Q.State and explain Le-Chatelier principle and predict the effect of temperature and pressure in the following reactions:
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Start your 14-day free trial to unlock the full solution →Le Chatelier's principle says a stressed equilibrium shifts to counteract the stress. Higher temperature favours NH3's reverse (endothermic) direction but NO's forward (endothermic) direction; higher pressure favours NH3's forward direction (fewer gas moles) but has no effect on the NO equilibrium (equal gas moles both sides).
Le Chatelier's principle (statement): If a system at dynamic equilibrium is subjected to a change in one of the factors that determines the equilibrium state — concentration, temperature, or pressure (volume) — the system responds by shifting its position of equilibrium in the direction that tends to counteract (partially undo) the applied change, until a new equilibrium is established.
(i) N2(g) + 3H2(g) <=> 2NH3(g); DeltaH = -92.4 kJ/mol (exothermic; forward reaction is a formation/synthesis of ammonia and is well known to release heat)
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Effect of temperature: Since the forward reaction is exothermic (releases heat), by Le Chatelier's principle, increasing the temperature adds 'heat' as a stress, so the equilibrium shifts in the direction that absorbs heat — the reverse (endothermic, decomposition of NH3) direction — decreasing the yield of NH3. Conversely, lowering the temperature favours the forward (exothermic) direction, increasing NH3 yield. (In the industrial Haber process, a moderate/optimum temperature, around 700 K, is used as a practical compromise between a good equilibrium yield and an acceptably fast reaction rate.)
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Effect of pressure: The forward reaction converts 4 moles of gas (1 mol N2 + 3 mol H2) into 2 moles of gas (2 mol NH3) — i.e. the forward direction reduces the total number of gas moles. By Le Chatelier's principle, increasing the pressure (which effectively decreases volume) shifts the equilibrium towards the side with fewer gas moles — the forward direction — increasing NH3 yield. (This is why the Haber process is run at high pressure, around 200 atm.)
(ii) N2(g) + O2(g) <=> 2NO(g); DeltaH = +180.7 kJ/mol (endothermic)
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