Standard entropies (S°)

Definition

Standard entropies ($S^\circ$) are the absolute entropy values of substances in their standard states at a specified temperature, typically 298.15 K (25°C). These values provide a reference for calculating changes in entropy during chemical reactions.

5 Must Know Facts For Your Next Test

1. Standard entropies are measured in units of J/(mol·K).
2. $S^\circ$ values are always positive because they represent the absolute entropy at standard conditions.
3. The Third Law of Thermodynamics states that the entropy of a perfect crystal at absolute zero is zero, thus giving a baseline for $S^\circ$ values.
4. Standard entropies can be used to calculate the Gibbs free energy change ($\Delta G^\circ$) using the equation $\Delta G^\circ = \Delta H^\circ - T \Delta S^\circ$.
5. $S^\circ$ values increase with molecular complexity and phase changes from solid to liquid to gas.

Review Questions

Related terms

Gibbs Free Energy ($G$): A thermodynamic potential that measures the maximum reversible work obtainable from a system at constant temperature and pressure.

$\Delta S$ (Change in Entropy): The difference in entropy between final and initial states of a process, indicating the degree of disorder or randomness.

$\Delta H$ (Change in Enthalpy): The heat absorbed or released during a chemical reaction at constant pressure.