(a) State Le Chatelier's principle (b) Use Le Chetelier's principle to deduce the conditions that favour a high yield of ammonia in the Haber process (c) Gi...

(a) Le Chatelier's principle

When a system at equilibrium is subjected to a change in conditions (concentration, temperature or pressure), the equilibrium shifts in the direction that tends to oppose or reduce the effect of that change.

(b) Conditions for a high yield of ammonia in the Haber process

The reaction is: \[N_{2(g)} + 3H_{2(g)} \rightleftharpoons 2NH_{3(g)} \quad \Delta H = -ve\]

• High pressure (about 200 to 250 atm): the forward reaction produces fewer gas molecules (4 volumes to 2 volumes), so raising the pressure shifts the equilibrium to the right, increasing the yield of ammonia.
• Low temperature: the forward reaction is exothermic, so a lower temperature favours a higher yield. However, too low a temperature makes the reaction very slow, so a moderate (compromise) temperature of about 450 to 500 °C is used together with an iron catalyst to reach equilibrium quickly.
• Continuous removal of ammonia (by liquefaction) drives the equilibrium forward.

(c) Chemical test for ammonia

Ammonia turns moist red litmus paper blue, and it gives dense white fumes of ammonium chloride when a glass rod dipped in concentrated hydrochloric acid is held in it: \[NH_{3(g)} + HCl_{(g)} \rightarrow NH_4Cl_{(s)}\]

(d) Observations with aqueous ammonia

• (i) Zinc chloride solution: a white gelatinous precipitate of zinc hydroxide forms, which dissolves in excess ammonia to give a colourless solution.
• (ii) Copper(II) tetraoxosulphate(VI) solution: a pale blue precipitate of copper(II) hydroxide forms, which dissolves in excess ammonia to give a deep royal-blue solution.

(e) H-N-H bond angle less than H-C-H

In methane the carbon atom has four bonding pairs of electrons and no lone pair, so the equal bond-pair repulsions give the ideal tetrahedral angle of 109.5°. In ammonia the nitrogen atom has three bonding pairs and one lone pair. A lone pair repels more strongly than a bonding pair, so it compresses the bonding pairs closer together, reducing the H-N-H angle to about 107°.

(f) Two uses of ammonia

• Manufacture of nitrogenous fertilizers such as ammonium salts and urea.
• Manufacture of trioxonitrate(V) acid (nitric acid) in the Ostwald process. (It is also used as a refrigerant.)