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22.8: Identifying Reaction Types

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    53962
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    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:

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

    CK12 Screenshot 22-8-1.png
    Figure \(\PageIndex{1}\): Thermite grenade demonstration.

    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.

    22.8: Identifying Reaction Types is shared under a CK-12 license and was authored, remixed, and/or curated by CK-12 Foundation via source content that was edited to conform to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.

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