Chapter 10 Laboratory: Reduction-Oxidation (Redox) Reactions 181

A reduction-oxidation reaction or redox reaction is a chemical reaction during which the

oxidation state of two or more of the reactants change. Note that the word “oxidation” in this

context has nothing to do with oxidation in the sense of combining with oxygen. Most redox

reactions do not involve oxygen at all.

The oxidation state of an atom or ion can be thought of as its electrical charge. In simple

terms, a neutral atom is in oxidation state 0. An ion with a positive charge has a positive

oxidation state overall (although one or more of the component atoms in that ion may

have nonpositive oxidation states. For example, if a neutral sodium atom (oxidation state 0,

Na^0 ) loses an electron, it has been oxidized to oxidation state 1+, and becomes a Na+ ion.

Conversely, a carbonate ion, CO 3 2–, is made up of one carbon atom in oxidation state 4+ and

three oxygen atoms, each in oxidation state 2–, for a net charge (oxidation state) of 2–.

Oxidation in the context of a redox reaction refers to the oxidation state of an atom being

increased. Conversely, reduction refers to the oxidation state of an atom being reduced. We

saw an example of a redox reaction in the preceding chapter, when we reacted aluminum

metal with hydrochloric acid to produce aluminum chloride and hydrogen gas. The balanced

equation for that reaction is:

2 Al(s) + 6 HCl(aq) → 2 AlCl 3 (aq) + 3 H 2 (g)

or, with the oxidation states of each species as a superscript:

2 Al^0 (s) + 6 H+(aq) + 6 Cl–(aq) → 2 Al3+(aq) + 6 Cl–(aq) + 3 H 20 (g)

In this reaction, two atoms of aluminum metal (oxidation state 0) each lose three electrons

and are oxidized to aluminum ions (oxidation state +3). The six electrons lost by the two

aluminum atoms combine with six hydrogen ions (oxidation state +1), reducing the hydrogen

ions to form six atoms of hydrogen gas (oxidation state 0), which then combine to form three

molecules of diatomic hydrogen gas (still in oxidation state 0). The six chloride ions (oxidation

state –1) do not change oxidation states, and so are the same on both sides of the equation.

10

Laboratory:

Reduction-Oxidation (Redox) Reactions