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Ammonia displacement

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					      What will you be learning in
      this chapter?


• Concept of reversible reactions
• Manufacture of ammonia by Haber
  Process
• Displacement of ammonia from its salts
      Concept of Reversible
      Reactions
• Many chemical reactions proceed in
  one direction only (which means they
  cannot be reversed)

• Example:
  2KOH + H2SO4 → K2SO4 + 2H2O


     This arrow symbolizes a non-
     reversible chemical reaction.
      Concept of Reversible
      Reactions
• However, certain chemical reactions
  can be reversed.

• Example:
  NH3 (g) + HCl (g) → NH4Cl (s)

• But NH4Cl decomposes on heating:
  NH4Cl (s) → NH3 (g) + HCl (g)
      Concept of Reversible
      Reactions



• A double arrow sign is used to
  indicate a reversible reaction.

• NH3 (g) + HCl (g)         NH4Cl (s)
      Concept of Reversible
      Reactions

NH3 (g) + HCl (g)        NH4Cl (s)

         Forward Reaction
         NH3 (g) + HCl (g) → NH4Cl (s)

         Reverse Reaction
         NH4Cl(s) → NH3 (g) + HCl (g)
      What will you be learning in
      this chapter?


• Concept of reversible reactions.
• Manufacture of ammonia by Haber
  Process
• Displacement of ammonia from its salts
     Manufacture of Ammonia




                         Fritz Haber

• Ammonia is manufactured industrially by
  Haber Process.
 Manufacture of Ammonia by
 Haber Process

• N2 (g) + 3H2 (g)            2NH3 (g)

• Forward reaction is an exothermic
  reaction. (ΔH = -92.4 kJ mol-1)

• Since it is a reversible reaction, reaction
  conditions must be controlled to achieve
  maximum yield of ammonia at minimum
  cost.
Manufacture of Ammonia by
Haber Process

• The conditions required for making
  ammonia is governed by Le Chatelier’s
  Principle.
1.   Effects of Temperature on the
     Yield of Ammonia




 • High temperatures are needed to
   overcome the chemical inertness of
   N2 gas.
1.   Effects of Temperature on the
     Yield of Ammonia




 • But high temperatures favour the
   reverse reaction and decrease the
   yield of ammonia.
1.   Effects of Temperature on the
     Yield of Ammonia




 • Low temperatures increase the yield
   of ammonia but rate of reaction will
   be very slow.
1.   Effects of Temperature on the
     Yield of Ammonia




 • Compromise temperature:
   450C
2.   Effects of Pressure on the Yield of
     Ammonia




 • At low pressures, the rate of reaction
   for the production of ammonia will be
   very slow.
2.   Effects of Pressure on the Yield of
     Ammonia




 • High pressures will increase the yield
   of ammonia. But high pressures
   require costly equipment.
2.   Effects of Pressure on the Yield of
     Ammonia




 • Compromise Pressure:
   250 atmospheres (≈ 350 kPa)
3.   Effects of Catalysts on the Yield of
     Ammonia

 • Even at high pressure and
   temperature, the rate of reaction is
   still very slow due to the chemical
   inertness of nitrogen gas.

 • A catalyst is used to speed up the
   reaction.
3.   Effects of Catalysts on the Yield of
     Ammonia

 • Osmium is the best catalyst for the
   reaction but it is very expensive.

 • Iron catalyst is used instead.
Conditions for Haber Process

• Temperature: 450C
• Pressure: 250 atm
• Finely Divided Iron catalyst
Flow Scheme for Haber Process

Nitrogen from
                        Mixture of gases
    fractional
                         compressed to      CONVERTER
distillation of air                         450C
                          200-300 atm
                          COMPRESSOR        250 atm
        N2 and H2 are                       Finely divided
        mixed in                            Iron catalyst
        proportion of
        1:3
        respectively      Unreacted H2
                         and N2 pumped        CONDENSER
Hydrogen from            back for further      Liquid NH3
  cracking of               reaction
  petroleum
Haber Process

• Only about 10-15% of the nitrogen
  and hydrogen combine to form
  ammonia.
      What will you be learning in
      this chapter?


• Concept of reversible reactions.
• Manufacture of ammonia by Haber
  Process
• Displacement of ammonia from its salts
Displacement of Ammonia from its
Salts

• Cross reference to Textbook, Chapter
  11 Acids & Bases, Page 177

• Basically, whenever an ammonium
  salt is heated with an alkali react
  with an alkali, ammonia gas is
  displaced from the ammonium salt.
Displacement of Ammonia from its
Salts

• alkali + ammonium salt
   ammonia + water + metal salt

• Example:
  Ca(OH)2 (aq) + 2NH4Cl (s)
   2NH3 (g) + 2H2O (l) + CaCl2 (aq)
END OF CHAPTER
Extra Self-Study Learning
 Materials On Fertilisers
Nitrogenous Fertilisers

   • Nitrogenous fertilisers are
     compounds that produce
     either NH4+ or NO3- ions in
     the soil.

   • Plants need nitrogen to
     manufacture plant proteins
     which are necessary for
     growth and cell repair.
              Nitrogen Cycle
• How plants obtain their nitrogen in nature.
Common types of nitrogenous
fertilisers
            Do you know that ammonium
            nitrate is also commonly used
            as an explosive?


 Nitrogenous      Formula       Comment
  fertilisers
Ammonium        NH4NO3       Contains 35%
Nitrate                      by mass of
                             nitrogen
Common types of nitrogenous
fertilisers




 Nitrogenous       Formula     Comment
  fertilisers
Ammonium        (NH4)2SO4    Contains
Sulphate                     21.2% by
                             mass of
                             nitrogen
Common types of nitrogenous
fertilisers

                Do you know that urea can be
                obtained naturally from the urine
                of mammals (including humans)?


 Nitrogenous         Formula          Comment
  fertilisers
Urea             CO(NH2)2          Contains 46.7% by
                                   mass of nitrogen
                                   but less soluble in
                                   water than other
                                   ammonium
                                   fertilisers
  Calculating nitrogen content of
             fertilisers


Percentage of nitrogen =

Sum of Ar of nitrogen atoms in formula
           Mr of compound
   Calculating nitrogen content of
              fertilisers
Example: Ammonium Nitrate NH4NO3

No of Nitrogen atoms = 2
Sum of Ar of Nitrogen atoms in formula = 2 x 14
                                       = 28
Mr of NH4NO3 = 14 + 4 + 14 + (3 x 16)
             = 80

Percentage of Nitrogen in NH4NO3 = (28/80) x 100
                                 = 35.0%
Pollution by Fertilisers

• Two main kinds of problems may arise
  due to the leaching of fertilisers in the
  drinking water supply or into rivers and
  lakes:

  – Water Pollution
  – Eutrophication

				
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