One mole of HI was sealed in a tube and heated to 440°C. At equilibrium, HI was found to be 50% dissociated. The Kc for the reaction 2HI ⇌ H₂ + I₂ is:
Correct answer: B. 0.25
- A. 1
- B. 0.25
- C. 0.5
- D. 0.625
Explanation
If 1 mole of HI is 50% dissociated, at equilibrium we have 0.5 mol HI, 0.25 mol H₂, and 0.25 mol I₂. K = [H₂][I₂]/[HI]² = (0.25) (0.25)/(0.5)² = 0.0625 / 0.25 = 0.25
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Reversible reactions reach dynamic equilibrium when forward and reverse rates become equal, and Le Chatelier's principle predicts the effect of changing concentration, pressure or temperature. The chapter also covers solubility product, the common ion effect, buffer action and the conditions used in Haber's process, with equilibrium shifts distinguished from changes in the equilibrium constant.
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