Hydrogen sulphide (H2S): Lab preparation, Kipp’s apparatus, Properties and Uses.

Laboratory preparation of Hydrogen sulphide (H₂S)

Hydrogen sulphide (H₂S) gas is prepared in the laboratory by treating iron sulphide with dil. H₂SO₄.

Procedure : Iron sulphide (FeS) pieces are kept in woulf’s bottle. Then a thistle funnel and delivery tube is fitted as shown in fig. When dilute H₂SO₄ is poured in woulf ’s bottle, reaction between FeS and dil. H₂SO₄ takes place and H₂S gas is evolved which is collected in the gas jar by upward displacement of air.

Test of H₂S : When a lead acetate paper is dipped in the jar containing H₂S gas, it turns to black due to formation of lead sulphide.

Intermittent supply of H₂S gas by Kipp’s apparatus :

(Working principle of Kipp’s apparatus )

{Intermittent = Stopping and starting at irregular intervals}

H₂S gas is frequently required for salt analysis in the laboratory. The special apparatus which is used for the preparation of H₂S is called Kipp’s apparatus.

Kipp’s apparatus consist of three bulbs A, B and C. The bulbs are interconnected with each other. The upper bulb C contains a long stem which reaches upto bulb A. When dil. H₂SO₄ is poured from bulb C, liquid level rises in bulb A and reaches upto bulb B just to cover iron sulphide. Then the reaction between iron sulphide and H₂SO₄ takes place to form H₂S gas. As H₂S gas is formed, the pressure inside bulb B increases and forces the dil. H₂SO₄ up into the upper bulb C. Then the contact between FeS and dil. H₂SO₄ breaks and the formation of gas is stopped (ceased). When the tap is opened H₂S comes out and the pressure decreases. As a result liquid level again rises at bulb B and H₂S gas start to evolve again. In this way H₂S gas is prepared intermittently by Kipp’s apparatus.

Physical Properties of H₂S :

1. It is colourless gas with rotten egg smell.
2. It is soluble in water.
3. It is poisonous gas.

   {In its low concentration it causes nausea, dizziness and headache. At high concentration it is fatal (i.e. deadly poisonous). Conc. of H2S in air 1 part in 1000 parts of air may be fatal.}

4. It is heavier than air{with vapour density 17}

Chemical Properties of H₂S :

 1. Acidic character of H₂S :

It acts as a weak diprotic (dibasic) acid. It ionizes in two steps to give two protons.

• H₂S reacts with base to form two series of salts. Eg.

• H₂S reacts with basic oxides to give salt.

2. H₂S as reducing agent :-

Sulphur in H₂S bears -2 oxidation state which in minimum oxidation state of sulphur. In chemical reaction, H₂S tends to get oxidized . So H₂S acts as reducing agent.

• It reduces halogens into halogen acids.

• It reduces sulphur dioxide to sulphur.

• It reduces ferric salt to ferrous salt.

• It reduces conc. HNO₃ to NO_2.

• It reduces conc. H₂SO₄ to SO₂.

• It reduces acidified potassium permanganate solution and pink colour of KMnO₄ is discharged.

• It reduces acidified K₂Cr₂O₇ solution and orange color of K₂Cr₂O₇ changes to green.

3. H₂S as analytical reagent in salt analysis ( Precipitation of metal sulphide) :

H₂S gas is used as analytical agent to detect group II basic radicals in acidic medium i.e. HCl and group IIIB basic radicals in in alkaline medium i.e. NH₄Cl and NH₄OH.

Group II basic radicals : Group II basic radicals consists of Cu ⁺⁺ , Hg ⁺⁺, Pb⁺⁺, Cd⁺⁺, Sn⁺⁺, As⁺⁺⁺, Sb⁺⁺⁺, Bi⁺⁺⁺. In qualitative salt analysis this group II radicals are precipitated in the form of their sulphide in acidic medium. By noting the colour of ppt., we can identify the metal cations.

Group IIIB basic radicals : Group IIIB basic radicals consist of Zn⁺⁺, Co⁺⁺, Ni⁺⁺, Mn⁺⁺. These are precipitated by passing H₂S in alkaline medium ( i.e. NH₄Cl + NH₄OH).

Structure of H₂S :

Uses of Hydrogen sulphide (H₂S) :

• It is used as analytical reagent in laboratory.
• It is used as reducing agent.
• It is used to prepare metallic sulphide, which can be used as pigment.

References :

• Shriver, D. F., Atkins, P. W., Inorganic Chemistry, Fifth Edition, Oxford university Press, 2010.
• Agrawal, S. K., Lal, K., Advanced Inorganic Chemistry, Fifth Revised Edition, Pragati Prakashan, Meerut, 2001.
• Cotton, F. A., and Wilkinson, G., Advanced Inorganic Chemistry, Fifth edition, John Wily and Sons, Singapore, 1995.
• Day, C.M., Selbin, J., Theoritical inorganic Chemistry, second edition, Affiliated East-West Press Pvt. Ltd., New Delhi, 2002.
• Mitra, L.A. , A Text Book of Inorganic Chemistry, Ghos and Company, 61^st edition, 1996.
• https://en.wikipedia.org/wiki/Kipp%27s_apparatus
• https://www2.humboldt.edu/scimus/HSC.54-70/Descriptions/Kipp%27sGasApp.htm