Spontaneity, Entropy, and Energy Release

Key Concepts

• Spontaneous reactions can occur without the system alone showing an entropy increase; the surroundings must be considered.
• The second law of thermodynamics: in all spontaneous reactions, entropy increases when both the system and its environment are taken into account.
• Energy released by reactions (kinetic energy, heat, light, sound) disperses into the surroundings, increasing their entropy.

Examples

• A reaction converting three gas molecules into two molecules of water vapor may appear to decrease the system's entropy, but the released energy increases the surroundings' entropy, yielding a net increase in the universe's entropy.
• The Hindenburg disaster illustrates how large amounts of kinetic energy can be released by a seemingly simple reaction.

Takeaways

• Physical and chemical changes tend to proceed toward lower potential energy, increased entropy, or both.
• Potential energy and entropy are used to determine spontaneity.
• The question of whether the reactions that led to chemical evolution were spontaneous is addressed in Section 2.4.

Spontaneity guidelines (from the text)

• Spontaneous processes satisfy that the total entropy of the system plus surroundings increases: \Delta S_{\text{universe}} > 0.
• Reactions tend toward lower potential energy, increased entropy, or both.
• These concepts help assess whether a reaction is spontaneous.