ORGANIC CHEMISTRY: UREA
Urea, which is correctly known as carboxamide, has the distinction of 
being the first organic compound to have been made in a laboratory 
(1828); it is the main nitrogenous excretion product of most mammals.
1. Urea is manufactured from ammonia and carbon dioxide;
The equation for this condensation reaction can be written as:
(a) Complete this first Table by inserting the correct numbers of bonds
broken and formed.
  Bonds broken  
 Energy absorbed 
    / kJ mol-¹
  Bonds formed  
 Energy released 
    / kJ mol-¹
     6 N-H 
      2334
     6 N-H 
      1156
     2 C=O 
      1490
     1 C=O 
       745
              
              
     2 C-N
       558
             
             
     2 O-H 
       926
           Total =      3824
           Total =      3785
                                                                    [2]
(b) Calculate the heat energy change (DH) for the above reaction.
Energy absorbed - energy released = 3824 - 3785 = DH +39 kJ mol-¹
                                                                    [2]
(c) Carefully explain two reasons why high temperatures are used in the
industrial process.  To increase the speed of reaction, because the
increased kinetic energies of the particles means that more have the 
required activation energy for successful collisions.  To increase the 
yield of urea, because the position of equilibrium moves to oppose the
temperature decrease of this endothermic reaction.
                                                                    [4]
2.  The nitrogenous fertilizers in commonest use are ammonium salts,
but others are occasionally used; e.g., urea and sodium amide (NaNH2).
(a) Complete this second Table by calculating the molar mass and the
percentage nitrogen of each compound.
 Name 
  Formula 
  M / mol g-¹ 
 % Nitrogen 
 Ammonium sulfate
  (NH4)2SO4
      132
    21.2
 Ammonium hydrogenphosphate
  (NH4)2HPO4
      128
    21.9
 Ammonium nitrate
  NH4NO3
       80
    35.0
 Urea
  (NH2)2CO
       60
    46.7
                                                                    [4]
Apart from its higher nitrogen content, suggest one reason why urea is
sometimes preferred to an ammonium salt as a fertilizer.  Higher 
thermal stability / Lower solubility in water (... so less leaching)
                                                                    [1]
(b) Various soil micro-organisms secrete urease; this nickel-containing
enzyme catalyzes the conversion of urea to ammonium carbonate in moist
soils. Construct the symbol equation for this reaction.
                                 Urease
       (NH2)2CO(s) + 2H2O(l) ———————————————® (NH4)2CO3(aq)                                                                   [2]
Suggest one pollutant which might inhibit the activity of this enzyme.
Heavy metal ions [e.g., Pb2+(aq) / ...] / Acid rain [i.e., H1+(aq)]
                                                                    [1]

3.  Urea's principal use is as a monomer in the syntheses of various 
condensation polymers that are thermosetting materials (i.e., they do
not soften on heating). One of the many versatile reaction schemes used 
is summarized below:
(a) Complete the equation below, which summarizes the conditions used 
to prepare the 'parent' polymer (i.e., R = H).
                                                                    [2]
(b) In contrast to addition polymerization, condensation polymerization
is an endothermic reaction; so, in principle, condensation polymers 
should be readily hydrolyzed to their respective monomers. Suggest and
explain one reason why, in practice, synthetic condensation polymers 
are usually stable to hydrolysis.  They have extremely low solubilities 
in water / high activation energies, so there will be few successful 
collisions between the particles.
                                                                    [2]
(c) Naturally occurring condensation polymers include fats, proteins,
carbohydrates, and nucleic acids; their hydrolysis is vitally important
in the transfer and recycling of chemical energy and nutrient ions 
between living organisms. For each of the following, name one group of 
enzymes involved in their hydrolysis as well as the name of the final
monomeric products.    Carbohydrases
Carbohydrates ——————————————————————————————® Monosaccharides
                Proteases / Peptidases                              [2]
Proteins ———————————————————————————————————® Amino acids
        Lipases                                                     [2]
Fats ————————————® Fatty acids and glycerol (... propane-1,2,3-triol)
                                                                    [3]
(d) Most synthetic condensation polymers, including polyesters such as 
Terylene, are thermosetting: but a few, including polyamides such as 
Nylon, are thermoplastic (i.e., they soften on heating and harden again 
on cooling). By contrast, nearly all synthetic addition polymers are 
thermoplastic; the most notable exception is poly(tetrafluoroethene), 
popularly known as Teflon, which is thermosetting. Name one addition
polymer, apart from poly(chloroethene), which is a thermoplastic.
Poly(ethene) / Poly(phenylethene) / Poly(isoprene) / ...
                                                                    [1]
4.  Chemists often use the patterns inherent in the Periodic Table to
justify the syntheses of compounds which should have similar properties
to one already known; e.g., NaCl ('common salt') ——» KCl, NaF, LiI, ...
Suggest the structural formulae of two compounds which might be
expected to show chemical or biological properties similar to urea.
                                                                    [2]

Dr. R. Peters Next Contents' List & Teacher's Notes