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oxidation = increase in oxidation number reduction = decrease in oxidation number

Returning to the reactions used to introduce this topic, they may now both be identified as redox processes. In the reaction between sodium and chlorine to yield sodium chloride, sodium is oxidized (its oxidation number increases from 0 in Na to +1 in NaCl) and chlorine is reduced (its oxidation number decreases from 0 in Cl 2 to –1 in NaCl). In the reaction between molecular hydrogen and chlorine, hydrogen is oxidized (its oxidation number increases from 0 in H 2 to +1 in HCl) and chlorine is reduced (its oxidation number decreases from 0 in Cl 2 to –1 in HCl).

Several subclasses of redox reactions are recognized, including combustion reactions in which the reductant (also called a fuel ) and oxidant (often, but not necessarily, molecular oxygen) react vigorously and produce significant amounts of heat, and often light, in the form of a flame. Solid rocket-fuel reactions such as the one depicted in [link] are combustion processes. A typical propellant reaction in which solid aluminum is oxidized by ammonium perchlorate is represented by this equation:

10 Al( s ) + 6 NH 4 ClO 4 ( s ) 4 Al 2 O 3 ( s ) + 2 AlCl 3 ( s ) + 12 H 2 O( g ) + 3 N 2 ( g )

Single-displacement (replacement) reactions are redox reactions in which an ion in solution is displaced (or replaced) via the oxidation of a metallic element. One common example of this type of reaction is the acid oxidation of certain metals:

Zn( s ) + 2 HCl ( a q ) ZnCl 2 ( a q ) + H 2 ( g )

Metallic elements may also be oxidized by solutions of other metal salts; for example:

Cu( s ) + 2 AgNO 3 ( a q ) Cu ( NO 3 ) 2 ( a q ) + 2 Ag( s )

This reaction may be observed by placing copper wire in a solution containing a dissolved silver salt. Silver ions in solution are reduced to elemental silver at the surface of the copper wire, and the resulting Cu 2+ ions dissolve in the solution to yield a characteristic blue color ( [link] ).

This figure contains three photographs. In a, a coiled copper wire is shown beside a test tube filled with a clear, colorless liquid. In b, the wire has been inserted into the test tube with the clear, colorless liquid. In c, the test tube contains a light blue liquid and the coiled wire appears to have a fuzzy silver gray coating.
(a) A copper wire is shown next to a solution containing silver(I) ions. (b) Displacement of dissolved silver ions by copper ions results in (c) accumulation of gray-colored silver metal on the wire and development of a blue color in the solution, due to dissolved copper ions. (credit: modification of work by Mark Ott)

Describing redox reactions

Identify which equations represent redox reactions, providing a name for the reaction if appropriate. For those reactions identified as redox, name the oxidant and reductant.

(a) ZnCO 3 ( s ) ZnO( s ) + CO 2 ( g )

(b) 2 Ga ( l ) + 3 Br 2 ( l ) 2 GaBr 3 ( s )

(c) 2 H 2 O 2 ( a q ) 2 H 2 O ( l ) + O 2 ( g )

(d) BaCl 2 ( a q ) + K 2 SO 4 ( a q ) BaSO 4 ( s ) + 2 KCl ( a q )

(e) C 2 H 4 ( g ) + 3 O 2 ( g ) 2 CO 2 ( g ) + 2 H 2 O ( l )

Solution

Redox reactions are identified per definition if one or more elements undergo a change in oxidation number.

(a) This is not a redox reaction, since oxidation numbers remain unchanged for all elements.

(b) This is a redox reaction. Gallium is oxidized, its oxidation number increasing from 0 in Ga( l ) to +3 in GaBr 3 ( s ). The reducing agent is Ga( l ). Bromine is reduced, its oxidation number decreasing from 0 in Br 2 ( l ) to –1 in GaBr 3 ( s ). The oxidizing agent is Br 2 ( l ).

(c) This is a redox reaction. It is a particularly interesting process, as it involves the same element, oxygen, undergoing both oxidation and reduction (a so-called disproportionation reaction) . Oxygen is oxidized, its oxidation number increasing from –1 in H 2 O 2 ( aq ) to 0 in O 2 ( g ). Oxygen is also reduced, its oxidation number decreasing from –1 in H 2 O 2 ( aq ) to –2 in H 2 O( l ). For disproportionation reactions, the same substance functions as an oxidant and a reductant.

(d) This is not a redox reaction, since oxidation numbers remain unchanged for all elements.

(e) This is a redox reaction (combustion). Carbon is oxidized, its oxidation number increasing from –2 in C 2 H 4 ( g ) to +4 in CO 2 ( g ). The reducing agent (fuel) is C 2 H 4 ( g ). Oxygen is reduced, its oxidation number decreasing from 0 in O 2 ( g ) to –2 in H 2 O( l ). The oxidizing agent is O 2 ( g ).

Check your learning

This equation describes the production of tin(II) chloride:

Sn( s ) + 2 HCl( g ) SnCl 2 ( s ) + H 2 ( g )

Is this a redox reaction? If so, provide a more specific name for the reaction if appropriate, and identify the oxidant and reductant.

Answer:

Yes, a single-replacement reaction. Sn( s ) is the reductant, HCl( g ) is the oxidant.

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Source:  OpenStax, Ucd bis2a intro to biology v1.2. OpenStax CNX. Sep 22, 2015 Download for free at https://legacy.cnx.org/content/col11890/1.1
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