JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Pharmacy II PHYSICAL PHARMACEUTICS-I Previous Question Paper (October/November-2020 - R17)

Dipole moment is a measure of the polarity of a chemical bond or a molecule. It arises due to the difference in electronegativity between two bonded atoms, resulting in a partial positive charge (δ+) and a partial negative charge (δ-) separated by a distance.

Mathematically, the dipole moment (μ) is defined as the product of the magnitude of the partial charge (q) and the distance (d) between the centers of the positive and negative charges:

The unit of dipole moment is the Debye (D), where 1 D = 3.336 × 10⁻³⁰ Coulomb-meters. A molecule is considered polar if it has a net non-zero dipole moment. In polyatomic molecules, the net dipole moment is the vector sum of the individual bond moments. For example, symmetric molecules like carbon dioxide have a zero net dipole moment because the bond moments cancel each other out, whereas water has a net dipole moment due to its bent geometry.

Dielectric constant (ε) is a measure of a substance's ability to store electrical energy in an electric field. It is defined as the ratio of the permittivity of the substance to the permittivity of a vacuum. It indicates the polarity of a solvent; solvents with high dielectric constants, such as water (ε ≈ 80), are effective at dissolving ionic compounds by reducing the electrostatic force of attraction between oppositely charged ions.

Dissociation constant (K_d) is an equilibrium constant that measures the propensity of a larger complex or molecule to separate reversibly into smaller components. For an electrolyte dissociating in a solvent, it is expressed as:

Where [A⁺] and [B⁻] are the concentrations of the dissociated ions and [AB] is the concentration of the undissociated molecule. A higher dissociation constant indicates a greater degree of dissociation, which is strongly influenced by the dielectric constant of the solvent; a higher dielectric constant facilitates the separation of ions, thereby increasing the dissociation constant.