Tests for hydrogen sulfide

1. It’s unpleasant smell of rotten eggs.

2. The blackening of filter paper moistened with a soluble lead(II)salt (e.g. nitrate) due to the formation of lead(II) sulfide.

PolysulFides.

The sulfur ability for chains formation is observed in numerous polysulfides. They are prepared by interaction of sulfides of alkaline metals or NH₄⁺ with sulfur in concentrated solutions (water solutions of sulfides simply dissolve sulfur):

Na₂S^-2 + S⁰ = Na₂S₂^-1

Na₂S + (n-1)S = Na₂S_n

These reactions can be considered as an oxidation of sulfides by sulfur. They usually form the mixtures of polysulfides: n = 2-8 (n=9 only in ((NH₄)₂S₉), more frequently n=2). At n growth the colour will change from yellow through orange to red. The richest in sulfur (NH₄)₂S₉ has intensive red colour.

Under the action of strong acids, polysulfides form sulfanes Н₂S_n:

Н₂S_n + 2НСl = Н₂S_n + 2NaCl

A heavy yellow oil-like liquid prepared is a mixture of sulfanes Н₂S_n. In the individual state sulfanes up to Н₂S₈ are isolated: Н₂S₂ (m.p. = -88⁰, b.p. = 75⁰), Н₂S₃ (m.p. = -52⁰), Н₂S₄ (m.p. = -85⁰), Н₂S₅ (m.p. = -50⁰). They are relatively stable only in a strongly acidic medium, under other conditions they decompose with elimination of sulfur.

Polysulfide and sulfane molecules are zig-zag chains made by atoms of sulfur, valence orbitals of which are in the state of sp³-hybridization:

The ends of these chains are the atoms of Н (H₂S_n) or atoms of alkali metals in polysulfides. The dissociation energy of S—S bonds is equal to 226 kJ/mol and is similar enough for all sulfanes.

Sulfanes are weak acids in water solution. However, they considerably exceed H₂S strength. Their strength grows at the increase of sulfur atoms number in the molecule, for example:

Н₂S Н₂S₄ Н₂S₅

K₁ = 8.7∙10^-8 2∙10^-4 3∙10^-4

K₂ = 1∙10^-14 5∙10^-7 2∙10^-6

At the increase of sulfur atoms number H—S bonds become more polarized, therefore, its dissociation is facilitated under the action of polar water molecules. In accordance with the increase of acid strength, hydrolysis of their salts appropriately diminishes:

n in Н₂S_n 2 3 4 5

Degree of hydrolysis, % 64.6 37.6 11.8 5.7

Like peroxides, polysulfides display reduction properties:

They can display oxidizing properties under certain conditions:

Sn⁺²S + Na₂S₂^-1 = Sn⁺⁴S₂ + Na₂S^-2.

oxidant

when heated, they disproportionate:

Na₂S₂^-1 = Na₂S^-2 + S.

Дата добавления: 2014-01-11