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In which of the following is it impossible to increase the rate of reverse reaction by increasing the concentration of reactant(s)?

Correct answer: A. Ca2+ (aq) + CO32- (aq) ⇋ CaCO3 (s)

  • A. Ca2+ (aq) + CO32- (aq) ⇋ CaCO3 (s)
  • B. N2 (g) + 2O2 (g) ⇋ 2NO2 (g)
  • C. 4NH3 (g) + 5O2 (g) ⇋ 4NO(g) + 6H2O(g)
  • D. H2 (g) + I2 (g) ⇋ 2HI(g)
  • E. Na2O2 (s) + H2O(l) ⇋ NaOH(aq) + H2O2 (aq)

Explanation

The rate of a reaction depends on the concentration of the substances involved in that specific step. In options B, C, and D, increasing reactants leads to more products, and because those products are gases, their increased concentration speeds up the reverse rate. However, in option A, the product is a solid CaCO3. Since the concentration of a pure solid is considered constant and doesn't change with its amount, the reverse rate (dissolution/decomposition of the solid) remains independent of reactant changes.

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About Chemical Equilibrium

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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