Identify oxidation and reduction by the transfer of electrons, assign oxidation numbers, and recognize oxidizing and reducing agents (MA STE HS-PS1-2, electron behavior in reactions).

What this topic is asking

Standard HS-PS1-2 explains reactions from the behavior of electrons. Oxidation-reduction (redox) reactions are the reactions where electrons actually move from one substance to another. Massachusetts high school chemistry expects you to recognize oxidation and reduction by electron transfer, assign oxidation numbers to track that transfer, and identify the oxidizing and reducing agents.

Oxidation and reduction as electron transfer

When zinc reacts with copper(II) ions, $\text{Zn} + \text{Cu}^{2+} \rightarrow \text{Zn}^{2+} + \text{Cu}$, the zinc atom loses two electrons (oxidized) and the copper ion gains them (reduced). The memory aid OIL RIG captures it: Oxidation Is Loss, Reduction Is Gain. Because electrons cannot simply vanish, oxidation and reduction are two halves of the same event.

Oxidation numbers

A working set of rules for a first course:

• A free element (such as $\text{Zn}$ or $\text{O}_2$) has an oxidation number of 0.
• A monatomic ion's oxidation number equals its charge ($\text{Na}^+$ is $+1$, $\text{Cl}^-$ is $-1$).
• Oxygen is usually $-2$, and hydrogen is usually $+1$.
• The oxidation numbers in a neutral compound sum to 0; in a polyatomic ion they sum to the ion's charge.

These rules let you find an unknown oxidation number, then compare before and after to see what was oxidized or reduced. The idea of charge on ions connects back to electron arrangement and valence electrons.

Oxidizing and reducing agents

The naming feels backwards at first. The reducing agent reduces the other substance, so it is itself oxidized. In $\text{Zn} + \text{Cu}^{2+} \rightarrow \text{Zn}^{2+} + \text{Cu}$, zinc is the reducing agent (it is oxidized and gives electrons), and the copper(II) ion is the oxidizing agent (it is reduced and takes electrons).

Redox all around us

Redox reactions run the everyday world: rusting (iron oxidized by oxygen), combustion (fuel oxidized, releasing energy), respiration in cells, batteries (which separate the two half-reactions to drive a current), and corrosion. Spotting a change in oxidation number is the quickest way to know a reaction is redox.

Try this

Q1. In $\text{Cl}_2 + 2\text{KBr} \rightarrow 2\text{KCl} + \text{Br}_2$, which element is reduced? [1]

• Cue. Chlorine (it goes from 0 in $\text{Cl}_2$ to $-1$ in KCl, a gain of electrons).

Q2. Find the oxidation number of manganese in $\text{MnO}_2$. Use O = $-2$. [1]

• Cue. Two oxygen give $-4$; the compound is neutral, so manganese is $+4$.