3: Identifying Oxidation-Reduction Reactions

A redox reaction must involve a change in oxidation number for at least two of the elements involved in the reaction. The oxidized element increases in oxidation number, while the reduced element decreases in oxidation number.

Example:

Sodium (Na) and chlorine (Cl) reacts to form sodium chloride (NaCl):

Na  +  Cl  →  NaCl

We know that pure elements, in this case the reactants Na and Cl, have the oxidation number 0. We also know that NaCl is an ionic compound consisting of sodium cations (Na+) and chloride anions (Cl-). The oxidation number of a monatomic ion is equal to its charge. Consequently, the sodium cation in NaCl has the oxidation number +1 and the chloride anion has the oxidation number -1.

As the oxidation number of sodium increases, sodium is oxidized, while the oxidation number of chlorine decreases, which means, chlorine is reduced.

Counterexample:

Let’s look at a counter example of a redox reaction. Sodium cations (Na+) and chloride anions (Cl-) can also react to form sodium chloride (NaCl):

Na-  +  Cl-  →  NaCl

If we assign the oxidation numbers,

we can see that they don’t change. There is no gain or loss of electrons. Consequently, this is not a redox reaction.

Further examples can be found here.

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Sources

Last modified: Thursday, 6 October 2016, 3:32 PM