Le-Chatelier principle

Le-Chatelier principle: if an equilibrium system is subjected to a change, reaction will occur in such a way as to counter act the imposed change.

Example. Manufacture of Ammonia:  N3 (g) + H2(g) → 2NH3(g) ; ΔH = -93.6kJ.

(i) Effect of pressure: The increases in pressure causes the-equilibrium to shift from left to right. Therefore forward reaction is favored. The forward reaction is accompanied by a decrease the number of moles. If the pressure is increased, the total volume will decrease and the number of moles per unit volume will increase. According to le-chateliers principle, therefore the equilibrium will shift in the direction in which there is a decrease in the number of moles i.e., in favor of the formation of ammonia. Thus higher the pressure, greater would be the yield of NH3.

(ii) Effect of temperature: Formation of NH3 is favored at low temperature since it is an exothermic reaction. Increase in temperature favors the reverse reaction resulting in a greater decomposition of NH3 into N2 and H2.

(iii) Effect of catalyst: In the manufacture of NH3 the yield Is poor at higher temperatures. At low temperature the reaction is very slow. To have a good yield of NH3 at a suitable rate, the reaction is performed at an intermediate temperature say 750 k in the presence of a catalyst.

(iv) Effect of pH value. A neutral and pure water possess an equal concentration of H+ ions and OH ions and their product is alv1ays constant: However when an acid is added in water, the concentration of H+ ions becomes greater than that of OH ions. Similarly when a when a base is added, the concentration of OH ions increases while that of H+ decreases. Thus an acid solution is one in which the [H+] is greater than 10-7 or a base is one in which the [H+] is less than 10-7.

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