Thermodynamics & Kinetic Control
Chemistry: IB Diploma Higher Level, topic 4.2.5
Thermodynamic & Kinetic Control
Reactions proceed due to a combination of favorable kinetics and thermodynamics. If one of these factors hinders the progression of a reaction, it is said to be controlling.
Thermodynamic control
- If there is enough energy in the system for the activation energy to be reached but the reaction does not proceed, then it is said to be under thermodynamic control.
- This means that it is not thermodynamically favorable for the reaction to proceed.
Examples of thermodynamic control
- A simple example is trying to use a stone as fuel for a fire. The fire provides more than enough energy for activation but it is not thermodynamically favorable for the stone to react with oxygen.
Kinetic control
- Kinetic control occurs when a reaction does not occur at a measurable rate despite being thermodynamically favorable.
- This is not because the position of equilibrium is to the left because the thermodynamics would favor a position to the right.
- The reason is that there is not sufficient energy to overcome the activation energy.
Examples of kinetic control
- Fossil fuels react with oxygen when they burn. These reactions are very thermodynamically favored, both enthalpically and entropically.
- So why does coal or oil or natural gas not spontaneously combust?
- There is not sufficient energy to start the reaction.
- In most cases, some sort of spark or fire is required to provide the activation energy.
Test yourself on Thermodynamics & Kinetic Control
When is a reaction under kinetic control?