# 22.8: Identifying Reaction Types


The reaction of copper wire with nitric acid produces a colorful mix of products that include copper (II) nitrate, nitrogen dioxide, and water. Copper salts are blue in solution, reflecting the rather unique arrangements of electrons in the $$d$$ orbital as the copper ionizes from metallic copper.

## Identifying Reaction Types

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

Single-replacement reactions are redox reactions because two different elements appear as free elements (oxidation number of zero) on one side of the equation and as part of a compound on the other side. Therefore, oxidation numbers must change.

$\ce{Zn} + 2 \ce{HCl} \rightarrow \ce{ZnCl_2} + \ce{H_2}\nonumber$

$$\ce{Zn}$$ is oxidized from $$\ce{Zn^0}$$ to $$\ce{Zn^{2+}}$$ and the $$\ce{H}$$ is reduced from $$\ce{H^+}$$ to $$\ce{H^0}$$.

Combustion reactions are redox reactions because elemental oxygen $$\left( \ce{O_2} \right)$$ acts as the oxidizing agent and is itself reduced.

$\ce{CH_4} + 2 \ce{O_2} \rightarrow \ce{CO_2} + 2 \ce{H_2O}\nonumber$

Most combination and decomposition reactions are redox reactions, since elements are usually transformed into compounds and vice-versa. The thermite reaction involves ferric oxide and metallic aluminum:

$\ce{Fe_2O_3} + 2 \ce{Al} \rightarrow \ce{Al_2O_3} + 2 \ce{Fe}\nonumber$

We see that the iron is reduced and the aluminum oxidized during the course of the reaction.

So what types of reactions are not redox reactions? Double-replacement reactions, such as the one below, are not redox reactions because ions are simply recombined without any transfer of electrons.

$\overset{+1}{\ce{Na_2}} \overset{+6}{\ce{S}} \overset{-2}{\ce{O_4}} \left( aq \right) + \overset{+2}{\ce{Ba}} ( \overset{+5}{\ce{N}} \overset{-2}{\ce{O_3}} ) \left( aq \right) \rightarrow 2 \overset{+1}{\ce{Na}} \overset{+5}{\ce{N}} \overset{-2}{\ce{O_3}} \left( aq \right) + \overset{+2}{\ce{Ba}} \overset{+6}{\ce{S}} \overset{-2}{\ce{O_4}} \left( s \right)\nonumber$

Note that the oxidation numbers for each element remain unchanged in the reaction.

Acid-base reactions involve a transfer of a hydrogen ion instead of an electron. Acid-base reactions, like the one below, are also not redox reactions.

$\overset{+1}{\ce{H}} \overset{-1}{\ce{F}} \left( aq \right) + \overset{-3}{\ce{N}} \overset{+1}{\ce{H_3}} \left( aq \right) \rightarrow \overset{-3}{\ce{N}} \overset{+1}{\ce{H_4^+}} \left( aq \right) + \overset{-1}{\ce{F^-}}\nonumber$

Again, the transfer of an $$\ce{H^+}$$ ion leaves the oxidation numbers unaffected. In summary, redox reactions can always be recognized by a change in oxidation number of two of the atoms in the reaction. Any reaction in which no oxidation numbers change is not a redox reaction.

## Summary

• A redox reaction must involve a change in oxidation number for two of the elements involved in the reaction.
• The oxidized element increases in oxidation number, while the reduced element decreases in oxidation number.
• Single-replacement reactions and combustion reactions are redox reactions.
• Most combination and decomposition reactions are redox reactions.
• Double-replacement reactions and acid-base reactions are not redox reactions.

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