Mercury, Hg2+, Hg22+

Most common oxidation states: +1, +2

M.P. -38.87 o

B.P. 356.57o

Density 13.546 g/cm3

Characteristics: Mercury is one of the few liquid elements. It dissolves in oxidizing acids, producing either Hg2+ or Hg22+, depending on which reagent is in excess. The metal is also soluble in aqua regia ( a mixture of hydrochloric and nitric acids) to form HgCl42-.

Mercury(I) Ion, Hg22+:

Mercury(I) compounds often undergo disproportionation, producing black metallic mercury and mercury(II) compounds.

Characteristic reactions of Hg22+:

Chloride Ion:

Soluble chlorides, including hydrochloric acid, precipitate white mercury(I) chloride, also known as calomel:

Hg22+(aq) + 2Cl-(aq) <==> Hg2Cl2(s)

Aqueous ammonia reacts with Hg2Cl2 to produce metallic mercury (black) and mercury(II) amidochloride (white), a disproportionation reaction:

Hg2Cl2(s) + 2NH3(aq) --> Hg(l) + HgNH2Cl(s) + NH4+(aq) + Cl-(aq)

Aqueous Ammonia:

Aqueous ammonia produces a mixture of a white basic amido salt and metallic mercury:

2Hg22+(aq) + 4NH3(aq) + NO3-(aq) + H2O(l) --> 2Hg(l) + Hg2ONH2NO3(s) + 3NH4+(aq)

The precipitate is not soluble in excess aqueous ammonia.

Sodium Hydroxide:

Black finely divided mercury metal and yellow mercury(II) oxide (HgO) are precipitated by NaOH:

Hg22+(aq) + 2OH-(aq) --> Hg(l) + HgO(aq) + H2O(l)

Reducing Agents:

Reducing agents, such as Sn2+ and Fe2+, reduce mercury(I) to the metal:

Hg22+(aq) + 2Fe2+(aq) --> 2Hg(l) + 2Fe3+(aq) 

Consult an activity series or a table of reduction potentials for other possible reducing agents.

No Reaction:

SO42- (unless solutions are concentrated; solubility of mercury(I) sulfate is 0.06 g per 100 mL of water at 25oC)


Mercury(II) Ion, Hg2+:

Characteristic reactions of Hg2+:

Chloride Ion:

No reaction is visible, but Hg(II) will be present as [HgCl4]2-.

Aqueous Ammonia:

Aqueous ammonia produces white amido salts whose composition depends on the mercury(II) salt present in the solution:

HgCl2(aq) + 2NH3(aq) <==> HgNH2Cl(s) + 2NH4+(aq) + Cl-(aq)

These salts are not soluble in excess aqueous ammonia, but do dissolve in acids:

HgNH2Cl(s) + 2H+(aq) + Cl-(aq) <==> HgCl2(aq) + NH4+(aq) 

Sodium Hydroxide:

A yellow precipitate of HgO is produced by NaOH:

Hg2+(aq) + 2OH-(aq) --> HgO(s) + H2O(l)

HgCl2(s) + 2OH-(aq) --> HgO(s) + H2O(l) + 2Cl-(aq)

The mercury(II) oxide precipitate is insoluble in excess hydroxide but is soluble in acids:

HgO(s) + 2H+(aq) <==> Hg2+(aq) + H2O(l)

Hydrogen Sulfide:

Hydrogen sulfide precipitates black mercury(II) sulfide, the least soluble of all sulfide salts.

Hg2+(aq) + H2S(aq) <==> HgS(s) + 2H+(aq)

[HgCl4]2-(aq) + H2S(aq) <==> HgS(s) + 2H+(aq) + 4Cl-(aq)

Mercury(II) sulfide is insoluble in 6 M HNO3 or 12 M HCl, even if heated. However, it is soluble in aqua regia (3:1 HCl:HNO3) and in hot dilute NaOH containing excess sulfide.

3HgS(s) + 12Cl-(aq) + 2NO3-(aq) + 8H+(aq) --> 3[HgCl4]2-(aq) + 2NO(g) + 3S(s) + 4H2O(l)

HgS(s) + S2-(aq) <==> [HgS2]2-(aq)

Tin(II) Chloride:

Tin(II) chloride reduces Hg(II) to Hg(I) or to metallic Hg, giving a white or gray precipitate:

2[HgCl4]2-(aq) + [SnCl4]2-(aq) <==> Hg2Cl2(s) + [SnCl6]2-(aq) + 4Cl-(aq)


No Reaction:

SO42- (may precipitate as a mixed sulfate-oxide - a basic sulfate - HgSO4.2HgO)

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