Page 170 - Chemistry ICSE Class IX

158 ICSE Chemistry – 9
volatile covalent compounds. Hydrogen and uorine combine even
in the dark at temperature as low as –250°C (23 K).
dark
H (g) + F (g) o 2HF(g)
2
2
Hydrogen reacts quite vigorously with chlorine when heated or
exposed to sunlight.
sunlight
H (g) + Cl (g) o 2HCl(g)
2
2
Bromine, which is less reactive, combines with hydrogen at
temperature above 400°C.
above 400°C
H (g) + Br (g) o 2HBr(g)
2
2
Hydrogen and iodine vapour combine under proper conditions in
the presence of catalyst. This reaction is reversible.
H (g) + I (g) 2HI(g)
2
2
ƒ 9KVJ PKVTQIGP Hydrogen reacts with nitrogen at about 450-500°C
and under high pressure (200-900 atm) in the presence of a catalyst
to give ammonia.
N (g) + 3H (g) 2NH (g) + heat
3
2
2
This reaction forms the basis of the Haber’s process for the
manufacture of ammonia from nitrogen and hydrogen.
ƒ 9KVJ UWNRJWT Hydrogen also combines directly when heated with
sulphur or bubbled through molten sulphur giving H S.
2
heat
H (g) + S o H S(g)
2
2
hydrogen sulphur hydrogen sulphide
or, H (g) + S (molten) o H S(g)
2
2
ƒ 9KVJ ECTDQP When heated over 1100°C, hydrogen directly combines
with carbon forming methane, CH .
4
If an arc is struck between carbon electrodes in an atmosphere of
hydrogen, acetylene (C H ) is formed.
2
2
1100°C
C(s) + 2H (g) o CH (g)
2
4
carbon hydrogen methane
electric arc
2C(s) + H (g) o C H (g)
2
2
2
carbon hydrogen acetylene
Reactions of hydrogen with metals
Highly electropositive metals, such as sodium, lithium and calcium, react
DĞƚĂůƐ ůŝŬĞ ŝƌŽŶ͕ ŶŝĐŬĞů͕ ƉĂůůĂĚŝƵŵ͕ ĞƚĐ͘ at high temperatures with hydrogen to form hydrides.
ƚĂŬĞ ƵƉ ŚLJĚƌŽŐĞŶ ĂŶĚ ĨŽƌŵ intersƟƟal In such compounds, each hydrogen atom accepts one electron to
Žƌ metallic hydrides͘
–
become a negative hydride ion, H . These hydrides are ionic compounds
and very unstable in water. These react with water giving hydroxide of the
metal and hydrogen gas.
'
+
2Na(s) + H (g) o 2NaH(s) o 2Na H –
2
sodium hydrogen sodium hydride
'
NaH(s) + H O(l) o NaOH(aq) + H (g)
2
2
'
2+
–
Ca(s) + H (g) o CaH (s) o Ca (H ) 2
2
2
calcium hydrogen calcium hydride
CaH (s) + 2H O(l) o Ca(OH) (aq) + 2H (g)
2
2
2
2