Ammonia - The Structure of Ammonia.

Ammonia

The Structure of Ammonia

The ammonia molecule has a pyramidal shape with a bond angle of 107º as shown above.

The central nitrogen atom has five outer electrons with an additional electron from each hydrogen. This gives a total of eight electrons, or four electron pairs which are arranged tetrahedrally. Three of these electron pairs are used as bond pairs, which leaves one lone pair of electrons. The lone pair of electrons repel more strongly than bond pairs, therefore the bond angle is not 109.5º as expected for a regular tetrahedral arrangement, but is measured at 107º.

Ammonia is a unidentate ligand. It can only donate one pair of electrons to a metal ion. It forms octahedral complexes with transition metals of the first row transition series. It also can act as a Lewis base as it can donate an electron pair. An example of this is the reaction between boron tetrafluoride and ammonia:

BF3 + NH3 → [F3B←NH3]

The arrow shows that a lone pair on the nitrogen atom of ammonia has been donated to boron. This is a co-ordinate bond.

Synthesis of Ammonia: The Haber Process:

Many chemicals need to be manufactured on a large scale for use in industry. Ammonia is one such chemical since ammonia can be readily oxidised to nitric acid, the discovery made by Fritz Haber in 1908 enabled Germany to manufacture both fertilisers – essential for food production, and explosives on a large scale throughout World War I. Ammonia is synthesised from nitrogen and hydrogen is the following chemical process:

N2 (g) + 3H2(g) ═ 2NH3(g) ∆H = -92kJ mol-1

Since ammonia is manufactured on a large scale, the manufacturer aims for the highest possible yield, in the shortest possible time, for the lowest possible cost. This is achieved by the following means: Pressures of 2.0 X 104 kPa are used. This can be explained by analysing the above equation; the equation for the synthesis of ammonia shows that four moles of gaseous reactants (one of nitrogen and three of hydrogen) form two moles of gaseous products (ammonia). Le Chatelier’s principle states that a system at equilibrium will react to oppose any change imposed upon it; therefore the synthesis of ammonia is favoured by high pressure. In practice, the pressure used is 2.0 X 104 kPa. Although higher pressures produce greater equilibrium yields of ammonia, the cost of generating such pressures is usually uneconomic; the pressure of 2.0 X 104 kPa is a good compromise between cost and equilibrium yield of ammonia.

Operating temperatures in the range of 650-720 K are used. This is because the ammonia synthesis reaction is an exothermic process (∆H = -92kJ mol-1). As the forward reaction gives out heat energy, the application of Le Chatelier’s principle correctly predicts that this reaction will be opposed by an increase in temperature. Therefore the best equilibrium yield of ammonia is obtained at a low temperature. However, at low temperatures the rate of reaction is low, and although a high equilibrium yield of ammonia can be achieved, it may take a long time to reach equilibrium. Increasing the temperature speeds up the rate of attainment of equilibrium, but reduces the equilibrium yield. A compromise is clearly necessary, hence the usual operating temperature is in the range 650-720 K.

A catalyst based on iron with potassium hydroxide added to it is used. The rates of both the forward and backward reactions are increased to the same extent by the use of a catalyst; hence the time taken for the reaction to reach equilibrium is reduced. The catalyst usually employed for ammonia is based on iron, with potassium hydroxide added to promote its activity.

Role of Ammonia in chemical tests

Ammonia solution can be used in a test for halide ions (Cl-, Br- and I-). Along with silver nitrate, ammonia solution can be used to identify the halide ions. This is done by precipitation of silver chloride, silver bromide and silver iodide when an aqueous solution containing the appropriate halide ion is treated with an aqueous solution of silver nitrate. The colours of the three silver salts formed with chloride, bromide and iodide ions, and their different solubilities in aqueous ammonia, can be used as a test for the presence of the halide. ...