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ChemistryNEET 2024Law of mass action, Kc and Kp, Le Chatelier

Equilibrium

In which of the following equilibria are KpK_p and KcK_c not equal?

  1. A.H2(g)+I2(g)⇌2HI(g)\mathrm{H_2(g) + I_2(g) \rightleftharpoons 2HI(g)}
  2. B.CO(g)+H2O(g)⇌CO2(g)+H2(g)\mathrm{CO(g) + H_2O(g) \rightleftharpoons CO_2(g) + H_2(g)}
  3. C.2BrCl(g)⇌Br2(g)+Cl2(g)\mathrm{2BrCl(g) \rightleftharpoons Br_2(g) + Cl_2(g)}
  4. D.PCl5(g)⇌PCl3(g)+Cl2(g)\mathrm{PCl_5(g) \rightleftharpoons PCl_3(g) + Cl_2(g)}✓

Solution

Kp=Kc(RT)ΔngK_p = K_c (RT)^{\Delta n_g}; they are equal only when Δng=0\Delta n_g = 0. For PCl5⇌PCl3+Cl2\mathrm{PCl_5 \rightleftharpoons PCl_3 + Cl_2}, Δng=2−1=1\Delta n_g = 2 - 1 = 1, so Kp≠KcK_p \neq K_c. The other three have equal moles of gas on both sides.

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