Physical Chemistry: Reaction Feasibility

Feasibility of Chemical Reactions

The feasibility of a chemical reaction refers to the likelihood of a reaction happening.
Just because a reaction is feasible, does not mean that it will happen quickly. The reaction could be kinetically hindered - meaning it happens slowly.
Feasibility can be predicted using energetics, entropy and the Gibbs free energy equation.

Enthalpy

The heat change at constant pressure during a reaction is known as enthalpy and represented by ΔH.
If a reaction takes in heat, i.e. it’s endothermic, ΔH is positive.
If a reaction gives out heat, i.e. it’s exothermic, ΔH is negative.
In terms of feasibility, exothermic reactions are more likely to occur because they release energy.

Entropy

Entropy describes the disorder or randomness in a system, represented as ΔS.
Increasing disorder (positive ΔS) is naturally favoured, so reactions where entropy increases are more likely to be feasible.
Solids have a lower entropy than liquids who in turn have a less entropy than gases. Therefore, reactions that produce a gas are often feasible due to an increase in entropy.

Gibbs Free Energy

Gibbs free energy, represented as ΔG, can be used to predict the feasibility of a reaction at any temperature.
The relationship is given by the equation ΔG = ΔH - TΔS, where ΔG is the change in free energy, ΔH is the change in enthalpy, T is the temperature in Kelvin, and ΔS is the change in entropy.
If ΔG is negative, a reaction is feasible at a specific temperature.
If ΔG is zero, a reaction is at equilibrium under the given conditions.
If ΔG is positive, a reaction is not feasible under the given conditions.

Calculating Feasibility

The feasibility of a reaction can be calculated by knowing ΔH, ΔS and the specific temperature of interest and then solving for ΔG.
Care should be taken with units when using the Gibbs equation. Usually enthalpy is given in kJ and entropy in J/K, so converting to consistent units is necessary.
Remember that temperature in this calculation must always be in Kelvin.

These key topics provide a basic understanding of reaction feasibility, which is essential for understanding and predicting whether a chemical reaction will occur under certain conditions.