Chemistry - Entropy and Gibbs Free Energy

To understand why some chemical reactions happen spontaneously (like a nail rusting) while others don't (like a pile of rust turning back into a nail), we look at two main factors: Entropy and Gibbs Free Energy.

1. Entropy (S): The Measure of Disorder

Entropy is often described as a measure of randomness or disorder in a system. In chemistry, it is more accurately described as the number of ways energy can be distributed among particles.

Low Entropy: Particles are organized, confined, or in a fixed position, like a solid crystal.

High Entropy: Particles are spread out, moving rapidly, or disorganized, like a gas.

The Second Law of Thermodynamics states that the total entropy of the universe is always increasing. This means nature naturally tends toward a state of higher disorder.

Factors that Increase Entropy:

Phase Changes: Moving from solid to liquid to gas.

Temperature: Increasing temperature makes particles move faster and more randomly.

Dissolving: Breaking a solid solute into ions in a solution.

Number of Moles: If a reaction produces more moles of gas than it started with, entropy increases.

2. Gibbs Free Energy (G): The Deciding Factor

While entropy tells us about disorder, it doesn't tell us the whole story. We also have to consider Enthalpy (H), which is the heat energy of the system.

Nature generally prefers low energy (exothermic reactions) and high disorder (increased entropy). Gibbs Free Energy combines these two ideas into one value to determine if a reaction is spontaneous, meaning it can occur without a continuous input of energy.

The Gibbs Equation:

Change in Gibbs Free Energy = (Change in Enthalpy) - (Temperature in Kelvin * Change in Entropy)

Change in G: Change in Free Energy (kJ/mol)

Change in H: Change in Enthalpy (Heat)

T: Temperature (in Kelvin)

Change in S: Change in Entropy

3. Is the Reaction Spontaneous?

The sign of the change in Gibbs Free Energy tells you if the reaction will "go" under specific conditions:

If the change in G is negative (less than 0): Energy is released and available to do work. The reaction is spontaneous.

If the change in G is positive (greater than 0): Energy must be added for the reaction to occur. The reaction is non-spontaneous.

If the change in G equals 0: The system has reached a state of balance called equilibrium.

The Tug-of-War:

Sometimes enthalpy and entropy disagree. For example, melting ice is endothermic (it absorbs heat, which nature usually resists), but it results in higher entropy because liquid water is more disordered than ice. At room temperature, the entropy gain is large enough to make the change in Gibbs Free Energy negative, so ice melts spontaneously.