Electrolysis & Ionic Compounds
Electrolysis is the decomposition of an ionic compound (the electrolyte), either molten or in aqueous solution, by passing a direct electric current through it. Positive cations migrate to the negative cathode where they are reduced, and negative anions migrate to the positive anode where they are oxidised.
- Electrolyte
- A liquid or gel containing ions that can be decomposed by electrolysis.
- Anode
- The positively charged electrode by which electrons leave a device.
- Cathode
- The negatively charged electrode by which electrons enter an electrical device.
Electrochemical Cell Builder & E°cell Calculator
Select any two half-cells below to calculate standard cell potential E°cell = E°(reduced) − E°(oxidised) and watch which way electrons flow.
Standard conditions: 298 K (25°C), 100 kPa pressure, 1.00 mol dm⁻³ ion concentrations.
E°cell = (+0.34) − (-0.76) = +1.10 V
- Anode (Oxidation: more negative E° = -0.76 V):
Zn loses electrons and dissolves into Zn²⁺(aq). Electrons flow out of the Zn electrode into the external wire.
+ Cathode (Reduction, more positive E° = +0.34 V):
Cu²⁺(aq) ions gain electrons at the surface to form solid Cu.
Key Concepts & Exam Definitions (5)
- Electrolyte
- A liquid or gel containing ions that can be decomposed by electrolysis.
- Anode
- The positively charged electrode by which electrons leave a device.
- Cathode
- The negatively charged electrode by which electrons enter an electrical device.
- Oxidation
- The loss of electrons during a reaction by a molecule, atom or ion.
- Reduction
- The gain of electrons by a substance in a chemical reaction.
Quick-Fire Flashcards (5)
Click any card to check the answer, then mark it as Known to track your progress.
Where do positive ions (cations) move during electrolysis?
What is produced at the cathode during the electrolysis of molten lead bromide?
In the electrolysis of aqueous solutions, when is hydrogen produced at the cathode?
What does Oil Rig stand for?
Why is cryolite used in the electrolysis of aluminium oxide?
Exam-Style Questions & Mark Schemes (3)
Write or think through your answer first, then click Check Mark Scheme to see the exact points examiners award marks for.
- Q1
Explain what happens at the anode during the electrolysis of molten sodium chloride.
Click “Check Mark Scheme” to compare your answer, Negative chloride ions (Cl-) are attracted to the positive anode. They lose electrons (oxidation) to form chlorine gas (Cl2).
- Q2
Write the half-equation for the production of aluminium at the cathode.
Click “Check Mark Scheme” to compare your answer, Al3+ + 3e- -> Al
- Q3
Why does solid sodium chloride not conduct electricity?
Click “Check Mark Scheme” to compare your answer, The ions are held firmly in a giant ionic lattice and cannot move to carry the charge.
Common Exam Questions (3)
What is electrolysis?
Electrolysis is the process of passing a direct electric current through a molten or dissolved ionic compound (an electrolyte) to break it down into elements.
Why must the electrolyte be molten or dissolved in water?
In a solid ionic lattice, the ions are held in fixed positions by strong electrostatic forces and cannot move. Melting the compound or dissolving it in water frees the ions to move and carry electrical charge to the electrodes.
What is an anode and a cathode?
The anode is the positive electrode, which attracts negatively charged anions (where oxidation occurs). The cathode is the negative electrode, which attracts positively charged cations (where reduction occurs).
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