This topic explains the energy changes that occur during phase changes. It covers molar heat of fusion, molar heat of vaporization, and how these relate to intermolecular forces.
Definition
Energetics of Phase Changes: The study of energy changes (in the form of heat) that occur when a substance changes from one phase to another.
Molar Heat of Fusion (ΔH_fus): The amount of heat required to convert one mole of a solid into its liquid state at its melting point.
Molar Heat of Vaporization (ΔH_vap): The amount of heat required to convert one mole of a liquid into its vapours at its boiling point.
Main Idea
Physical and chemical changes are accompanied by a change in energy in the form of heat. Physical energy change is a quantitative measurement of the strength of intermolecular forces. The change in energy at constant pressure is called the change in enthalpy (ΔH). When a substance undergoes a phase change, its temperature remains constant even when heat is added.
Core Concept
Enthalpy change (ΔH): The heat change at constant pressure.
Molar heat of fusion (ΔH_fus): Heat to melt 1 mole of solid.
Molar heat of vaporization (ΔH_vap): Heat to vaporize 1 mole of liquid.
During a phase change, temperature remains constant.
Heat of vaporization > Heat of fusion because evaporation causes larger changes in intermolecular distance and potential energy.
Detailed Explanation
Energy Changes in Phase Transitions
Physical and chemical changes are accompanied by a change in energy in the form of heat.
Physical energy change is a quantitative measurement of the strength of intermolecular forces.
The change in energy at constant pressure is called the change in enthalpy, denoted by ΔH.
It is expressed in kJ mol⁻¹.
When a substance undergoes a phase change (change of state), its temperature remains constant even when heat is added.
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