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Identifying Reaction Types

Discusses methods for identifying redox reactions from chemical equations.

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Identifying Reaction Types

The reaction of copper with nitric acid leaves a blue solution of copper 2+ ion behind

Credit: User:The mad scientist/Wikimedia Commons
Source: http://commons.wikimedia.org/wiki/File:CopperReaction.JPG
License: CC BY-NC 3.0

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.

Zn+2HClZnCl2+H2

Zn is oxidized from Zn0 to Zn2+ and the H is reduced from H+ to H0

Combustion reactions are redox reactions because elemental oxygen (O2) acts as the oxidizing agent and is itself reduced.

CH4+2O2CO2+2H2O

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:

Fe2O3+2AlAl2O3+2Fe

The thermite reaction involves the oxidation of aluminum and the reduction of iron

Credit: Courtesy of the U.S. Marines
Source: http://commons.wikimedia.org/wiki/File:USMC-07509.jpg
License: CC BY-NC 3.0

Thermite grenade demonstration.[Figure2]

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

Watch a video of the thermite 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.

Na2+1S+6O42(aq)+Ba+2(N+5O32)2(aq)2Na+1N+5O23(aq)+Ba+2S+6O42(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.

H+1F1(aq)+N3H+13(aq)N3H+14+(aq)+F1(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.

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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Image Attributions

  1. [1]^ Credit: User:The mad scientist/Wikimedia Commons; Source: http://commons.wikimedia.org/wiki/File:CopperReaction.JPG; License: CC BY-NC 3.0
  2. [2]^ Credit: Courtesy of the U.S. Marines; Source: http://commons.wikimedia.org/wiki/File:USMC-07509.jpg; License: CC BY-NC 3.0

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