Entropy is measured as the amount of disorder or randomness in a system, usually in units of joules per mole per kelvin (J/mol·K).
1. From heat changes (for reversible processes)
- If a small amount of heat is added or removed reversibly at a certain temperature, the change in entropy can be estimated.
- More heat spreading out at a given temperature → greater increase in entropy.
2. Using standard entropy values
- Chemists have measured standard molar entropy for many substances at 25°C and 1 atm.
- These values can be used to calculate entropy changes in reactions by comparing the entropy of products and reactants.
3. From probability (microscopic view)
- Entropy can also be understood as a measure of how many ways particles and energy can be arranged.
- More possible arrangements → higher entropy.
Simple Example:
- Ice melting: Solid water has low entropy because molecules are ordered.
- When it melts into liquid, molecules move more freely → entropy increases.
- This increase can be measured using the heat absorbed during melting.
In short: Entropy is measured either experimentally (from heat changes) or calculated (from standard entropy data or molecular arrangements).