Download Position of Hydrogen in the periodic table. Atomic number of

CHEMISTRY
HYDROGEN
STD -8
Position of Hydrogen in the periodic table.
Atomic number of hydrogen - 1
Number of valence electron - 1
It either loses one electron and behaves like an electropositive alkali metal ( group 1 – IA)
H – 1e- ---------- H+
Or gains one electron behaving like electronegative halogens (group 17 – VII A)
H + 1e- ---------- H+
So Hydrogen shows similarities with alkali metals and halogens.
OCCURRENCE:
In the free state:
Earth’s crust
0.98%
Earth’ s atmosphere
0.01%
Volcanic gases
0.025%
Atmosphere around the sun & the stars 01.1%
In the combined state:
* In plant and animal tissues.
* In water
* As a constituent of substances like acids, alkalies, petroleum products & organic substances.
GENERAL METHODS OF PREPARATION OF HYDROGEN
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LABORATORY METHOD OF PREPARATION OF HYDROGEN
REACTION: Zn + 2HCl  ZnCl2
+ H2
REACTANTS: Granulated zinc in flat bottom flask (X)
Dilute hydrochloric acid added through thistle funnel ( Y)
PROCEDURE: Granulated zinc is placed in the flask and hydrochloric acid is added slowly from the
thistle funnel .A brisk effervescence is seen with the evolution of hydrogen gas.
* Hydrogen forms an explosive mixture with air hence the apparatus is air tight to prevent any
leakage of the gas.
* A naked flame should not be brought near the apparatus since it may cause an explosion if the gas
leaks.
* The lower end of the funnel should below the level of the acid in the flask thereby minimizing the
chances of any leakage of the hydrogen gas through the thistle funnel.
* Granulated zinc is the commercial zinc obtained from molten zinc.It may contain some impurities
which has a slight catalyzing effect on the reaction. Addition of traces of copper sulphate also
enhances the speed of the reaction.
* The preferred acid is dilute sulphuric or hydrochloric acid .Nitric acid being a strong oxidizing
agent oxidises the hydrogen formed to water and is therefore not used.
* Even though hydrogen is lighter than air it is not collected by downward displacement of air as it
forms an explosive mixture with air.
* Hydrogen is collected by downward displacement of water as it is almost insoluble in water.
* Granulated zinc on reaction with different acids imparts traces of gaseous impurities which are
removed by the passage through different solutions :
-- Arsine (AsH3 ) and phosphine (PH 3 ) through silver nitrate solution
-- Hydrogen sulphide (H2S ) through lead nitrate solution
-- Nitrogen dioxide, carbon dioxide and sulphur dioxide through KOH solution.
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-- Moisture using a drying agent i.e. fused calcium chloride.
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USES OF HYDROGEN
* Hydrogenation reactions: Addition of
hydrogen to organic compounds in presence of
catalyst e.g. Pt or Ni under pressure at about
2000 C is called hydrogenation. Vegetable oils
turn to semi solid fats by hydrogenation.
* Passage of hydrogen under high pressure
over powdered coal in presence of a catalyst at
a suitable temp. is called hydrogenation of
coal.
* Hydrogen when passed over heated metal
oxides of less active metals like Zn, Fe, Pb and
Cu reduces them to free metals.- a process
useful in metallurgy.
* Oxygen burns in the atmosphere of hydrogen
to produce an oxy- hydrogen flame which is
used for welding and cutting metals.
* In oxidation and reduction reactions
REASONS
Certain metals like Pt, Ni , Pd & Au readily
adsorb large volumes of H2 on their
surface.This phenomenon is called occlusion.
It leads to the conversion of coal to a product
similar to petroleum containing higher
percentage of hydrogen.
H2 acts as a reducing agent.
ZnO + H2----- Zn + H2O
Fe2 O3 + 3 H2 - 2 Fe + 3H2 O
Hydrogen- oxygen mixture on burning
produces an exothermic reaction and the temp.
of the flame is around 28000
H2S + Cl2 ---- S( oxidized product) + 2 HCl
Br2 + H2S ---- 2 HBr ( reduced product) + S
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