Enthalpy of Phase Transition — Core Principles

Core Principles

Enthalpy of phase transition, also known as latent heat, is the energy absorbed or released when a substance changes its physical state (solid, liquid, gas) at constant temperature and pressure. This energy is used to overcome or establish intermolecular forces, not to change the kinetic energy of particles, hence the constant temperature.

Key transitions include fusion (melting, solid to liquid, $Delta H_{fus} > 0$), vaporization (boiling, liquid to gas, $Delta H_{vap} > 0$), and sublimation (solid to gas, $Delta H_{sub} > 0$). Their reverse processes (freezing, condensation, deposition) are exothermic, having negative enthalpy values.

The magnitude of these enthalpy changes is primarily determined by the strength of intermolecular forces. Hess's Law relates these, stating $Delta H_{sub} = Delta H_{fus} + Delta H_{vap}$. These concepts are vital for understanding energy changes in various natural and industrial processes, and for solving numerical problems in NEET that often combine specific heat calculations with phase change enthalpies.

Important Differences

vs Specific Heat Capacity

Aspect	This Topic	Specific Heat Capacity
Definition	Enthalpy of Phase Transition (Latent Heat)	Specific Heat Capacity
What it measures	Heat absorbed/released during a phase change at constant temperature.	Heat absorbed/released to change the temperature of a substance without a phase change.
Temperature change	No temperature change occurs during the process.	Always involves a change in temperature.
Units (common)	Joules per mole (J/mol) or Joules per gram (J/g).	Joules per gram per Kelvin (J/g·K) or Joules per mole per Kelvin (J/mol·K).
Energy utilization	Energy used to overcome/establish intermolecular forces.	Energy used to increase/decrease the kinetic energy of particles.
Formula (for calculation)	$q = n cdot Delta H_{transition}$ (or $q = m cdot L$)	$q = m cdot c cdot Delta T$

Enthalpy of phase transition, or latent heat, quantifies the energy involved in changing a substance's physical state (e.g., melting, boiling) without altering its temperature. This energy primarily addresses intermolecular forces. In contrast, specific heat capacity measures the energy required to change a substance's temperature without a phase change, directly affecting the kinetic energy of its particles. The key distinction lies in whether the energy causes a change in state at constant temperature or a change in temperature at constant state.