Redox Reactions at Electrodes
Electrolysis uses a direct electric current to break down an ionic substance, forcing a non-spontaneous chemical reaction. At the two electrodes, ions are converted into neutral atoms or molecules through the gain (reduction) or loss (oxidation) of electrons.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- At the cathode (the negative electrode), positively charged ions (cations) are attracted. They gain electrons and are reduced.
- At the anode (the positive electrode), negatively charged ions (anions) are attracted. They lose electrons and are oxidised.
- A simple way to remember the electrode processes is 'An Ox, Red Cat': Oxidation occurs at the Anode, and Reduction occurs at the Cathode.
- To remember the electrode charges in electrolysis, use the mnemonic 'PANIC': Positive Anode, Negative Is Cathode.
- The movement of ions through the electrolyte and electrons through the external circuit completes the electrical circuit.
Formulae
Cation^(n+) + n e^- → Atom/Molecule This general form represents reduction at the cathode, where a positive ion gains 'n' electrons to become a neutral substance.
Anion^(m-) → Atom/Molecule + m e^- This general form represents oxidation at the anode, where a negative ion loses 'm' electrons to become a neutral substance.
Definitions
- Reduction
- The gain of electrons by a chemical species, resulting in a decrease in oxidation state. It occurs at the cathode.
- Oxidation
- The loss of electrons from a chemical species, resulting in an increase in oxidation state. It occurs at the anode.
- Cation
- A positively charged ion that migrates towards the negatively charged cathode during electrolysis.
- Anion
- A negatively charged ion that migrates towards the positively charged anode during electrolysis.
Worked example
During the electrolysis of molten aluminium oxide (Al2O3), aluminium metal is formed at one electrode and oxygen gas is formed at the other. Identify which substance is formed at the cathode and state whether this process is oxidation or reduction.
- 1
First, identify the ions present in molten Al2O3.
These are aluminium cations (Al3+) and oxide anions (O2-).
- 2
The cathode is the negative electrode.
It attracts the positively charged cations.
In this case, the Al3+ ions are attracted to the cathode.
- 3
At the cathode, the Al3+ ions gain electrons to become neutral aluminium atoms (Al).
The half-equation is Al3+ + 3e^- → Al.
- 4
The gain of electrons is defined as reduction.
- 5
Therefore, aluminium metal is formed at the cathode via reduction.
Answer: Aluminium metal is formed at the cathode. The process is reduction.
Common mistakes
- ×Mixing up oxidation and reduction. Remember 'OIL RIG': Oxidation Is Loss, Reduction Is Gain (of electrons).
- ×Incorrect stoichiometry for diatomic molecules. Anions like Cl^- or O2- form molecules (Cl2, O2) at the anode, which requires balancing both the atoms and the charge, e.g., 2Cl^- → Cl2 + 2e^-. Forgetting the '2' is a common source of 'off by a factor' errors.
- ×Misremembering which electrode is positive/negative. Stick to 'PANIC' (Positive Anode, Negative Is Cathode) for electrolytic cells.
No-calculator tips
- ✓Always check that the total charge is conserved in your half-equations. The sum of charges on the reactant side must equal the sum of charges on the product side. For example, in 2O2- → O2 + 4e^-, the charge on the left is 2*(-2) = -4, and on the right is 0 + 4*(-1) = -4. It balances.
- ✓Before writing any equations, quickly sketch a diagram and label the anode (+), cathode (-), and the direction of ion movement (cations to cathode, anions to anode). This helps to prevent simple mix-ups.