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Chemistry LibreTexts

15.6: Controlling the Products of Reactions

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  • Both kinetic and thermodynamic factors can be used to control reaction products.

    Conceptual Problems

    1. A reaction mixture will produce either product A or B depending on the reaction pathway. In the absence of a catalyst, product A is formed; in the presence of a catalyst, product B is formed. What conclusions can you draw about the forward and reverse rates of the reaction that produces A versus the reaction that produces B in (a) the absence of a catalyst and (b) the presence of a catalyst?
    2. Describe how you would design an experiment to determine the equilibrium constant for the synthesis of ammonia: \[N_{2(g)} + 3H_{2(g)} \rightleftharpoons 2NH_{3(g)}\] The forward reaction is exothermic (ΔH° = −91.8 kJ). What effect would an increase in temperature have on the equilibrium constant?
    3. What effect does a catalyst have on each of the following?
      1. the equilibrium position of a reaction
      2. the rate at which equilibrium is reached
      3. the equilibrium constant?
    4. How can the ratio Q/K be used to determine in which direction a reaction will proceed to reach equilibrium?
    5. Industrial reactions are frequently run under conditions in which competing reactions can occur. Explain how a catalyst can be used to achieve reaction selectivity. Does the ratio Q/K for the selected reaction change in the presence of a catalyst?

    Numerical Problems

    1. The oxidation of acetylene via \(2C_2H_{2(g)}+5O_{2(g)}\rightleftharpoons 4CO_{2(g)} +2H_2O_{(l)}\) has \(ΔH° = −2600\; kJ\). What strategy would you use with regard to temperature, volume, and pressure to maximize the yield of product?
    2. The oxidation of carbon monoxide via \(CO_{(g)} + 1/2 O_{2(g)} \rightleftharpoons CO_{2(g)}\) has ΔH° = −283 kJ. If you were interested in maximizing the yield of \(CO_2\), what general conditions would you select with regard to temperature, pressure, and volume?
    3. You are interested in maximizing the product yield of the system: \[2SO_{2(g)}+O_{2(g)} \rightleftharpoons 2SO_{3(g)}\] with \(K = 280\) and \(ΔH° = −158\; kJ\). What general conditions would you select with regard to temperature, pressure, and volume? If \(SO_2\) has an initial concentration of 0.200 M and the amount of \(O_2\) is stoichiometric, what amount of \(SO_3\) is produced at equilibrium?

    Numerical Answer

    1. Use low temperature and high pressure (small volume).