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Chemical Equilibrium, Law Of Mass-action : Effect Of Addition Of Products Of Dissociation


The effect of adding an excess of one of the products of dissociation at constant volume is obvious from the law of mass-action. In the case of the dissociation of hydrogen iodide

equilibrium constant KI

addition of excess of H2 or I2 vapour will cause an increase of [H2] or [I2] respectively (since V is constant), hence to maintain the value of K, [HI] must also increase, i.e., the extent of dissociation is diminished. The same effect is produced by adding excess of Cl2 or PCl3 vapour to partly dissociated PCl5, since in this case

equilibrium constant PCl<sub>5</suv>

and increase of [PCl3] or [Cl2] must be followed by an increase of [PCl5] in order to maintain K constant.

The effect of adding an excess of one of the products of dissociation at constant pressure, however, requires more detailed examination.

Let 2 mols of HI be heated at 444°, then:

= 0.01895;

γ/(1 - γ) = ±√0.0758 = ±0.2754; γ = 0.216

Now suppose a further 1 mol of H2 be added, the pressure remaining constant, and let γ change to γ'. Then:

2HI(2(1 - γ')) <=> H2(γ' + 1) + I2(γ),

hence the total number of mols is

n = 2(1 - γ') + γ' + 1 + γ' = 3. The total volume is V' = nRT/p = 3RT/p;

HI concentration; H<sub>2</sub> concentration; I<sub>2</suv> concentration.

= 0.01895;

γ' = 0.063 (the negative root being inadmissible). Hence the dissociation is reduced. Now consider the case of PCl5. Let 1 mol be heated at 200° at 1 atm. pressure and let a further 2 mols of PCl3 vapour be added at constant pressure. If γ' is the degree of dissociation (previously 0.485),

PCl5(1 - γ') <=> PCl3(2 + γ') + Cl2(γ')

V' = nRT/p = (3 + γ')RT/p = (3 + γ')RT (p = 1 atm);

pcl5 concentration; pcl3 concentration; cl2 concentration;

equilibrium constant for pcl5 = 0.00793.

Now R = 0.08208 litre atm.: T = 273 + 200 = 473; RT = 38.824;

= 0.3079; x = 0.306.

the other root being inadmissible. Hence the dissociation is reduced.

This was shown by Wurtz (1873): if PCl5 is volatilised into an atmosphere of PCl3 at atmospheric pressure, the dissociation is largely suppressed, and only a very pale greenish colour, due to chlorine, is seen.

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Chemical Equilibrium, Law Of Mass-action
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