KEY2CHEM

Driving Change: Ways to Make Thermodynamically Unfavorable Processes Favorable

Many important processes (including charging a cell phone battery or photosynthesis) are not thermodynamically favorable. However, they can be made to occur through different methods. One method uses application of external energy (often light) to drive a thermodynamically unfavorable process to occur. Alternatively, a thermodynamically favorable reaction can be coupled with an unfavorable one; the favorable process provides sufficient free energy to “drive” the unfavorable process.

Example 1.

A process has $$\Delta G^\circ = 25 \text{ kJ/mol}$$. What is true about this process?

A. It is thermodynamically favorable on its own.

B. It could be thermodynamically favorable if external energy in excess of $$25 \text{ kJ/mol}$$ were applied.

C. It could be thermodynamically favorable if coupled to another reaction with $$\Delta G^\circ = -10\text{ kJ/mol}$$.

Solution

B. It could be thermodynamically favorable if external energy in excess of $$25 \text{ kJ/mol}$$ were applied.

A thermodynamically favorable process has $$\Delta G^\circ < 0$$. A non-favorable process can occur if external energy in excess of the positive $$\Delta G^\circ$$ value is applied.

Example 2.

A thermodynamically favorable process has $$\Delta G^\circ$$ ____ $$0$$ and a process that is not thermodynamically favorable has $$\Delta G^\circ$$ ____ $$0$$.

A. $$<, >$$

B. $$>, <$$

C. $$>, =$$

Solution

A. $$<, >$$

A thermodynamically favorable process has $$\Delta G^\circ < 0$$and a process that is not thermodynamically favorable has $$\Delta G^\circ > 0$$.

Example 3.

The re-charging of a laptop battery is not thermodynamically favorable. What is true about this re-charging process?

A. $$\Delta G ^\circ< 0$$

B.$$\Delta G^\circ > 0$$

C.$$\Delta G^\circ = 0$$

Solution

B. $$\Delta G^\circ > 0$$

A process that is not thermodynamically favorable has $$\Delta G^\circ > 0$$. Re-charging a battery requires input of external energy (from the electricity at the wall, for example) to drive the process forward.