Which of the following half reactions correctly represents a reduction process in a redox reaction?

Introduction / Context:
Redox reactions involve both oxidation and reduction processes. A half reaction shows either the oxidation part, where electrons are lost, or the reduction part, where electrons are gained. Recognising which half reaction is a reduction process is a fundamental skill in balancing redox equations and understanding electron transfer. This question asks you to select the half reaction that correctly represents reduction, based on the change in oxidation state and electron flow.

Given Data / Assumptions:

• Reduction is defined as a gain of electrons by a species.
• Oxidation is defined as a loss of electrons by a species.
• The half reactions are written in terms of ions and electrons.
• We assume standard oxidation state conventions for metals and their cations.

Concept / Approach:
In the electron transfer view of redox reactions, oxidation is loss of electrons and reduction is gain of electrons. This is often remembered by the phrase OIL RIG (Oxidation Is Loss, Reduction Is Gain). In a reduction half reaction, electrons appear on the left side of the arrow and the oxidation state of the element being reduced decreases. In an oxidation half reaction, electrons appear on the right side and the oxidation state increases. Therefore, among the options, we must identify the half reaction where electrons are added to a higher positive oxidation state to form a lower or neutral state.

Step-by-Step Solution:
Step 1: Analyse option A, Fe2+ + e- -> Fe3+. Here iron goes from +2 on the left to +3 on the right while losing an electron overall, which is an increase in oxidation state and therefore oxidation, not reduction. Step 2: Analyse option B, Au3+ + 3e- -> Au. Gold cations with charge +3 gain three electrons to form neutral gold atoms. The oxidation state decreases from +3 to 0, which is a gain of electrons and is a reduction process. Step 3: Analyse option C, Ag -> Ag+ + e-. Silver atoms lose electrons to become silver ions; the oxidation state goes from 0 to +1, so this is oxidation, not reduction. Step 4: Since only option B shows a gain of electrons and a decrease in oxidation state, it is the only correct representation of reduction among the listed half reactions.

Verification / Alternative check:
A quick way to verify is to look at where the electrons appear in the equation. In a reduction half reaction, electrons are on the left side of the arrow because the species is accepting electrons. In option B, electrons are indeed on the left and the charge on gold decreases. In options A and C, the electrons appear on the right, indicating that those species are losing electrons and are therefore being oxidised. This simple visual check aligns perfectly with the formal oxidation state analysis and confirms that option B is the correct answer.

Why Other Options Are Wrong:
- Fe2+ + e- -> Fe3+: Iron goes from +2 to +3, so it loses electrons overall and is oxidised, not reduced.
- Ag -> Ag+ + e-: Silver goes from 0 to +1, which is again a loss of electrons and an oxidation process.
- All of the above half reactions: Because A and C clearly represent oxidation, not all of them can be reduction processes.
- None of the above half reactions: Option B is a valid reduction half reaction, so this choice is incorrect.

Common Pitfalls:
Students often confuse oxidation and reduction because both processes always occur together in a full redox reaction. A useful tip is to focus on one species at a time and track its electrons or oxidation state. Remember OIL RIG and also the idea that reduction means the charge is reduced or becomes more negative as electrons are gained. Checking the side of the equation where electrons appear is a very fast method to classify half reactions correctly in multiple choice exams.

Final Answer:
The half reaction that correctly represents reduction is Au3+ + 3e- -> Au.