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Hcl notes

HYDROGEN CHLORIDE – HCl
Hydrogen chloride gas is prepared in the following methods.
By synthesis or direct combination:
H2 + Cl2 2HCl
Moisture acts as catalyst.
This reaction is not carried out in direct sunlight because it is explosive.
Laboratory method – from NaCl and Conc. Sulphuric acid
NaCl + H2SO4 NaHSO4 + HCl
Conc.
2NaCl + H2SO4 Na2SO4 + HCl
Conc.
Reactants : Sodium chloride (rock salt) and Conc. H2SO4
Sodium Chloride is preferred because it is cheap and easily available.
Conc. H2SO4 is used because it is non-volatile. Conc.HNO3 not
preferred because it is a volatile acid.
Products : Sodium bisulphate and Hydrogen chloride gas
Temperature : less than 2000C
If it is more than 2000C Fuel is wasted, the glass apparatus may tend to
crack and the product Sodium sulphate form a hard crust which is
difficult to remove from the round bottom flask.
Precautions : The reactants must be heated slowly –to control the evolution
of Hydrogen chloride gas.
The lower end of the thistle funnel should be below the surface
of the acid – otherwise HCl gas escape out through it
Purification : HCl gas is purified by passing it through Conc. Sulphuric acid (drying
agent) removes moisture from HCl gas.
CaO and P2O5 are not used because they react with HCl as shown:
CaO + 2HCl CaCl2 + H2O
2P2O5 + 3HCl POCl3 + 3HPO3
Collection : Upward displacement of air because HCl gas is heavier than air and
highly soluble in water.
Identification : A glass rod dipped in Ammonia solution is kept near the gas jar then gives
Dense white fumes of Ammonium chloride.
Physical properties of Hydrogen chloride :
 HCl is a colourless gas with pungent chocking odour.
 Heavier than air and highly soluble in water.
Fountain experiment – to prove the high solubility of HCl gas in water
HCl gas in the round bottom flask dissolves in
water and creates vacuum in round bottom flask and
due to the higher pressure on the litmus solution
pushes it into the round bottom flask. The litmus
solution emerges out of the jet tube as red fountain.
Chemical properties:
 HCl gas is non-combustible
 HCl gas undergoes thermal dissociation (reversible thermal decomposition)
2HCl H2 + Cl2
 Reacts with Ammonia gas to form Ammonium chloride fumes.
HCl + NH3 NH4Cl
 Active metals displace Hydrogen from Hydrogen chloride gas.
Zn + 2HCl ZnCl2 + H2
Fe + 2HCl FeCl2 + H2
Mg + 2HCl MgCl2 + H2
Preparation of Hydrochloric acid:
Hydrochloric acid is prepared by dissolving Hydrogen chloride in water.
 Direct absorption of HCl gas in water using delivery tube causes back-suction.
 Hence special funnel arrangement is used to prepare Hydrochloric acid.
Special funnel arrangement –prevents back-suction of water
–provides a large surface area for absorption of HCl gas.
HCl when dissolves in water ionizes to give H+ and Cl –
. H+ combines with H2O and forms
Hydronium ion which is responsible for the acidic nature of HCl solution.
HCl + H2O H3O
+
+ Cl –
 Reactions of dil. HCl
1. dil. HCl + active metal H2 + salt
dil. 2HCl + Mg H2 + MgCl2
dil. 2HCl + Zn H2 + ZnCl2
2. dil. HCl + base Salt + water
dil. HCl + NaOH NaCl + H2O
dil. 2HCl + CuO CuCl2 + H2O
3. dil. HCl + metal carbonate/bicarbonate salt + water + CO2
dil.2HCl + Na2CO3 2NaCl + H2O +CO2
4. dil. HCl + metal sulphite/bisuphite salt + water + SO2
dil. HCl + NaHSO3 NaCl + H2O + SO2
5. dil. HCl + metal sulphide salt + H2S
dil. 2HCl + CuS CuCl2 + H2S
black blue
dil. 2HCl + FeS FeCl2 + H2S
6. dil.HCl + AgNO3 HNO3 + AgCl
white ppt.
7. dil. 2HCl + Pb(NO3)2 2HNO3 + PbCl2
white ppt.
8. dil. 2HCl + Na2S2O3 2NaCl + H2O + SO2 + S
(yellow particles)
 Reactions of Conc. HCl with oxidising agents:
MnO2 + 4HCl MnCl2 + H2O + Cl2
2KMnO4 + 16HCl 2KCl + 2MnCl2 + 8H2O + 5Cl2
K2Cr2O7 + 14HCl 2KCl + 2CrCl3 + 7H2O + 3Cl2
PbO2 + 4HCl PbCl2 + 2H2O + Cl2
Pb3O4 + 8HCl 3PbCl2 + 4H2O + Cl2

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