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:
\Delta S = \frac{q_\text{rev}}{T}
Where:
- ΔS = change in entropy
- qrev = 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:
\Delta G = \Delta H - T \Delta S
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)