Thermodynamics (A Level only) (AQA A-Level Chemistry): Revision Notes

5.1.8 Reaction Feasibility

Factors Affecting Reaction Feasibility

The feasibility of a chemical reaction depends on the balance of enthalpy change ($\Delta H$) and entropy change ($\Delta S$), which together determine the Gibbs free energy change ($\Delta G$) using the equation:

$$\Delta G = \Delta H - T \Delta S$$

A reaction is feasible if $\Delta G$ is zero or negative.

Feasibility Scenarios Based on $\Delta H$ and $\Delta S$

Condition	$\Delta H$	$\Delta S$	Effect on $\Delta G$	Feasibility
Always Feasible	Negative (-)	Positive (+)	$\Delta G$ is always negative	Reaction is feasible at all temperatures
Never Feasible	Positive (+)	Negative (-)	$\Delta G$ is always positive	Reaction is not feasible at any temperature
Feasible at Low Temperatures	Negative (-)	Negative (-)	$\Delta G$ becomes negative as temperature decreases	Reaction feasible at low temperatures only
Feasible at High Temperatures	Positive (+)	Positive (+)	$\Delta G$ becomes negative as temperature increases	Reaction feasible at high temperatures only

By understanding how $\Delta H$ and $\Delta S$ affect $\Delta G$, you can predict the conditions under which a reaction will proceed. This helps in determining suitable temperatures for reactions in various practical applications, from industrial synthesis to biochemical processes.