Unit 9: Topic 6 - Coupled Reactions

Question 1

Give some examples of thermodynamically unfavorable reactions that proceed when in contact with an external source of energy.

Answer 1

One example of an external source allowing an unfavorable reaction to proceed is electricity. For example, assume the reaction Y + Z –> Y(+) + Z- is a spontaneous one. Electricity can be used for the reverse nonspontaneous reaction to allow it to proceed; a battery can be connected to an electrolytic cell to push electrons from the ions. This is similar to charging a battery using electricity.
Another example is the use of light to convert carbon dioxide and water into glucose. The reverse reaction, the combustion of glucose in oxygen, is highly favorable. However, light also makes this reverse reaction favorable, and it is seen in the process of photosynthesis in plants.

Question 2

How can coupled reactions make non spontaneous reactions spontaneous?

Answer 2

Coupled reactions are a combination of a nonspontaneous reaction with a spontaneous reaction to form an intermediate. For example, the reaction Cu2S –> 2Cu + S is unfavorable, with ∆G = 86.2 kJ. However, coupling this reaction with S + O2 –> SO2, ∆G = -300.1 kJ, will form a favorable reaction. Since ∆G behaves according to Hess’s Law, the reaction Cu2S + O2 –> 2Cu + SO2, ∆G = -213.9 kJ, is favorable.