Cambridge O Level Chemistry · Syllabus 5070 · Electrochemistry
Reduction
What is Reduction?
The gain of electrons by a species. In electrolysis it happens at the cathode, where positive ions arriving from the electrolyte take electrons from the electrode and are released as neutral atoms; in a half-equation the electrons are written on the left-hand side.
This definition is part of the Electrochemistry chapter in Cambridge O Level Chemistry.
Common mistakes with Reduction
- 4. "Oxidation is gain of electrons." Repair Oxidation is loss of electrons and happens at the anode; reduction is gain of electrons and happens at the cathode — write the full words, then check the side your electrons are on. Taught in Lesson B.
- 8. "Connect the spoon you want to plate to the positive terminal." Repair Metal ions are reduced onto the object, and reduction happens at the cathode, so the object being plated is the negative cathode and the coating metal is the positive anode. Taught in Lesson I.
Questions students ask about Reduction
What is the difference between the anode and the cathode?
In a simple electrolytic cell the anode is the positive electrode, joined to the positive terminal of the power supply, and the cathode is the negative electrode, joined to the negative terminal. Oxidation is loss of electrons and happens at the anode, where anions arrive and give up electrons. Reduction is gain of electrons and happens at the cathode, where cations arrive and take electrons. Write the full words before any mnemonic, because the exam marks the words, and remember that these signs belong to this powered cell.
How do you write an ionic half-equation for an electrode?
Write the ion on the left and the product on the right, then add the electrons on the correct side: on the left when the ion gains them (reduction at the cathode, for example \(\mathrm{Pb^{2+}} + 2e^- \rightarrow \mathrm{Pb}\)) and on the right when the ion loses them (oxidation at the anode, for example \(2\mathrm{Br^-} \rightarrow \mathrm{Br_2} + 2e^-\)). Then make two checks: count the atoms of each element on both sides, and add up the total charge on each side counting each \(e^-\) as \(-1\). Both must match.
Which electrode do you connect the object to when electroplating?
The object being plated is made the negative cathode. Metal ions from the electrolyte are reduced onto the object, and reduction happens at the cathode, so connecting the object to the positive terminal would be wrong. The coating metal is made the positive anode, and the electrolyte is a solution containing ions of the coating metal. Electroplating is done for two reasons, and a question asking why wants both: to improve the appearance of a metal object and to improve its resistance to corrosion.