As you can see, iron is oxidized, and oxygen is reduced. The result is that iron is
converted into the reddish-brown iron(II) oxide commonly called rust.
Oxidizing and Reducing Agents
There are two other terms that you should be familiar with: oxidizing agent (or
oxidant) and reducing agent (or reductant). An oxidizing agent causes another
species to be oxidized by undergoing reduction. A reducing agent—by itself
being oxidized—causes another substance to be reduced. Consider the two redox
reactions we examined earlier.
• Fe + 2HCl → FeCl 2 + H 2
• 4NH 3 + 5O 2 → 4NO + 6H 2 OIn the first reaction, Fe (which is oxidized) is the reducing agent, and HCl
(which contains the species being reduced, H) is the oxidizing agent.
In the second reaction, NH 3 (which contains the species being oxidized, N) is the
reducing agent and O 2 (which is reduced) is the oxidizing agent. Note that
oxygen (O 2 ) is an excellent oxidizing agent, and fluorine (F 2 ) is also a powerful
oxidizing agent. The active metals make strong reducing agents.
Different substances will have different reactivity levels based on how easily
they can be oxidized when placed in water or acid. For instance, lithium will
very easily lose an electron to form Li+, and thus is a strong reducing agent. On
the other hand, a transition metal like copper does not give up its electrons
nearly as readily, and is a very weak reducing agent that cannot be oxidized by
water (but can still be oxidized by some strong acids).
The activity series is a qualitative series that compares the reactivity of the
various metals with water or acids. The higher the metal is on the activity series,
the more vigorously it will react with water/acid. The metals at the top are thus
very strong reducing agents, while those at the bottom are much weaker