# 15.6: Controlling the Products of Reactions

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?