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Identifying Reaction Types

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Identifying Reaction Types

What’s that blue stuff?

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 element (oxidation number of zero) on one side of the equation and as part of a compound on the other side. Therefore, its oxidation number must change.

\text{Zn}+2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2

Zn is oxidized from Zn 0 to Zn 2+ and the H is reduced from H + to H 0

Combustion reactions are redox reactions because elemental oxygen (O 2 ) acts as the oxidizing agent and is itself reduced.

\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}

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:

\text{Fe}_2\text{O}_3+2\text{Al} \rightarrow \text{Al}_2\text{O}_3+2\text{Fe}

Thermite grenade demonstration.

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

Watch a video of the thermite reaction:

http://www.youtube.com/watch?v=CWMATrOatRw

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{\underset{\mathrm{+1}}{}}{\text{Na}_2} \overset{\underset{\mathrm{+6}}{}}{\text{S}} \overset{\underset{\mathrm{-2}}{}}{\text{O}_4}(aq) + \overset{\underset{\mathrm{+2}}{}}{\text{Ba}}(\overset{\underset{\mathrm{+5}}{}}{\text{N}} \overset{\underset{\mathrm{-2}}{}}{\text{O}_3})_2(aq) \rightarrow 2 \overset{\underset{\mathrm{+1}}{}}{\text{Na}} \overset{\underset{\mathrm{+5}}{}}{\text{N}} \overset{\underset{\mathrm{-2}}{}}{\text{O}}_3(aq) + \overset{\underset{\mathrm{+2}}{}}{\text{Ba}} \overset{\underset{\mathrm{+6}}{}}{\text{S}} \overset{\underset{\mathrm{-2}}{}}{\text{O}_4}(s)

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{\underset{\mathrm{+1}}{}}{\text{H}} \overset{\underset{\mathrm{-1}}{}}{\text{F}}(aq) + \overset{\underset{\mathrm{-3}}{}}{\text{N}} \overset{\underset{\mathrm{+1}}{}}{\text{H}}_3(aq) \rightarrow \overset{\underset{\mathrm{-3}}{}}{\text{N}} \overset{\underset{\mathrm{+1}}{}}{\text{H}}_4 \! ^+(aq) + \overset{\underset{\mathrm{-1}}{}}{\text{F}} \! ^-(aq)

Again, the transfer of an 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

  • Criteria for determining whether or not a reaction involves oxidation and reduction are listed.

Practice

Indicate which of the following reactions are redox reactions and which are not:

http://dreamspinnerlearning.com/_Media/Redox%20reactions%20Worksheet.doc

You do not need to write explanations or balance the equations.

Review

  1. Why is the Zn + HCl reaction a redox reaction?
  2. Why is the sodium sulfate + barium nitrate reaction not a redox reaction?
  3. Does the transfer of H + affect oxidation numbers?

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