CHEMISTRY

TOPIC: Hydrogen Sulphide

By Kingsley Idiagbor, B.Sc.(Hon), PGDCs, NCE, MNSM

Hydrogen Sulphide,also known as hydrogen sulfide occurs naturally in petroleum deposits and is also an air pollutant. It is easily identified by its characteristic smell.

Laboratory Preparation

Hydrogen Sulphide is prepared by the action of dilute hydrochloric acid on iron(II) sulphide. The liberated gas is collected above warm water (since it readily dissolves in cold water) or by downward delivery.

Equation of reaction: FeS + HCl-----------------> FeCl₂ + H₂S

Test

The prepared gas can be recognized by any of the following ways:

Smell: This gas has a characteristic rotten-egg smell;

Action on Litmus: Hydrogen gas turns litmus paper red (somehow pink);

Action on Lead ethanoate paper: The gas turns lead ethanoate paper black.

Physical Properties

• Hydrogen sulphide is colourless gas with rotten-gas smell;
• It turns litmus paper red, somehow pinkish; because of the acidic nature of the gas;
• The gas is about 1.25 times denser than air, hence it can be collected by downward delivery

Chemical Properties

Action on water: Hydrogen sulphide can dissolve in water to form a very weak acid;

Thus: H₂S + H₂O ---------------------> HS + H₂

Acidic behaviour: The gas, when dissolved in water, shows the normal acidic properties:

• It reacts with trioxocarbonates (IV) to liberate carbon (IV) oxide gas;

H₂S + Na₂CO₃ -------------> Na₂S + H₂O + CO₂

• It undergoes neutralization reaction to form salt (a sulphide) and water only;

H₂S + 2NaOH ----------------> Na₂S + 2H₂O

• Metals higher than hydrogen in the activity series can displace the gas from the acid;

Zn + H₂S ----------------------->ZnS + H₂

Action on Lead Ethanoate: Hydrogen sulphide reacts with lead ethanoate paper to from black precipitate of Lead (II) sulphide:

(CH₃COO)₂Pb + H₂S---------->PbS + CH₃COOH

Action on Lead trioxonitrate: In the absence of lead ethanoate, hydrogen sulphide can also form black precipitate of lead (II) sulphide with Lead (II) trioxonitrate (V) paper.

Pb (NO₃)₂ + H₂S---------->PbS + HNO₃

As reducing agent: H₂S acts as reducing agent by decolorizing acidified KMnO₄ and depositing yellow sulphur in the process.

H₂S + 2KMnO₄ + H₂SO₄------> K₂SO₄ +2MnSO₄ + H₂O + S

It also reduces sulphur (IV) oxide to sulphur. Thus: 2H₂S + SO₂ --> 2H₂O + 3S

Reaction with metals: H₂S reacts with metals to form the corresponding metallic sulphide while hydrogen gas is evolved. This is, of course, with the exception of such metals that are found below hydrogen in the activity series. Thus:

H₂S + Zn ------------> ZnS + H₂

H₂S + Mg ------------> MgS + H₂

H₂S + Na ------------> NaS + H₂

KIPP'S APPARATUS

This apparatus is used in the laboratory for the intermittent supply of gases prepared by the action of an aqueous solution on a solid reactant. Example of such a gas is Hydrogen Sulphide. Other gases that can be produced using the Kipp's apparatus include hydrogen and carbon (IV) oxide.

Mechanism

Kipp's Apparatus is made up of three chambers. In the intermittent preparation of H₂S gas, hydrochloric acid is poured through the thistle funnel and chamber A to the chamber C. Whenever the gas is required, the tap is opened. This causes the volume of the chamber B to increase.

In accordance with the statements of Boyle's Law, the pressure decreases. The HCl acid rises from the chamber C to the chamber B, in order to raise the pressure to be at par with the atmospheric pressure. The acid will subsequently react with the iron sulphide in the chamber B and the gas is produced.

If the gas is no longer needed, the tap is closed. Since the H₂S is still being produced for a while, there is high pressure and so the HCl acid falls back to the chamber C in order to permit the pressure to fall to the atmospheric pressure. The HCl is no longer in contact with the iron sulphide (FeS) that is in the chamber B and so the reaction ceases. There is also the consequent stop in the production of the H₂S gas.

Uses

The major use of hydrogen sulphide is as an analytic reagent.

It is also used in the manufacture of metallic sulphides.

Copyright © Kingsley Idiagbor 2 September 2005

All rights reserved. No part of this lesson may be reproduced, transmitted by any means or stored in an electronic retrieval system without first obtaining permission from the owner of the copyright. Violators are liable to civil prosecution under the applicable Federal laws.

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