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.