Entropy: definition in chemistry
Entropy: A measure of how spread out, or dispersed, the energy and particles of a system are, often described loosely as disorder. Its symbol is S.
Entropy increases when a solid melts, a liquid evaporates or a gas spreads out, because the particles and their energy can then be arranged in many more ways. The second law of thermodynamics states that the total entropy of a system and its surroundings increases in any spontaneous process. Together with enthalpy, entropy decides whether a reaction can happen on its own, through the Gibbs free energy equation, ΔG = ΔH − TΔS.
Examples
- Ice melting — liquid water has higher entropy than ice
- Perfume spreading through a room
- Dissolving salt in water usually increases entropy
- Water vapor — about 189 J/(mol·K), compared with 70 for liquid water
On the periodic table
The gaseous elements, found mostly at the upper right of the table, have the highest standard entropies, while hard solids such as diamond have the lowest.