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4.2 Classifying chemical reactions  (Page 7/34)

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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 )
 

Watch a brief video showing the test firing of a small-scale, prototype, hybrid rocket engine planned for use in the new Space Launch System being developed by NASA. The first engines firing at
3 s (green flame) use a liquid fuel/oxidant mixture, and the second, more powerful engines firing at 4 s (yellow flame) use a solid mixture.

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, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9
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