What is Spontaneity? - Definition, Types, Gibbs Energy

A spontaneous process is one that takes place naturally under given conditions without continuous external intervention once initiated. For example, ice melting at room temperature or heat flowing from a hot object to a cold object are spontaneous processes. Spontaneity is related to the feasibility of a process, not its speed. Some spontaneous reactions may occur very slowly, while some non-spontaneous processes require energy to proceed.

Types of Spontaneous Processes

In thermodynamics, processes are classified based on whether they occur naturally or require external energy.

1. Spontaneous Process

A spontaneous process is one that occurs on its own under given conditions without continuous external energy.

• Takes place naturally
• Moves in a definite direction
• Leads to greater stability (often an increase in entropy)
• May be slow or fast

Examples:

• Flow of heat from hot to cold body
• Melting of ice at room temperature
• Expansion of gas in a vacuum

2. Non-Spontaneous Process

A non-spontaneous process is one that does not occur on its own and requires external energy to take place.

• Does not occur naturally
• Requires continuous supply of energy
• Opposite of spontaneous process

Examples:

• Flow of heat from cold to hot body
• Freezing of water at room temperature
• Electrolysis of water

Entropy

To understand Spontaneity properly, we use the concept of entropy. Entropy is a measure of the degree of disorder or randomness in a system. It tells us how spread out or disordered the particles are.

• More disorder , higher entropy
• Less disorder , lower entropy

Entropy Change (ΔS)

Entropy change is represented as ΔS.

• If ΔS > 0, entropy increases (more disorder) → process is favorable
• If ΔS < 0, entropy decreases (more order) → process is less favorable

For a reversible process:

Where:

• ΔS = change in entropy
• q_rev​ = heat absorbed in a reversible process
• T = temperature in Kelvin

Examples:

Melting of ice: Disorder increases, ΔS is positive, spontaneous at room temperature

Factors Affecting Spontaneity

In Spontaneity, whether a process occurs naturally depends mainly on two important thermodynamic factors: enthalpy (ΔH) and entropy (ΔS).

1. Enthalpy Change (ΔH)

Enthalpy represents the heat change during a reaction.

• If ΔH is negative (exothermic reaction), heat is released, process is more likely to be spontaneous
• If ΔH is positive (endothermic reaction), heat is absorbed, process is less likely to be spontaneous

Example:
Combustion of fuel releases heat (ΔH < 0), so it is spontaneous.

2. Entropy Change (ΔS)

Entropy measures the disorder of a system.

• If ΔS is positive, disorder increases, favors spontaneity
• If ΔS is negative, disorder decreases, does not favor spontaneity

Example:
Melting of ice increases disorder, so ΔS > 0.

3. Temperature (T)

Temperature plays an important role in deciding spontaneity.

• At high temperature, processes with increase in entropy (ΔS > 0) become more favorable
• At low temperature, exothermic processes (ΔH < 0) are more favorable

Gibbs Energy

Gibbs Free Energy (G) is a thermodynamic quantity that helps us predict whether a chemical or physical process can occur spontaneously at constant temperature and pressure. It combines the concepts of enthalpy (ΔH) and entropy (ΔS) into a single term that indicates the “useful energy” available to do work.

The change in Gibbs free energy is given by the formula:

Where:

• ΔG = change in Gibbs free energy
• ΔH = change in enthalpy (heat absorbed or released)
• ΔS = change in entropy (disorder)
• T= absolute temperature in Kelvin

Spontaneity and Gibbs Free Energy:

• If ΔG < 0 , the process is spontaneous.
• If ΔG > 0 the process is non-spontaneous.
• If ΔG = 0, the system is in equilibrium.

Example:

• Melting of ice at 0°C , ΔG = 0 (equilibrium)
• Melting of ice at 25°C , ΔG < 0 (spontaneous)
• Freezing of water at 25°C , ΔG > 0 (non-spontaneous)

Related Articles:

• Hess Law
• First Law