HYDROGEN SULPHIDE

HYDROGEN SULPHIDE
It is mostly found in volcanic gases, sulphur springs, coal gas and gases formed formed during the decay of organic matter containing sulphur.
PREPARATION
It is prepared in the laboratory and commercially by the action of a dilute acid on a metallic sulphide like iron(ii) sulphide.
The gas is easily liquefied and may be purchased in this form in steel cylinders.
KIPP’S APPARATUS
It is used to prepare intermittent or immediate gas just when needed in the laboratory.
It is used to produce frequently used gases like SO2 and CO
PHYSICAL PROPERTIES

1. It is a colourless gas with a repulsive smell like that of a rotten egg.
2. It is very poisonous.
3. It is about 1.18 times denser than air.
4. It is moderately soluble in water to form a very weak acidic solution.
5. It burns with a pale blue flame.
   CHEMICAL PROPERTIES
6. As an acid:
   It ionises slightly in water to form a dibasic acid. When bubbled through an alkali, a normal salt and water are formed. However, when present in excess, hydrogen sulphide can react with sodium or potassium hydroxide to form the corresponding acid salt.
7. As a precipitating agent:
   Many metals form insoluble sulphides which are usually coloured. When hydrogen sulphide is bubbled through solutions of salts of such metals, the coloured sulphides are precipitated. Paints often change colour because of the reaction of a metallic pigment with the hydrogen sulphide in the surrounding air.
8. Reaction with oxygen:
   It does NOT support combustion but it burns in a plentiful supply of air with a bright blue flame to produce sulphur (iv) oxide.
   As a reducing agent:
   It is a strong reducing agent. It oxidises itself to sulphur.
   (A) With tetraoxomanganate (vii): When hydrogen sulphide is bubbled through a solution of acidified KMnO4, the purple KMnO4 solution disappears with a yellow deposit of sulphur observed. The colour change is due to the reduction of the purple tetraoxomanganate (vii) ion to the very pale pink mangenese (ii) ion.
   (B) With Heptaoxodichromate (vi) :
   When hydrogen sulphide is bubbled through a solution of acidified potassium Heptaoxodichromate (vi) solution, the solution changes from orange to green. Sulphur is also deposited at the same time. The colour change is due to the reduction of the orange Heptaoxodichromate (vi) ion to the green chromium (iii) ion.
   (C) With the halogens:
   Hydrogen sulphide is oxidized by the halogens to sulphur. Moisture must be present for the reactions to occur.
   (D) With iron (iii) chloride:
   Hydrogen sulphide reduces the brown-yellow solution of iron(iii) chloride to a green solution of iron(iii) chloride. The hydrogen sulphide itself is oxidized to sulphur and hydrogen chloride.
   (E). With sulphur (iv) oxide: Hydrogen sulphide is a stronger reducing agent than sulphur (iv) oxide. It reduces sulphur (iv) oxide to sulphur in the presence of moisture.

TEST FOR HYDROGEN SULPHIDE
Using lead(ii) trioxonitrate (v) : Moisten a piece of filter paper with lead (ii) trioxonitrate (v) solution and drop it into a gas jar of the unknown gas. If the gas is hydrogen sulphide, the paper turns black due to the formation of black lead (ii) sulphide. Lead(ii) ethanoate too gives the same reaction.
Pb(NO3)2(aq) + H2S(g) to give PbS(s) + HNO3(aq)

USES

1. It is used in the analysis of ores and metals.
2. The presence of a certain metal is often indicated by the colour of its sulphide.