external circuit?
By the movement of electrons
How is charge transferred at the electrodes during electrolysis?
By the loss or gain of electrons
How is charge transferred in the electrolyte during electrolysis?
By the movement of ions
What happens during electrolysis of aqueous copper(II) sulfate with carbon/graphite electrodes?
• Cathode: Copper metal forms (reddish-brown deposit) • Anode: Oxygen gas forms (bubbles) • Observation: Blue colour of solution fades
What happens during electrolysis of aqueous copper(II) sulfate with copper electrodes?
• Cathode: Copper is deposited. • Anode: Copper dissolves into solution. • Observation: Blue colour remains the same.
What substances are formed at each electrode during electrolysis?
• Cathode: Metals or hydrogen • Anode: Non-metals (except hydrogen)
How can you predict the products of electrolysis of a binary compound in the molten state?
• Metal ion → metal at the cathode • Non-metal ion → non-metal at the anode
What is a hydrogen–oxygen fuel cell?
A cell that uses hydrogen and oxygen to produce electricity, with water as the only chemical product
advantages of hydrogen–oxygen fuel cells compared with petrol engines? (3)
• Only produces water (no CO₂ pollution) • More energy efficient • Less air pollution
disadvantages of hydrogen–oxygen fuel cells compared with petrol engines? (3)
• Hydrogen storage is difficult and dangerous • Hydrogen is expensive to produce • Requires special infrastructure
What happens at the cathode during electrolysis?
Reduction - ions gain electrons
What happens at the anode during electrolysis?
Oxidation - ions lose electrons
half-equation for hydrogen forming at the cathode
2H^+ + 2e^- —> H_2
half-equation for oxygen forming at the anode
4OH^- —> O2 + 2H_2O + 4e^-
half-equation for copper forming at the cathode
Cu^2+ + 2e^- —> Cu
half-equation for chlorine and bromine forming at the anode
2Cl^- —> Cl_2 + 2e^- 2Br^- —> Br_2 + 2e^-
What type of ions move to the cathode?
Positive ions (cations)