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Equilibrium — Chemistry STD 11 Science — Question

Gujarat BoardEnglish MediumSTD 11 ScienceChemistryEquilibrium3 Marks
Question
On the basis of Le Chatelier principle explain how temperature and pressure can be adjusted to increase the yield of ammonia in the following reaction.
$\text{N}_2\text{(g)}+\text{3H}_2\text{(g)}\rightleftharpoons\text{2NH}_3\text{(g)} \ \ \ \ \ \ \ \Delta\text{H}= – 92.38\text{kJ mol}^{–1}$
What will be the effect of addition of argon to the above reaction mixture at constant volume?
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Answer

$\text{N}_2\text{(g)}+\text{3H}_2\text{(g)}\rightleftharpoons\text{2NH}_3\text{(g)} \ \ \ \ \ \ \ \Delta\text{H}= – 92.38\text{kJ mol}^{–1}$ Effect of temperature: All chemical reactions are accompanied by the liberation or uptake if heat. If we regard heat as a reactant or product in an endothermic or exothermic reaction respectively, we can use the Le Chatelier principle to predict the direction in which an increase or decrease in temperature will shift the equilibrium state. Therefore for the give reaction we can write, $\text{N}_2\text{(g)}+\text{3H}_2\text{(g)}\rightleftharpoons\text{2NH}_3\text{(g)} \ \ \ \ \ \ \ \Delta\text{H}= – 92.38\text{kJ mol}^{–1}$ The Le Chatelier principle tells us that a net reaction will occur in the direction that will partially counteract this change. Since the reaction is exothermic, a shift of the equilibrium to the left will take place. Thus, by decreasing the temperature yield of $NH_3$​​​​​​​ can be increased. Effect of pressure:
$\Delta\text{n}_\text{g}=\text{n}_\text{g}\text{(products)}-\text{n}_\text{g}\text{(reactants)}$ $= 2 - (1 + 3) = 2 - 4 = -2$ Since $\Delta\text{n}_\text{g}$ is negative, if the pressure is increased,The equilibrium will shift in the forward direction according to the Lr Chatelier principle, because doing this will decrease the total number of moles of gases and hence the pressure which will decrease the effect of increasing pressure. Thus, high pressure would increase the yield of ammonia. Addition of argon at constant volume The addition of argon at constant volume would not change the partial pressures of any substance, $N_2, H_2$​​​​​​​ or $NH_3​​​​​​​$​​​​​​​. Hence, the equilibrium is not disturbed and there would not be any effect of addition of argon at constant Volume.

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