5.3 Energetics of Phase Changes

Physical and chemical changes are accompanied by a change in energy, typically in the form of heat. The energy change associated with a physical process provides a quantitative measure of the strength of intermolecular forces. The change in energy at constant pressure is called the enthalpy change, denoted by $\Delta H$. It is expressed in units of $kJmo{l}^{-1}$. A key characteristic of a phase change is that the temperature of the substance remains constant while heat is added or removed.

---

5.3.1 Molar Heat of Fusion and Molar Heat of Vaporization

Molar Heat of Fusion ($\Delta H_f$)

Molar heat of fusion is the amount of heat required to convert one mole of a solid into its liquid state at its melting point. It represents the energy needed to overcome the forces holding the particles in the fixed crystal lattice.

The molar heat of fusion for ice is $+6.02kJmo{l}^{-1}$. The positive sign indicates that this is an endothermic process (heat is absorbed).

${\mathrm{H}}_2{\mathrm{O}}_{(\mathrm{s})}\to {\mathrm{H}}_2{\mathrm{O}}_{(\mathrm{l})},\Delta H_f=+6.02kJmo{l}^{-1}$

Molar Heat of Vaporization ($\Delta H_v$)

Molar heat of vaporization is the amount of heat required to convert one mole of a liquid into its vapor (gas) state at its boiling point. This energy is used to overcome the intermolecular attractions that hold the liquid molecules together.

The molar heat of vaporization for water is $+40.7kJmo{l}^{-1}$.

${\mathrm{H}}_2{\mathrm{O}}_{(\mathrm{l})}\to {\mathrm{H}}_2{\mathrm{O}}_{(\mathrm{g})},\Delta H_v=+40.7kJmo{l}^{-1}$

Molar Heat of Sublimation ($\Delta H_s$)

Some solids directly convert into vapors without passing through the liquid phase. The heat required to convert one mole of a solid directly into its vapors at a specific temperature and pressure is called the molar heat of sublimation. According to Hess's Law, the heat of sublimation is the sum of the heat of fusion and heat of vaporization.

$\Delta H_s=\Delta H_f+\Delta H_v$

---

5.3.2 Energy Changes and Intermolecular Forces

When a solid melts, there is a relatively small change in the intermolecular distances and potential energy of the particles (atoms, molecules, or ions). However, when a liquid evaporates, the particles undergo a large change in their intermolecular distances as they move far apart in the gas phase. This results in a significant increase in their potential energy.

Consequently, the heat of vaporization is much greater than the heat of fusion for the same substance because more energy is required to completely separate the molecules than to simply allow them to move past one another in the liquid state.

The strength of these forces also determines the physical state at room temperature. For example, in Halogens→, larger atoms have stronger London dispersion forces.

---

5.3.3 Liquid Crystals

In 1888, the Austrian botanist Friedrich Reinitzer discovered a unique state of matter while studying an organic compound, cholesteryl benzoate. He observed that when this solid was heated, it first turned into a milky, turbid liquid at $145^{\circ }C$ and then into a clear liquid at $179^{\circ }C$. Upon cooling, the reverse process occurred. This turbid intermediate state is known as a liquid crystal.

The liquid crystal state exists between the melting temperature and a "clearing" temperature.

$\mathrm{Solid}\mathrm{Crystal}\rightleftharpoons \mathrm{Liquid}\mathrm{Crystal}\rightleftharpoons \mathrm{Clear}\mathrm{Liquid}$

Properties of Liquid Crystals:

• Structure: Molecules in a liquid crystal have some degree of order, resembling solid crystals, but they can also flow like liquids. This state is often found in substances with long, rod-like molecules.
• Anisotropy: Like solid crystals, they can exhibit anisotropic properties (properties that vary with direction), such as optical properties. In contrast, normal liquids are isotropic (properties are the same in all directions).
• Types: Based on the degree and type of molecular ordering, liquid crystals are classified as nematic, smectic, and cholesteric.

---

5.3.4 Uses of Liquid Crystals in Daily Life

The unique ability of liquid crystals to change their molecular structure in response to external factors like electric fields and temperature makes them highly useful.

1. Displays: Used in the displays of digital watches, calculators, laptops, and computer monitors (LCDs).
2. Medical Diagnostics: Cholesteric liquid crystals are sensitive to temperature and are applied to the skin to locate veins, arteries, infections, and tumors, which are often warmer than surrounding tissues.
3. Spectroscopy: Nematic liquid crystals are used as solvents in Nuclear Magnetic Resonance (NMR) spectroscopy to help resolve complex molecular structures.
4. Chromatography: Used as a stationary phase in chromatographic separations.
5. Advanced Technology: Employed in oscillography systems and modern television displays.
6. Industrial Polymers: Liquid crystal polymers are used in fire-resistant coatings and for protecting multi-fibre optical cables.

---

5.3.5 Differentiation of Liquid Crystals from Pure Liquids and Crystalline Solids