KEY2CHEM

Varying Reaction Conditions

A reaction that is not thermodynamically favored may produce large amounts of product if the initial set of conditions that will proceed toward equilibrium. This could mean that the concentration of reactants is higher than the equilibrium value, and the concentration of products is lower than the equilibrium value. As such, the process will be thermodynamically favorable in the direction that proceeds toward the equilibrium values.

Conversely, a process that is thermodynamically favor may not generate large quantities of product. This could be attributed to the initial set of reaction conditions (starting with a mixture that contains mostly product, for example, will not be thermodynamically favorable to proceed additionally toward equilibrium). Alternatively, the reaction could be under “kinetic control” instead of “thermodynamic control”. That means that, although a process is thermodynamically favored, there are kinetic constraints (such as a large activation energy barrier) that prevent the reaction from proceeding at a measurable rate. This does not imply the reaction has reached equilibrium; it just means the reaction is too slow to achieve a large quantity of product on a measurable scale.

Example 1.

A reaction has $$\Delta G^\circ < 0$$ but produces only a very small quantity of product under a given set of conditions. What is true about the reaction?

A. It has a very small equilibrium constant.

B. It may have a large activation barrier to generate products.

C. It is not thermodynamically favorable.

Solution

B. It may have a large activation barrier to generate products.

A reaction with $$\Delta G < 0$$ is thermodynamically favorable. However, the reaction may not generate significant product if it has a large activation barrier to surpass. This means the reaction is under kinetic control rather than thermodynamic control.

Example 2.

A process is not thermodynamically favorable. Which modification could increase the amount of products formed from the process?

A. Increase the activation energy barrier.

B. Increase the amount of reactant in the initial conditions.

C. Increase the rate constant of the process.

Solution

B. Increase the amount of reactant in the initial conditions.

Changing the initial set of conditions for a process that is not thermodynamically favorable (such as increasing the amount of reactant that could be converted into product) can increase the yield of product from a reaction.

Example 3.

A thermodynamically favorable reaction is said to be under kinetic control. Which statement about the reaction is true?

A. The rate constant of the process influences how much product will form.

B. The equilibrium constant determines how much product will form.

C. The initial quantities control how much product will form.

Solution

A. The rate constant of the process influences how much product will form.

A process under kinetic control is controlled by the kinetics (including rate constant and activation energy barrier) of the process, instead of by the thermodynamic quantities (equilibrium constant, $$\Delta G$$, etc.).