A Reaction Must Be Spontaneous If It Is

A Reaction Must Be Spontaneous If It Is… Thermodynamically Favorable!

A chemical reaction's spontaneity isn't about how fast it happens, but whether it will happen under a given set of conditions. This is determined by thermodynamics, specifically by changes in Gibbs Free Energy (ΔG). A reaction will be spontaneous if and only if it is thermodynamically favorable, meaning it leads to a decrease in Gibbs Free Energy. Let's delve into the specifics.

Understanding Gibbs Free Energy (ΔG)

Gibbs Free Energy (G) is a thermodynamic potential that measures the maximum reversible work that may be performed by a thermodynamic system at a constant temperature and pressure. The change in Gibbs Free Energy (ΔG) during a reaction indicates the spontaneity of that reaction. ΔG is calculated using the following equation:

ΔG = ΔH - TΔS

Where:

• ΔG is the change in Gibbs Free Energy (kJ/mol)
• ΔH is the change in enthalpy (kJ/mol) – essentially the heat absorbed or released during the reaction. Exothermic reactions (heat released, ΔH < 0) are favored.
• T is the temperature in Kelvin (K)
• ΔS is the change in entropy (kJ/mol·K) – a measure of disorder or randomness. Reactions that increase disorder (ΔS > 0) are favored.

Spontaneity and the Sign of ΔG

The sign of ΔG dictates the spontaneity of a reaction:

• ΔG < 0 (negative): The reaction is spontaneous under the given conditions. It will proceed in the forward direction without external intervention.
• ΔG > 0 (positive): The reaction is non-spontaneous under the given conditions. It will not proceed in the forward direction without external input (like adding energy). The reverse reaction, however, will be spontaneous.
• ΔG = 0 (zero): The reaction is at equilibrium. The forward and reverse reaction rates are equal.

Factors Affecting Spontaneity: Enthalpy and Entropy

Both enthalpy (ΔH) and entropy (ΔS) play crucial roles in determining spontaneity.

• Enthalpy (ΔH): Exothermic reactions (ΔH < 0), which release heat, tend to be spontaneous because they are energetically favorable. The system moves to a lower energy state.

• Entropy (ΔS): Reactions that increase the disorder or randomness of the system (ΔS > 0) are favored entropically. Think of a solid dissolving into a liquid – the particles become more dispersed, increasing entropy.

Temperature's Role

The temperature (T) acts as a weighting factor in the Gibbs Free Energy equation. At low temperatures, the enthalpy term (ΔH) dominates, while at high temperatures, the entropy term (TΔS) becomes more significant. This means that a reaction that is non-spontaneous at low temperatures might become spontaneous at high temperatures if the entropy increase is sufficiently large.

Examples:

• Combustion of Methane: This is a highly exothermic reaction (ΔH < 0) and leads to an increase in entropy (ΔS > 0). Therefore, ΔG is significantly negative, making it highly spontaneous.

• Melting of Ice: This is an endothermic reaction (ΔH > 0), but the increase in entropy (ΔS > 0) is substantial. At temperatures above 0°C, the TΔS term outweighs ΔH, resulting in a negative ΔG and spontaneous melting.

In Conclusion:

A reaction is spontaneous if and only if its Gibbs Free Energy change (ΔG) is negative. This spontaneity is a consequence of the interplay between enthalpy (ΔH) and entropy (ΔS) changes, modulated by temperature (T). Understanding these thermodynamic principles is crucial for predicting the direction and feasibility of chemical reactions.