Spontaneous Processes

Core Concept

A spontaneous process is one that proceeds on its own without a continuous external input of energy, once it has begun. Spontaneity describes the direction a process will go under a given set of conditions — it says nothing about how fast it goes. A process that requires a continuous outside driving force is nonspontaneous (its reverse is spontaneous). Chemists originally guessed that exothermic reactions (ΔH < 0) were always spontaneous, but many endothermic processes — ice melting, salt dissolving, water evaporating — occur spontaneously, which shows that another factor (entropy) must also matter.

Key Ideas

• Spontaneity is directional, not temporal. "Spontaneous" ≠ "instant" or "fast." Rusting of iron and diamond → graphite conversion are spontaneous but unimaginably slow.
• Exothermic is a tendency, not a law. Many spontaneous processes release heat (combustion, freezing water below 0 °C), but endothermic processes (melting above 0 °C, dissolving NH₄NO₃ in water) are also spontaneous.
• Conditions matter. Water freezing is spontaneous below 0 °C; the reverse (melting) is spontaneous above 0 °C. A process can flip direction when temperature changes.
• Driving force. A spontaneous process increases the entropy of the universe; a nonspontaneous process can be driven only by a continuous input of energy (e.g., electrolysis, charging a battery).
• ΔG is the master criterion. The thermodynamic quantity that combines enthalpy and entropy, Gibbs free energy (ΔG = ΔH − TΔS), is negative for a spontaneous process.

Equations and Variables

How It Works

1. A system sits in some initial state with a certain enthalpy and entropy.
2. The universe as a whole tends toward a state of greater dispersal of energy (higher entropy).
3. A process is spontaneous if it increases the entropy of the universe (Second Law, developed in a later note): ΔS_universe > 0.
4. Enthalpy contributes indirectly: an exothermic process dumps heat into the surroundings, raising their entropy, which favors spontaneity.
5. Gibbs free energy packages both effects into one number. When ΔG = ΔH − TΔS is negative, the process is spontaneous at that temperature.

Worked Example

Predict spontaneity at room temperature for each process and state the sign of ΔH and ΔS.

1. Water freezing at −5 °C. Spontaneous below 0 °C. ΔH < 0 (heat released as bonds form); ΔS < 0 (liquid → ordered solid). Despite the unfavorable entropy change, the low temperature lets the exothermic term dominate, so ΔG < 0.
2. Ice melting at +25 °C. Spontaneous. ΔH > 0 (heat absorbed); ΔS > 0 (solid → less-ordered liquid). Here the entropy term dominates, so ΔG < 0 even though the process is endothermic.
3. Water decomposing to H₂ and O₂ at 25 °C. Nonspontaneous; the reverse (H₂ + O₂ → H₂O) is spontaneous. Driving H₂O → H₂ + O₂ requires continuous electrical work (electrolysis).

This example shows the same substance can be spontaneous in one direction below 0 °C and the other above it — temperature decides.

How it works

1. A system sits in some initial state with a certain enthalpy and entropy.
2. The universe as a whole tends toward a state of greater dispersal of energy (higher entropy).
3. A process is spontaneous if it increases the entropy of the universe (Second Law, developed in a later note): ΔS_universe > 0.
4. Enthalpy contributes indirectly: an exothermic process dumps heat into the surroundings, raising their entropy, which favors spontaneity.
5. Gibbs free energy packages both effects into one number. When ΔG = ΔH − TΔS is negative, the process is spontaneous at that temperature.

Common confusions

• "Spontaneous means fast." — Wrong. Spontaneity is a statement about direction, not rate. Iron rusting is spontaneous but takes years.
• "Only exothermic reactions are spontaneous." — Wrong. Ice melting and NH₄NO₃ dissolving are endothermic yet spontaneous because their entropy increases.
• "A spontaneous process must release energy." — Wrong. It must increase the entropy of the universe; a spontaneous process may actually absorb heat.
• "If a reaction is slow, it must be nonspontaneous." — Wrong. Diamond → graphite is spontaneous but kinetically frozen.
• "Spontaneous and nonspontaneous are absolute properties of a reaction." — Wrong. They depend on conditions (T, P, concentrations); water freezing flips direction at 0 °C.

Quick review

• Spontaneous processes proceed without continuous external energy; nonspontaneous ones need a constant driving force.
• Spontaneity indicates direction, not speed.
• Exothermic tendency (ΔH < 0) does not by itself guarantee spontaneity.
• Temperature can reverse a process's spontaneous direction (freezing vs. melting).
• The real criterion is ΔS_universe > 0, packaged as ΔG = ΔH − TΔS < 0.