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.
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Standard entropies are measured in units of J/(mol·K).
$S^\circ$ values are always positive because they represent the absolute entropy at standard conditions.
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.
Standard entropies can be used to calculate the Gibbs free energy change ($\Delta G^\circ$) using the equation $\Delta G^\circ = \Delta H^\circ - T \Delta S^\circ$.
$S^\circ$ values increase with molecular complexity and phase changes from solid to liquid to gas.
Review Questions
What is the unit of measurement for standard entropies?
How does the Third Law of Thermodynamics relate to standard entropies?
Why are standard entropy values always positive?
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.