Oxidation numbers and redox reactions: assign oxidation numbers using the standard rules, and identify oxidation, reduction, and the oxidizing and reducing agents in a reaction.

What this topic is asking

The Core Curriculum asks you to assign oxidation numbers using the standard rules and to use them to identify oxidation (loss of electrons), reduction (gain of electrons), and the oxidizing and reducing agents in a reaction. This is the foundation of the whole redox topic and a steady source of Part A and Part B-2 questions.

Assigning oxidation numbers

These rules let you find the oxidation number of any atom by setting up a simple sum. For an unknown atom in a compound, assign the known values (hydrogen, oxygen, group ions), set the total to the overall charge, and solve. The element with the higher electronegativity in a bond is given the negative oxidation number.

Oxidation and reduction

To decide what is oxidized or reduced, compare each element's oxidation number before and after the reaction. An increase means oxidation; a decrease means reduction. In $\text{Zn} + \text{Cu}^{2+} \rightarrow \text{Zn}^{2+} + \text{Cu}$, zinc goes from $0$ to $+2$ (oxidized) and copper goes from $+2$ to $0$ (reduced).

Oxidizing and reducing agents

This naming trips many students up. The substance that is reduced is the oxidizing agent, and the substance that is oxidized is the reducing agent. In the zinc-copper example, $\text{Cu}^{2+}$ (reduced) is the oxidizing agent and zinc (oxidized) is the reducing agent.

Try this

Q1. Determine the oxidation number of nitrogen in $\text{NO}_3^-$. [1 point]

• Cue. Oxygen is $-2$ ($3 \times -2 = -6$); the ion is $-1$, so $N + (-6) = -1$, giving $N = +5$.

Q2. In a reaction, an element's oxidation number decreases. State whether it is oxidized or reduced. [1 point]

• Cue. Reduced (a decrease in oxidation number means a gain of electrons).