Nitrogenous fertilizers

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Unit 31

• Nitrogenous fertilizers sulphuric acid

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How do green plants make their own food?

• By Photosynthesis
• Green plants make use of carbon dioxide and
  water, in the presence of light and chlorophyll,
  make glucose (starch) and oxygen.

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What are needed for healthy plant?

• Green plants make their food in photosynthesis.
  Any more things needed?
• Mineral salts (elements) are needed. What are
  they?
• They are nitrogen, phosphorus, and potassium.
• These are removed from the soil when crops are
  harvested.
• Thus, fertilizers have to be added for restoring
  minerals to the soil.

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Why these elements are good for plants?
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Types of fertilizers

• Natural fertilizers - compost (decaying organic
  matter) and manure.
• Artificial fertilizers - a mixture of compounds
  containing the elements, nitrogen, phosphorus and
  potassium. (e.g., ammonium sulphate or amonium
  nitrate)

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Changing nitrogen gas (in air) into compounds

• How?
• In thunderstorm (lightning) (with electric
  spark), nitrogen and oxygen combine to form
  nitrogen oxide.
• N2(g) O2(g) ? 2NO(g)
• Nitrogen monoxide then combines with oxygen in
  air to form nitrogen dioxide.
• 2NO(g) O2(g) ? 2NO2(g)

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Changing nitrogen gas into compounds

• Nitrogen dioxide dissolves in rainwater to form
  dilute nitric acid.
• 2NO2(g) H2O(l) ? HNO2(aq) HNO3(aq)
• 4NO2(g) O2(g) 2H2O(l) ? 4HNO3(aq)
• Sources of nitrate ions (natural fertilizers).

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Nitrogen in soil

• Nitrogen-fixing bacteria in the roots of peas and
  beans for changing nitrogen into nitrates.

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Nitrogenous fertilizers
Nitrogenous compound Physical state Molar mass (g mol-1) solubility
ammonia, NH3 gas 17 very high
ammonium nitrate, NH4NO3 solid 80 extremely high
ammonium sulphate, (NH4)2SO4 solid 132 high
ammonium hydrogenphosphate, (NH4)2HPO4 solid 132 low
Urea, (NH2)2CO solid 60 high
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Percentage by mass of nitrogen in fertilizers

• Calculate the percentage by mass of nitrogen in
  ammonia nitrate.
• Formula of ammonium nitrate ?
• Mass of 1 mole of ammonium nitrate ?
• Mass of nitrogen in 1 mole of ammonium nitrate ?
• Mass percentage of nitrogen in ammonium nitrate
  ??

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Percentage by mass of nitrogen in ammonium
sulphate

• Formula of ammonium sulphate ?
• Molar mass of ammonium sulphate ?
• Mass percentage of nitrogen in ammonium sulphate
  ??

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Calculation example

• 1 kg of ammonium sulphate is sold at a price of
  2. Calculate the cost of buying 1 kg of nitrogen
  by buying ammonium sulphate.

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NPK label
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Nitrogen Number

• The mass percentage of nitrogen in the
  fertilizer.
• If the nitrogen number of a fertilizer is 15,
• Calculate the mass of nitrogen in 1000 g
  fertilizer.

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Phosphorus Number

• the mass percentage of phosphorus in the form of
  P2O5 present in the fertilizer.
• If the NPK label of a fertilizer is 16.8.24.,
  calculate the mass percentage of phosphorus in
  the fertilizer.

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Potassium number

• the mass percentage of potassium in the form of
  K2O present in the fertilizer.
• If the NPK label of a fertilizer is 16.8.24.,
  calculate the mass percentage of potassium in the
  fertilizer

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How to prepare a fertilizer (ammonium sulphate)
in the laboratory?

• How??
• What chemicals are needed?
• What are their formulae?
• Write an equation for the reaction involved.

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Experimental Steps (Procedures)

• Pipette 25.0 cm3 ammonia solution into a conical
  flask.
• Add a few drops of methyl orange (indicator).
  Colour in alkali yellow
• Fill the burette with dilute sulphuric acid.
• Titrate the ammonia solution with dilute
  sulphuric acid until the methyl indicator turns
  from ______ to ______ . (I.e. the end point is
  reached.)

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Experimental Steps (Procedures)

• Mix 25.0 cm3 of ammonia solution with the
  required volume of dilute sulphuric acid without
  any indicator.
• Warm to saturate the salt solution.
• Allow it to crystallize slowly to give large
  crystals of ammonium sulphate.
• Filter off the crystals and dry crystals between
  filter paper.

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Haber Process

• Industrial Manufacture of ammonia
• By the direct combination of nitrogen and
  hydrogen
• In the presence of fine divided iron catalyst.
• N2(g) 3H2(g) ? 2NH3(g)

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What is a catalyst?

• Name an enzyme (a biological catalyst).
• A catalyst is a substance that will alter /
  change (usually speed up) the rate of a chemical
  reaction.
• Catalysts are usually transition metals or
  compounds of transition metals.

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Flow diagram of Haber Process
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Raw materials of Haber Process

• Nitrogen from the fractional distillation of
  liquid air
• Hydrogen from the reaction of steam with
  methane or naphtha.
• CH4(g) H2O(g) ? CO(g) 3H2(g)

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Finely divided iron catalyst

• Fine-divided in powder form
• Why ?
• To Increase the surface area of the catalyst.

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Poisoning of the catalyst

• Impurities, such as sulphides and carbon
  monoxide, adhere on the surface of the solid
  catalyst. poisoning of the catalyst.
• Raw materials, nitrogen and hydrogen are
  purified, before passing over the red-hot
  catalyst.

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Reaction Conditions of Haber Process

• Nitrogen to hydrogen volume ratio 13
• Compressed to a pressure of 200 atmospheres.
• Pass over the red-hot iron catalyst at 500oC.
• N2(g) 3H2(g) ? 2NH3(g) Exothermic
• Percentage yield 15

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Ways to improve (increase) the yield of ammonia

• By removing ammonia (product) from the reaction
  mixture.
• By liquefying ammonia or by dissolving ammonia in
  water.
• Removing ammonia and the recycle of unreacted
  nitrogen and hydrogen. Increase the yield of
  ammonia. (More ammonia)
• Heat released in the reaction is absorbed by the
  heat exchanger for heating the incoming new
  reactants.

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Preparing ammonia in the laboratory

• Ammonia a weak alkali.
• Prepared by heating a salt of weak alkali with a
  strong alkali.
• Heating ammonium salt with a strong alkali.
• NH4Cl NaOH ? NH3 NaCl H2O
• A colourless gas with irritating smell which
  turns moist (wet) red litmus paper blue is given
  out.
• Ammonia gas is poisonous.

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Heating ammonium salt with sodium hydroxide
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Test for ammonia gas

• With a characteristic irritating smell (no a
  chemical test)
• Turns moist red litmus paper blue.
• NH3(aq) H2O(l) ? NH4(aq) OH-(aq)
• Put a glass rod wetted with concentrated
  hydrochloric acid near a gas jar (tube) of
  ammonia gas. A dense white fume of ammonium
  chloride is formed.
• NH3(g) HCl(g) ? NH4Cl(s)

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Ammonia solution as a weak alkali

• What is a weak alkali?
• Partly ionized in aqueous solution.
• NH3(aq) H2O(l) ? NH4(aq) OH-(aq)
• What do you see when limited amount of aqueous
  ammonia is added into copper(II) sulphate
  solution?
• Pale blue precipitate is formed.
• Cu2(aq) 2OH-(aq) ? Cu(OH)2(s)

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Uses of ammonia gas

• To fix atmospheric nitrogen into nitrogen
  compounds which are used as fertilizers.

A good solvent for grease. Used as a glass
(window) cleaner.
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Uses of ammonia gas

• Used as a freezing agent (refrigerant) in
  refrigerators.
• An important starting chemical for preparing
  nitric acid and fertilizers.

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Industrial manufacture of nitric acid (Ostwald
Process)

• Oxidation number of nitrogen in ammonia ??
• Oxidation number of nitrogen in nitric acid ???
• By catalytic oxidation of ammonia
• Ammonia is oxidized catalytically by oxygen in
  the presence finely divided platinum catalyst.
• 4NH3(g) 5O2(g) ? 4NO(g) 6H2O(g)

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Industrial manufacture of nitric acid (Ostwald
Process)

• Nitrogen monoxide reacts with oxygen (in air) to
  form nitrogen dioxide.
• 2NO(g) O2(g) ? 2NO2(g)
• Nitrogen dioxide is then dissolved in water in
  the presence of oxygen to form nitric acid
• 4NO2(g) O2(g) 2H2O(l) ? 4HNO3(aq)

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Uses of nitric acid

• For making fertilizers.
• For making monomers of the polymer nylon.
• For making explosives.
• For making drugs.

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Contact Process for the manufacture of
concentrated sulphuric acid

• Raw materials ???
• Sulphur dioxide, SO2
• By burning sulphur in air
• S(s) O2(g) ? SO2(g)
• By roasting metal sulphide ore
• 2ZnS(s) 3O2(g) ? 2ZnO(s) 2SO2(g)
• Oxygen from air

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Contact Process

• To prevent the poisoning of the catalyst by
  purifying the raw materials.
• Catalyst used
• Finely divided platinum expensive, (more
  efficient)
• Or vanadium(V) oxide, V2O5 (cheaper)

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Reaction conditions

• Sulphur dioxide and oxygen mixed in the ratio of
  2 to 1.
• Temperature 450oC
• A pressure of one or two atmospheres.
• 2SO2(g) O2(g) ? 2SO3(g) Exothermic
• Percentage yield of SO3 90

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Flow diagram of contact process
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Increasing the yield of sulphur trioxide

• By removing sulphur trioxide.
• Not by dissolving sulphur trioxide in water as
  the reaction is highly exothermic and vaporize
  the acid.
• The acid fume is hazardous to the operator and
  harmful to the machinery.

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Removing sulphur trioxide

• Dissolving sulphur trioxide in concentrated
  sulphuric acid to form oleum (fuming sulphuric
  acid)
• SO3(g) H2SO4(l) ? H2S2O7(l)
• Oleum is then carefully diluted in right
  proportion to give concentrated sulphuric acid
• H2S2O7(l) H2O(l) ? 2H2SO4(l)

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Uses of sulphuric acid

• For the manufacturing of fertilizers.
• For the manufacture of soapless detergents and
  dyestuffs..

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Paint Additives

• For the manufacture of paint additives
• Metal sulphates are soluble in water except
  calcium sulphate, barium sulphate and lead(II)
  sulphate
• Mixing a soluble calcium / barium salt solution
  with a soluble sulphate solution.
• Ca2(aq) SO42-(aq) ? CaSO4(s)
• Ba2(aq) SO42-(aq) ? BaSO4(s)

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Uses of concentrated sulphuric acid