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Difficulty: HardFactors Affecting Rates of Reaction

Consider the reversible industrial synthesis of ammonia gas: N2(g)+3H2(g)⇌2NH3(g)ΔH=−92 kJ mol−1N_{2(g)} + 3H_{2(g)} \rightleftharpoons 2NH_{3(g)} \quad \Delta H = -92\text{ kJ mol}^{-1}. A chemical engineer introduces a finely divided iron catalyst into the reaction vessel while maintaining constant temperature and pressure. Which of the following best describes the effect of adding the catalyst on the system?

  1. A
    It selectively lowers the activation energy of the forward reaction, shifting the equilibrium position to the right and increasing the yield of NH3NH_3.
  2. B
    It increases the average kinetic energy of the reactant molecules, causing an increase in the equilibrium constant (KcK_c) and a higher yield of NH3NH_3.
  3. It lowers the activation energy for both the forward and reverse reactions equally, increasing their rates and reducing the time taken to reach equilibrium without altering the final yield of NH3NH_3.Answer
  4. D
    It shifts the position of equilibrium to the left because the forward reaction is exothermic, thereby favoring the decomposition of NH3NH_3.

Answer

Adding a catalyst lowers the activation energy for both forward and reverse reactions by equal amounts, increasing their rates and reducing the time needed to reach dynamic equilibrium without altering the equilibrium yield of ammonia.
A catalyst functions by offering an alternative pathway with a lower activation energy barrier. In a reversible system, this reduction in activation energy applies equally to both the forward and reverse directions. Consequently, the rates of both reactions increase by the same proportion, enabling the system to reach dynamic equilibrium faster without changing the equilibrium composition or the yield of products.

Step-by-Step Solution

1
Analyze the function of a catalyst in chemical kinetics.
A catalyst provides an alternative reaction pathway with a lower activation energy (EaE_a).
Lowering EaE_a allows a greater fraction of reactant molecules to possess sufficient energy to undergo effective collisions per unit time.
2
Evaluate the symmetry of activation energy reduction in reversible reactions.
The catalyst lowers the activation energy barrier for both the forward reaction and the reverse reaction by the exact same amount (ΔEaΔ E_a).
Since the initial reactants and final products remain in the same energy states, the difference between the forward and reverse activation energy barriers (ΔHΔ H) is unchanged.
3
Determine the impact of the catalyst on chemical equilibrium and product yield.
The rates of both forward and reverse reactions increase by the same factor, so the equilibrium position and equilibrium constant (KcK_c) remain unchanged; only the time to reach equilibrium decreases.
Equal acceleration of both directions leaves the relative equilibrium concentrations of products and reactants unaffected.

Key Concept

Effect of Catalysts on Reaction Kinetics and Chemical Equilibrium
Estimated Time:1m 30s
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Topics

Factors Affecting Rates of Reaction