HYDROGEN
Occurrence
Hydrogen is found only in very small amounts in the atmosphere and in volcanic gases.
Combined with carbon, hydrogen is found in natural gas, Kerosene, gasoline and other
petroleum products. It is also a constituent of most other organic compounds.
Laboratory preparation of hydrogen
Hydrogen is prepared in the laboratory by reacting certain metals with dilute mineral
acids, water or steam. The three methods commonly used for the preparation of hydrogen
in the laboratory are:
1. Action of zinc on an acid: Dilute hydrochloric acid or tetraoxosulphate(VI) acid
attacks metallic zinc with the liberation of hydrogen gas.
Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
Zn(s) + H2SO4(aq) + H2(g)
2. Action of sodium on cold water: Sodium liberates hydrogen from cold water. This
reaction is very vigorous and should be carried out with extreme care using a small
piece of sodium.
2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
3. Action of iron on steam: Iron, at red heat liberates hydrogen from steam.
3Fe(s) + 4H2O(g)
Fe3O4(s) + 4H2(g)
Industrial preparation of hydrogen
1. From water gas (Bosch process): When steam is passed over red hot coke
(carbon) at about 1200⁰C, a mixture of carbon(II) oxide and hydrogen known as
water gas is produced.
H2O(g) + C(s) → CO(g) + H2(g)
2. From methane: Steam is mixed with methane (obtained from natural gas) and
passed over nickel catalyst at about 800⁰C. The mixture of carbon(II) oxide and
hydrogen produced is known as synthesis gas.
Ni
CH4(g) + H2O(g)
CO(g) + 3H2(g)
800⁰C
Hydrogen and activity series
The metals which can displace hydrogen from water or acids vary in their degree
of activity.
Potassium and sodium reacts vigorously with cold water to displace hydrogen,
while calcium reacts slowly. Heated magnesium displaces hydrogen from steam,
aluminum, zinc and iron will only do so at red heat, while lead and copper do not
react at all.
With dilute hydrochloric or tetraoxosulphate(VI) acid, potassium and sodium react too
violently for the reaction to be carried out safely in the Laboratory. Calcium and
magnesium reacts vigorously, while aluminum, zinc and iron reacts moderately, with the
rate of reaction (as measured by the rate of release of hydrogen bubbles) being the fastest
with aluminum and the slowest with iron. Lead and copper do not react with dilute acids.
Note: Lead does not show any reaction with the dilute acids because the lead salts formed
are insoluble and form a protective coating over the metal.
Physical properties of hydrogen
1.
2.
3.
4.
5.
Hydrogen is a colourless, odourless and tasteless gas.
It is neutral to litmus
It is relatively insoluble in water
Hydrogen is the lightest known gas
It has a very low boiling point of -253⁰C
Chemical properties of hydrogen
1. Combination reaction
a. With metals: hydrogen combines directly with the active metals to form ionic
hydrides.
2Na(s) + H2(g) → 2NaH(s)
b. With oxygen: pure hydrogen burns with a pale blue flame as it combines with
oxygen to produce steam.
2H2(g) + O2(g) → 2H2O(g)
c. With halogens: Hydrogen combines directly with the halogens to produce
halides.
Cl2(g) + H2(g) → 2HCl(g)
2. Reducing action: Hydrogen is a strong reducing agent. It reduces the oxide of
copper, lead, iron and zinc to the respective metals when they are heated in
steam of the gas. At the same time, hydrogen itself is oxidized to form water.
CuO(s) + H2(g) → Cu(s) + H2O(g)
Test for Hydrogen
Insert a lighted splinter into a test tube containing the unknown gas. If the gas is hydrogen,
it will burn with a pop sound.
Isotopes of hydrogen
Hydrogen exist in three isotopic forms, namely:
1. hydrogen or protium 11𝐻,
2. Heavy hydrogen or Deuterium 21𝐻
3. Tritum 31𝐻
Assignment: State five uses of hydrogen