Revision Notes: Study Of The First Element – Hydrogen
Study of the First Element - Hydrogen
Hydrogen is the first element of the periodic table. It was discovered by Cavendish and named by Lavoisier.
Position of hydrogen in the periodic table
Hydrogen belongs to the first group and the first period of the periodic table.
Although the properties of hydrogen should be similar to those of the other members of the first group, this is not the case.
Some of the properties of hydrogen resemble the properties of Group IA elements (Alkali metals), and some of the properties resemble the properties of Halogens (VIIA).
Thus, hydrogen was put at the top of the periodic table so that the symmetry of the modern periodic table is not disturbed.
Dual Nature of Hydrogen
Hydrogen has an electronic configuration of 1.
Hydrogen loses 1 electron and behaves like electropositive alkali metals [Group 1 (IA)].
Hydrogen gains 1 electron and behaves like electronegative halogens [Group 17 (VIIA)].
Similarity of Hydrogen with Alkali Metals and Halogens
Similarity of hydrogen with alkali metals [Group 1 (IA)]
Similarity of hydrogen with halogens [Group 17 (VIIA)]
Electronic configuration
Electronic configuration = 1. Thus, there is 1 electron in the outermost valence shell. Examples:
H = 1; Li = 2, 1; Na = 2, 8, 1; K = 2, 8, 8, 1
One electron less than the
nearest noble gas. Examples:
H = 1 (He = 2)
F = 2, 7 (Ne = 2, 8)
Cl = 2, 8, 7 (Ar = 2, 8, 8)
Ion formation
Electropositive character exhibited.
Examples:
-
1+
H - 1e→ H
-
1+
Li - 1e→ Li
-
1+
Na - 1e→ Na
Electronegative character
exhibited.
Examples:
-
1-
H + 1e→ H
-
1-
F + 1e→ F
-
1-
Cl + 1e→ Cl
Valency
Electrovalency of one exhibited. H , Li , Na
1+ 1+ 1+
Electrovalency and covalency
exhibited.
Hydrogen forms
NaH (electrovalent)
4
CH(covalent)
Chlorine forms
NaCl (electrovalent)
4
CCl(covalent)
Reactions
Strong affinity for non-metals. Examples: O, S, Cl
Hydrogen forms H
2
2
2
2
O, HS, HCl Sodium forms NaO, NaS, NaCl
__
Reducing agent
Acts as a reducing agent.
Hydrogen:
2
2
CuO + H→ Cu + HO
Sodium:
2
CuO + 2Na → Cu + NaO
__
Atomicity
__
Diatomic molecules are formed (Two atoms linked by a single bond).
Hydrogen
H:H or H-H → H2
Chlorine
Cl:Cl or Cl-Cl → Cl2
Preparation of Hydrogen - General Methods
1. General methods - From cold water and metals
Metals react with cold water to form metal hydroxide and liberate hydrogen gas.
Reactions:
2
2
2K + 2HO → 2KOH + H↑
2
2
2Na + 2HO → 2NaOH + H↑
2
2
2
Ca + 2HO → Ca(OH)+ H↑
2. General methods - From boiling water/steam and metals
Magnesium
Mg + H2O → MgO + H2↑ (boiling water)
Aluminium
2Al + H2O → Al2O3 + 3H2↑ (steam)
Zinc
Zn + H2O → ZnO + H2↑ (steam)
Iron
Fe + 4H2O Fe3O4 + 4H2↑ (steam)
Mg, Al, Zn and Fe do not react with cold water. They react with boiling water and liberate hydrogen gas, but the reaction is very slow.
Mg, Al, Zn and Fe react with hot steam in the heated state and form the corresponding oxide and hydrogen gas.
Iron reacts with steam, and the reaction is reversible.
3. General methods - From acids
Magnesium
Mg + 2HCl → MgCl2 + H2↑
Aluminium
2Al + 3H2SO4 → Al2(SO4)3 + 3H2↑
Zinc
Zn + H2SO4 → ZnSO4 + H2↑
Iron
Fe + 2HCl → FeCl2 + H2↑
4. General methods - From concentrated alkalis
Aluminium
2Al + 6NaOH → 2Na3AlO3 +3 H2↑ (Sodium aluminate) 2Al + 2KOH + 2H2O → 2KAlO2 +3 H2↑ (Potassium meta aluminate)
Zinc
2
2
2
Zn + 2NaOH → NaZnO+ H↑
(Sodium zincate)
2
2
2
Zn + 2KOH → KZnO+ H↑
(Potassium zincate)
Lead
2
2
2
Pb + 2NaOH → NaPbO+ H↑
(Sodium plumbate)
Preparation of Hydrogen - Laboratory Method
Laboratory method by using granulated zinc, dilute hydrochloric acid or dilute sulphuric acid.
Reactions:
Zn + 2HCl → ZnCl2 + H2↑ (dilute) Zn + H2SO4 → ZnSO4 + H2↑ (dilute)
Collection:
Hydrogen gas is collected by the downward displacement of water.
Manufacture of Hydrogen - By the Electrolysis of Water
Hydrogen is commercially obtained by the electrolysis of water.
Water is a poor conductor of electricity. Thus, a less volatile acid such as sulphuric acid is added to water to make it a good conductor of electricity. This is called acidulated water.
On passing electric current through this acidulated water, water dissociates.
H2O H+ + OH-
H+ , being positively charged, moves towards the cathode (negatively charged electrode).
At cathode,
H+ + e -→ H
H + H → H2
At anode,
OH- - e -→ OH
OH + OH → H2O+O
O + O → O2
Hence, water dissociates to give hydrogen and oxygen by passing an electric current through acidulated water.
2H2O → 2H2 + O2
Bosch Process
The Bosch process consists of the following steps.
Step 1:
Steam is passed over hot coke (at 1000°C) in a special type of furnace called a converter to form carbon monoxide and hydrogen gas. This mixture is called water gas.
Step 2:
Excess of steam is mixed with water gas, and the entire mixture is passed over heated ferric oxide and chromic oxide. Ferric oxide acts as a catalyst and chromic oxide as a promoter.
Step 3:
In this step, carbon dioxide is removed from the reaction mixture. The mixture of carbon dioxide and hydrogen is forced through cold water under pressure at 30 atmospheric pressure or through caustic potash solution, which dissolves carbon dioxide leaving behind hydrogen gas.
2KOH + CO2 → K2CO3 + H2O
Step 4:
In this last step, the mixture is passed through ammoniacal solution of cuprous chloride solution so as to dissolve carbon monoxide. Thus, hydrogen gas is obtained.
CuCl + CO + 2H2O → CuCl.CO.2H2O
Physical Properties of Hydrogen
Colourless, odourless and tasteless gas.
Non-poisonous.
Solubility is very low.
It can be liquefied.
Vapour density is 1, much lighter than air.
Chemical Properties of Hydrogen
It is neutral to litmus.
It is combustible but does not support combustion.
Nascent hydrogen: Fresh hydrogen formed at the time of generation is called nascent hydrogen.
Reaction with oxygen
Hydrogen burns with a pop sound in oxygen and burns with a pale blue flame forming water. 2H2 + O2 → 2H2O
Reaction with Chlorine
H2 + Cl2 2HCl
Reaction with Hydrogen (Haber process)
Three volumes of hydrogen and one volume of nitrogen react at temperature 450-500°C at a pressure of 200-900 atm in the presence of finely divided iron (catalyst) and molybdenum (promoter).
Reaction with sulphur
Hydrogen gas when passed through molten sulphur reacts to give another gas, hydrogen sulphide.
H2 + S → H2S
Reaction with metals
Dry hydrogen when passed over heated metals, such as Na, K and Ca, reacts to give their corresponding hydrides.
2K + H2
→
2KH
2Na + H2
→
2NaH
Ca + H2
→
CaH2
Reaction with metallic oxides
Hydrogen reduces metal oxides to give metals; thus, hydrogen is a reducing agent.
Fe2O3
+
3H2
→
2Fe
+ 3H2O
Fe3O4
+
4H2
→
3Fe
+ 4H2O
PbO
+
H2
→
Pb
+ H2O
CuO
+
H2
→
Cu
+ H2O
Uses of Hydrogen
As a fuel in oxy-hydrogen blow torch
In meteorological balloons
In the manufacture of ammonia
In the manufacture of hydrogenated oils
For producing artificial petrol from coal
In the extraction of metals
As a reducing agent
