National Chemistry

Fertilisers

 

  Increased world population has led to the need for more efficient food production.

      Growing plants require nutrients (from compounds of K, P and N). Fertilisers are  substances which restore the essential elements for plant growth to the soil.

 

Useful fertilisers - because they are soluble and contain essential elements are    ammonium

salts, potassium salts, nitrates and phosphates. Ammonia and nitric acid are  important

compounds in the manufacture of fertilisers. Different crops need different   fertilisers containing different amounts of potassium, nitrogen and phosphorous.

 

 Nitrogen can be put into the soil naturally by

      (i) decomposition of plant and animal protein (remains)

      (ii) nitrifying bacteria in root nodules of some plants convert atmosphere (free) N2(g) into nitrogen compounds

 

      In the nitrogen cycle plants obtain nitrogen from the soil while dead plants and animals replace the nitrogen by decomposition. However, the harvesting of food removes the chance of nitrogen returning to the soil naturally.

 

 HABER PROCESS -  manufactures ammonia

                                                           Iron catalys ; 400oC

                  NITROGEN+HYDROGEN  ----------------------> AMMONIA

                                                            HIGH PRESSURE

 

             N2    +   3H2          <-------     2NH3

 

The Haber process is one of the most important reactions in the production of fertilisers and is an example of a reversible reaction. That is; as the nitrogen and hydrogen make ammonia, the ammonia in turn breaks down to hydrogen and nitrogen. The proportions of each that exist depends on carefully controlled reaction conditions

 

 

Ammonia is used to produce nitric acid which is then used to make nitrate compounds (eg ammonium nitrate).

 

    NH3    --------     HNO3     ------neutralisation--  NH4NO3

 

      Ammonia gas (NH3) is colourless, soluble, has a characteristic smell and is an alkaline (pH7) gas (pH paper = blue). Ammonia is used to make ammonium compounds and to produce nitric acid. The Haber process is carried out at a moderately high temperature  to ensure a faster (though less) yield of ammonia.  (film)

 

Ammonia can be prepared in a lab by heating  an ammonium compound with sodium   hydroxide

 (this is similar to acid + alkali  - it forms salt + water as well as ammonia

                     NH4Cl         +           NaOH -----> NaCl           +      H2O         +        NH3

 

Nitrogen gas is not very reactive and will only react with oxygen in the presence of a   great deal of energy (eg from a lightning storm or in a car engine) - therefore to produce  the brown nitrogen dioxide (NO2)by reacting nitrogen and  oxygen is very expensive.

      When NO2 is present in the air it dissolves to produce nitric acid (acid rain) which helps replace nitrogen in the soil but also lowers the pH (makes acidic).

 

      Industrially nitrogen dioxide is produced by oxidising ammonia in the presence of a catalyst - part of the OSWALD process to produce nitric acid.

 

            N2       +    3H2 ------ >      NH3          Haber produced ammonia

 

            NH3  +     O2 ------ >         NO2          Oswald produces nitrogen dioxide

 

 

Heat is used to start this reaction but does not need to be supplied once reaction has  started as the reaction supplies its own heat - is exothermic

 

   NO2     +     H2O ----in the air---->      HNO3          (Nitric acid produced in air = acid rain)

 

In the lab oxidation of ammonia this is carried out hsing hot platinium catalyst. This is heated at the start but does not need heated during the reaction as heat is given out which continues to make the platinum catalyst glow hot.

 

Haber uses an Iron catalyst while Oswald uses a Platinium catalyst

Rememeber by alphabetical order HI (Haber/Iron) and OP (Owald/Platinium)

 

 Calculate the percentage of an element in a compound

What is the percentage of nitrogen in NH4NO3 ?

step 1 : mass of 1 mole (2 x N) + (4 x H) + (3 x O)

                                     (2 x 14) + (4 x 1) + (3 x 16)

                                        28 + 4  +48 = 80g

 

step 2 : How much nitrogen is present ( 2 x N) = (2 x 14) = 28

 

step 3: 28g out of 80g is nitrogen  ie   28

                                                            80

 

step 4: remember it is a percentage:   28   x  100     =  

                                                            80

 

another example

 

           

National 4 and 5

Fertilisers

 

These notes are for the Scottish National Chemistry course taught in fourth year in most Scottish schools.

 

The notes available here are concise notes. They are NOT to be considered as material to learn from - they are for revision.

 

 The notes are arranged under the following topics

 

National 3        Unit 1

                          Unit 2

                          Unit 3

 

National 4 and 5  Unit 1        Rates of Reaction

                                                  Atomic Structure

                                                  Bonding and Properties

                                                  Acids and bases

 

National 4 and 5  Unit 2       Fuels and Homologous Series

                                                Consumer Products

                                                Carbohydrates

 

National 4 and 5  Unit 3         Metals

                                                   Plastics

                                                   Fertilisers

                                                   Nuclear Chemistry

                                                  Chemical analysis

 

 

 

 

 

 

 

 

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