Glossary
Absolute Entropy (S°)
The entropy of a substance at standard conditions (1 atm and 298 K, not 273 K as stated in the notes, which is 0°C). It represents the inherent disorder of a substance at a specific temperature.
Example:
The absolute entropy of gaseous water (steam) is significantly higher than that of liquid water at the same temperature, reflecting its greater molecular freedom and disorder.
Change in Entropy (ΔS°)
The difference in disorder between the products and reactants in a chemical reaction or physical process under standard conditions.
Example:
For the combustion of methane, the change in entropy is positive because a solid and a gas react to form more moles of gas, increasing the overall disorder of the system.
Complexity (Entropy Prediction)
A factor used to predict the sign of ΔS°, where larger, more complex molecules generally have higher entropy than smaller, simpler ones due to more possible vibrational and rotational states.
Example:
Comparing methane (CH₄) and octane (C₈H₁₈), octane has a higher entropy due to its greater complexity and more ways its atoms can vibrate and rotate.
Entropy (S)
A thermodynamic property that measures the disorder or randomness of a system, indicating the number of possible microscopic arrangements for a given macroscopic state.
Example:
When a solid sugar cube dissolves in water, its particles spread out and become more disordered, leading to an increase in the system's entropy.
Negative ΔS°
Indicates that a reaction or process leads to a decrease in the system's disorder or randomness, resulting in a more ordered state.
Example:
The formation of snow crystals from water vapor in the atmosphere exhibits a negative ΔS° as the highly disordered gas molecules arrange into an ordered solid structure.
Number of Moles (Entropy Prediction)
A factor used to predict the sign of ΔS°, where an increase in the total number of moles of gas from reactants to products typically leads to a positive ΔS°.
Example:
In the decomposition of hydrogen peroxide (2H₂O₂(l) → 2H₂O(l) + O₂(g)), the increase in the number of moles of gas (from zero to one) suggests a positive ΔS°.
Phase Changes (Entropy Prediction)
A factor used to predict the sign of ΔS°, where transitions from more ordered phases (solid) to less ordered phases (liquid, gas) generally result in positive ΔS°.
Example:
Melting ice into liquid water is an example where phase changes dictate a positive ΔS°, as the molecules gain translational freedom.
Positive ΔS°
Indicates that a reaction or process leads to an increase in the system's disorder or randomness.
Example:
The sublimation of dry ice (solid CO2 turning directly into gaseous CO2) has a positive ΔS° because the molecules gain much more freedom of movement.
State Function
A property of a system that depends only on its current state, not on the pathway taken to reach that state.
Example:
Whether you climb a mountain directly or take a winding path, your change in altitude (a state function) from base to summit remains the same.