The molar heat of vaporization of ethanol is 39.3 kJ / mol,...

The molar heat of vaporization of ethanol is $39.3 \mathrm{kJ} / \mathrm{mol}$, and the boiling point of ethanol is $78.3^{\circ} \mathrm{C}$. Calculate $\Delta S$ for the vaporization of 0.50 mole of ethanol.

Key Concepts

Molar Heat of Vaporization

This is the amount of energy required to convert one mole of a liquid into vapor at constant temperature and pressure. It reflects the strength of intermolecular forces within the liquid, and is a key thermodynamic property used to understand phase transitions.

Boiling Point

The boiling point is the specific temperature at which a liquid’s vapor pressure equals the ambient pressure, leading to a phase transition from liquid to gas. It is crucial to express this temperature in Kelvin when performing thermodynamic calculations.

Entropy Change

Entropy, a measure of disorder or randomness in a system, increases during the vaporization process as the more ordered liquid becomes a disordered gas. The change in entropy can be calculated by dividing the heat absorbed during the phase transition by the absolute temperature at which the transition takes place.

Thermodynamic Relationship for Phase Transitions

The relation ?S = ?H_rev/T is used to determine the entropy change during a reversible process, such as vaporization at the boiling point. This formula directly connects the energy input during the process with the resultant increase in molecular randomness.

Molar Basis in Thermodynamic Calculations

Using a molar basis, such as joules per mole or kilojoules per mole, ensures that thermodynamic properties like enthalpy and entropy are consistently calculated. When a specified number of moles undergo a phase change, this concept allows scaling from a molar quantity to the total energy or entropy change for the sample.

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