Chapter 26 Hydrogen

7/28/2019 Chapter 26 Hydrogen

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Chapter 26 Hydrogen

26.1 Occurrence

Uncombined hydrogen does not occur in nature to any appreciable extent.

Hydrogen occurs in vast quantities in a combined state in compounds such as water, acids, and many organic

substances.

26.2 Industrial preparation of hydrogen

(1)From hydrocarbonsMethane can be passed with steam over nickel catalyst at 800C and 30atm.Carbon monoxide and hydrogen is formed.

This process is called steam reforming of methane. Therefore, methane is the raw materials in Haber process.

CH4(g) + H

2O(g)

Ni

800C 30atmCO(g) + 3H

2(g)

(2)By electrolysis(a) Hydrogen is obtained as a by-product in the electrolytic manufacture of chlorine from brine using diaphragm cell.

In the electrolysis of brine, sodium hydroxide, chlorine and hydrogen are produced.

Ionization of electrolyte: Possible reactions if diaphragm cell is not used:

At cathode:At anode:

Uses of these three important chemicals

Sodium hydroxide Chlorine Hydrogen

(b) Hydrogen can also be obtained by the electrolysis of water containing sulphuric acid. Water is a weak electrolyteand acid is added to increase the conductivity of the electrolyte.

Ionization of electrolyte:

At cathode:

At anode:

What is the volume ratio of the gases produced?

Calculate the time req

26.3 Laboratory preparation of hydrogen

Two methods can be used to prepare hydrogen in laboratory

(1) action of dilute acids on metals(2) action of steam on heated iron(1) Preparation of hydrogen by the action of dilute acids on metalsPut some pieces of zinc into a flat-bottomed flask. Add dilute hydrochloric acid by using a thistle funnel. Zinc start to

dissolve and effervescence occurs. Hydrogen produces is then collected over water.

Zn(s) + 2HCL(aq) ZnCl2(aq) + H

2(g)


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Dilute nitric acid, is not used for preparation of hydrogen by reaction with a metal as it is a strong oxidizing agent and

will produce hydrogen only when reacting with magnesium.

Sodium and potassium are never used with a mineral acid to prepare hydrogen, as the reactions are extremely violent.

Preparation of hydrogen by the action of steam on heated iron

It is a reversible reaction and needs to be displaced from equilibrium in order to collect a high yield of hydrogen.

Heat the tube with a plug of loosely packed wool soaked in water under the iron fillings, gently at first. By moving the

flame of the Bunsen burner occasionally backwards and forwards, steam will be generated. Keep the flow of steam

constant, and collect the hydrogen by displacement of water.

Fe(s) + H2O(g) Fe

3O

4(s) + H

2(g)

Uses of hydrogen

(1) For filling balloons.(2) In the hardening of oils to make margarine. Oils are heated to 180C and finely divided nickel is added as a

catalyst. The oil combines with hydrogen and is converted to a fat, which is solid at ordinary temperature and is

used in the manufacture of margarine.

(3) In the manufacture of ammonia by Habers process.(4) In the synthesis of hydrochloric acid.


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Test for hydrogen

It burns with a pop sound when a burning splint is applied to the hydrogen.

Properties of hydrogen

It is invisible, neutral and if pure, possesses no smell and burns steadily in air. It is much less dense than air.

If a gas-jar full of hydrogen (B) is held under a gas-jar full of air (A), and the covers of both are removed, after a short

time (approximately 15 seconds) the upper gas-jar may be removed and tested for hydrogen by applying a lighted

splint. The contents of the gas-jar will explode (or burn), showing that hydrogen has passed upwards displacing the air

in the top gas-jar to form a mixture. A similar test applied to the bottom gas-jar will produce no explosion or burning,

showing that the hydrogen previously in the bottom jar has all been displaced by the air from the upper jar.

Pure hydrogen burns steadily in air. A lighted splint applied to an inverted gas-jar of hydrogen will cause the gas toburn steadily around the edges of the jar. The product if combustion of hydrogen is water.

2H2(g) + O

2(g) 2H

2O(l)

The burning of hydrogen to form water

Hydrogen is generated by the action of fairly concentrated hydrochloric acid on zinc.

Zn(s) + 2H+(aq) Zn

2+(aq) + H

2(g)

The gas then passes through a U-tube containing anhydrous calcium chloride in order to dry the gas, the hydrogen is

burnt at a jet and the vapours are cooled by coming in to contact with a can kept cool by water.

2H2(g) + O

2(g) 2H

2O(l)

When a test-tube full of hydrogen burns quietly on the application of a flame to it, light the jet and allow theflame to burn so that it just does not touch the cool can.

Moisture will condense on the can and drop off into a dish which is placed below to receive the liquid.

Test for the presence of waterAction on anhydrous copper(ll) sulphate

Allow a drop of the liquid fall on to anhydrous copper(ll) sulphate. A blue patch on the white solid(with hissing sound

and development of heat) indicates that water is present, but does not prove the liquid to be pure water.

CuSo4(s) + 5H

2O(l) CuSo

4.5H

2O(s)

Test for pure water

Boiling-point of pure water

If the atmospheric pressure is 760mmHg the thermometer should register 100C.

These two tests together prove the liquid to be pure water. An additional, but less convenient test, is to find the

freezing point, which should be 0C ate 760mmHg pressure.

Isotopes of hydrogen


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Ordinary hydrogen or protium H Heavy hydrogen or deuterium D- Deuterium oxide is known as heavy water D2O. It resembles ordinary water but has a higher density (about

1.10gcm-3

at room temperature.)

Tritium T

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Chapter 26 Hydrogen