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19.1 Occurrence, preparation, and properties of transition metals  (Page 7/27)

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4Ag ( s ) + 8CN − ( a q ) + O 2 ( g ) + 2H 2 O ( l ) ⟶ 4 [ Ag ( CN ) 2 ] − ( a q ) + 4OH − ( a q )
2Ag 2 S ( s ) + 8CN − ( a q ) + O 2 ( g ) + 2H 2 O ( l ) ⟶ 4 [ Ag(CN) 2 ] − ( a q ) + 2S ( s ) + 4OH − ( a q )
AgCl ( s ) + 2CN − ( a q ) ⟶ [ Ag ( CN ) 2 ] − ( a q ) + Cl − ( a q )
This figure contains two images. The first is of a small clump of bronze-colored metal with a very rough, irregular surface. The second shows a layer-like region of silver metal embedded in rock.
Naturally occurring free silver may be found as nuggets (a) or in veins (b). (credit a: modification of work by “Teravolt”/Wikimedia Commons; credit b: modification of work by James St. John)

The silver is precipitated from the cyanide solution by the addition of either zinc or iron(II) ions, which serves as the reducing agent:

2 [ Ag ( CN ) 2 ] − ( a q ) + Zn ( s ) ⟶ 2Ag ( s ) + [ Zn ( CN ) 4 ] 2− ( a q )

Refining redox

One of the steps for refining silver involves converting silver into dicyanoargenate(I) ions:

4Ag ( s ) + 8CN − ( a q ) + O 2 ( g ) + 2 H 2 O ( l ) ⟶ 4 [ Ag ( CN ) 2 ] − ( a q ) + 4OH − ( a q )

Explain why oxygen must be present to carry out the reaction. Why does the reaction not occur as:

4Ag ( s ) + 8CN − ( a q ) ⟶ 4 [ Ag ( CN ) 2 ] − ( a q ) ?

Solution

The charges, as well as the atoms, must balance in reactions. The silver atom is being oxidized from the 0 oxidation state to the 1+ state. Whenever something loses electrons, something must also gain electrons (be reduced) to balance the equation. Oxygen is a good oxidizing agent for these reactions because it can gain electrons to go from the 0 oxidation state to the 2− state.

Check your learning

During the refining of iron, carbon must be present in the blast furnace. Why is carbon necessary to convert iron oxide into iron?

Answer:

The carbon is converted into CO, which is the reducing agent that accepts electrons so that iron(III) can be reduced to iron(0).

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Transition metal compounds

The bonding in the simple compounds of the transition elements ranges from ionic to covalent. In their lower oxidation states, the transition elements form ionic compounds; in their higher oxidation states, they form covalent compounds or polyatomic ions. The variation in oxidation states exhibited by the transition elements gives these compounds a metal-based, oxidation-reduction chemistry. The chemistry of several classes of compounds containing elements of the transition series follows.

Halides

Anhydrous halides of each of the transition elements can be prepared by the direct reaction of the metal with halogens. For example:

2Fe ( s ) + 3Cl 2 ( g ) ⟶ 2FeCl 3 ( s )

Heating a metal halide with additional metal can be used to form a halide of the metal with a lower oxidation state:

Fe ( s ) + 2FeCl 3 ( s ) ⟶ 3FeCl 2 ( s )

The stoichiometry of the metal halide that results from the reaction of the metal with a halogen is determined by the relative amounts of metal and halogen and by the strength of the halogen as an oxidizing agent. Generally, fluorine forms fluoride-containing metals in their highest oxidation states. The other halogens may not form analogous compounds.

In general, the preparation of stable water solutions of the halides of the metals of the first transition series is by the addition of a hydrohalic acid to carbonates, hydroxides, oxides, or other compounds that contain basic anions. Sample reactions are:

NiCO 3 ( s ) + 2HF ( a q ) ⟶ NiF 2 ( a q ) + H 2 O ( l ) + CO 2 ( g )
Co ( OH ) 2 ( s ) + 2HBr ( a q ) ⟶ CoBr 2 ( a q ) + 2H 2 O ( l )

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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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