Spontaneous and Non‑Spontaneous Reactions

A spontaneous process (or natural process) occurs autonomously, transitioning from a non‑equilibrium state to an equilibrium state without any external intervention. Such processes are unidirectional, irreversible, and real. Below are several examples of spontaneous processes.

• (i) Water flow: Water flows from a higher elevation to a lower elevation. The reverse (lifting water uphill) cannot occur without external work.
• (ii) Acid‑base neutralisation: The reaction between a strong acid and a strong base is spontaneous and proceeds to completion.

• (iii) Redox displacement: Zinc metal added to copper sulphate solution displaces copper, and the blue colour fades spontaneously.

A reaction is termed spontaneous if it requires an initial input of energy but then proceeds autonomously. For example, the combustion of coal and hydrocarbons in air is spontaneous once ignited. A piece of coal does not ignite in air independently; however, a spark triggers the reaction, and then it continues spontaneously to completion.

Non‑Spontaneous Processes

A non‑spontaneous process is the inverse of a spontaneous process. It does not occur on its own and is not observed in nature without external energy input. Reversible processes represent a boundary between spontaneous and non‑spontaneous behaviour. Certain non‑spontaneous processes can be made to occur by supplying energy from an external source. Examples include:

• (i) Pumping water to a higher elevation (requires work).
• (ii) Transfer of heat from the cold interior of a refrigerator to the warm surroundings (requires electrical work).
• (iii) Reaction of nitrogen with oxygen to form nitric oxide – this does not occur under ordinary conditions but happens when energy is supplied by lightning.

Fig. 1: Spontaneous reactions proceed without external energy input after initiation, while non‑spontaneous reactions require continuous energy supply.

Thermodynamic Criterion for Spontaneity

In thermodynamics, the spontaneity of a process at constant temperature and pressure is determined by the change in Gibbs free energy (\(\Delta G\)):

• If \(\Delta G < 0\), the process is spontaneous in the forward direction.
• If \(\Delta G > 0\), the process is non‑spontaneous (the reverse reaction is spontaneous).
• If \(\Delta G = 0\), the system is at equilibrium.

Entropy (\(\Delta S\)) and enthalpy (\(\Delta H\)) both influence spontaneity. Many spontaneous reactions are exothermic (\(\Delta H < 0\)), but some endothermic processes can also be spontaneous if the entropy increase is large enough.

Additional Examples of Spontaneous Processes

• Dissolution of salt in water (at room temperature).
• Diffusion of perfume molecules in air.
• Rusting of iron in moist air (slow but spontaneous).
• Expansion of a gas into a vacuum.

Examples of Non‑Spontaneous Processes

• Electrolysis of water to produce hydrogen and oxygen (requires electrical work).
• Formation of glucose from carbon dioxide and water in photosynthesis (requires light energy).
• Charging a rechargeable battery.

Key Takeaways

• Spontaneous processes occur naturally without external energy input after initiation.
• Non‑spontaneous processes require continuous energy input from an external source.
• The sign of \(\Delta G\) (Gibbs free energy change) determines spontaneity under constant temperature and pressure.
• Examples include water flow, neutralisation, combustion (spontaneous) and pumping water, electrolysis (non‑spontaneous).