Hydrogen makes up about 1% of the earth’s crust. It is freely found 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’s also a constituent of most other organic substances, including proteins, carbohydrates and fats which are essential components of all living matter.
Hydrogen has only one electron in its shell. It therefore needs one other electron to complete its duplet and so resemble Helium nucleus.
It has an oxidation number of 1 in mosr of its compounds except in hydride where it is -1.
Hydrogen is liberated when certain metals react with dilute mineral acids , water or steam. It is also given off when Tin, Zinc or Aluminium reacts with hot concentrated solutions of sodium or potassium hydoxide.
- Action of Zinc on an acid:
Dilute hydrochloric acid or tetraoxosulphate (vi) acid attacks metallic zinc with the liberation of hydrogen gas. No heating is necessary.
Zn(s) + 2 H2SO4 (aq) gives ZnSO4(aq) + H2(g)
- Action of Sodium on cold water:
Sodium liberates hydrogen from cold water. This reaction is vigorous and should be carried out with extreme care using a small piece of sodium.
2 Na(s) + 2H2O(l) gives 2 NaOH(aq) + H2(g)
- Action of iron on steam:
Iron, at red heat liberates hydrogen from steam. Iron(ii) diiron(iii) oxide, Fe3O4 is formed at the same time.
- Hydrogen is colourless, tasteless and odourless
- It is neutral to litmus paper.
- It is relatively insoluble in water.
- Hydrogen is the lightest known substance.
- It has a very low boiling point of – 253 oC
Hydrogen is an unusual element. It has a single valence electron like Group 1 alkali metals, yet it is clearly a gas with non-metallic properties like Group 7 ( halogens). Thus, it can behave as a group 1 element by loosing an electron or as a group 7 element by gaining an electron to form an hydride ion.
The chemical behaviour of hydrogen can be explained by its tendency to acquire the stable duplet electronic configurations of helium.
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 , Aluminium, Zinc and Iron reacts moderately. Lead and Copper do not react with dilute acids.
- From water gas (Bosch process)
In this process, large quantities of hydrogen are produced from cheap raw materials, namely water and coke. When steam is passed over red-hot coke ( carbon) at about 1200 oC, a mixture of carbon (ii) oxide and hydrogen known as water gas . One mole of hydrogen and one mole of synthesis gas makes up the water gas.
- From Methane:
The Bosch process is now being replaced by a similar process which uses cheap hydrocarbons like methane instead of coke. Steam is mixed with methane (obtained as a natural gas) and passed over a nickel catalyst at about 800 oC. The mixture of carbon (ii) oxide and hydrogen produced is known as synthesis gas . Three moles of hydrogen and one mole of carbon(ii) oxide makes the synthesis gas.
- By electrolytic method:
Pure hydrogen is obtained as a by-product in the electrolysis of brine for the manufacture of sodium hydroxide and chlorine.
HYDROGEN AND THE ACTIVITY SERIES
In the laboratory preparation of hydrogen some metals could displace hydrogen from water and acids. There are, however, other metals like copper and silver which cannot displace hydrogen from water or acids. Metals above hydrogen are electropositive metals which will displace hydrogen from water and acids while those below hydrogen will NOT be able to displace hydrogen from water and acids.wat
- COMBINATION REACTIONS
a. With metals:
It combines with other metals to form ionic hydrides e.g NaH
b. With Oxygen:
It combines with oxygen to form steam.
c. It combines directly with the halogens to form hydrogen halides like HBr and HCl
- REDUCING ACTION
Copper (ii) oxide reacts with Hydrogen to give copper and water, thus hydrogen behaving as a reducing agent which becomes oxidized.
TEST FOR HYDROGEN
Insert a lighted splint into a test-tube containing an unknown gas. If it’s hydrogen gas, it gives a pop sound when ignited in air.
Precaution: It should be carried out with small quantities of hydrogen gas.
- It is used in the manufacture of ammonia, hydrochloric acid and methanol.
- It is used in the manufacture of margarine. Hydrogen of oils in the presence of nickel as catalyst gives margarine.
- It is used in filling balloons because it is the lightest gas. However, its high flammable nature limits its use.
- Hydrogen is a constituent of many gaseous fuels such as water gas and coal gas. Liquid hydrogen is used as rockel fuel.
- It can be used as oxy-hydrogen flames to produce high temperatures of over 2000 oC that can melt metals.
ISOTOPES OF HYDROGEN
It exists in three isotopic forms namely protium, deuterium and tritium with relative atomic masses of 1, 2 and 3 respectively.
The alkali and alkali earth metals like sodium and calcium form ionic hydrides with hydrogen. They are crystalline solids with high melting points that conducts electricity when molten. They readily react with water to form hydroxides and liberate hydrogen gas. Boron and aluminium form complex covalent hydrides which are important reducing agents like lithium tetrahydridoaluminate (iii) and sodium tetrahydridoborate