Enthalpy of Phase Transition — Prelims Strategy

Prelims Strategy

To excel in NEET questions on Enthalpy of Phase Transition, a systematic approach is crucial:

1
Master the Basics: — Clearly understand the definitions of enthalpy of fusion, vaporization, and sublimation, and their reverse processes. Know that endothermic processes have positive $\Delta H$ and exothermic processes have negative $\Delta H$.
2
Heating/Cooling Curves: — Visualize the process on a heating/cooling curve. Identify distinct regions: heating/cooling within a phase (slope) and phase transitions (plateaus). This helps in breaking down complex problems into manageable steps.
3
Formulas and Units: — Memorize and correctly apply the two main formulas: $q = mcDelta T$ for temperature changes (where $c$ is specific heat capacity) and $q = nDelta H_{transition}$ (or $q = m \cdot L$) for phase changes (where $L$ is latent heat per gram). Pay meticulous attention to units. If $\Delta H$ is given in $\text{kJ/mol}$, convert mass to moles, or if given in $\text{J/g}$, use mass in grams. Ensure final answers are in the requested units (e.g., kJ).
4
Step-by-Step Calculation: — For numerical problems involving multiple steps (e.g., ice to steam), calculate the heat for each step separately and then sum them up. This minimizes errors. For example, for ice at $-10^\circ\text{C}$ to steam at $110^\circ\text{C}$, you'd have five steps: heating ice, melting ice, heating water, vaporizing water, heating steam.
5
Conceptual Clarity: — Understand why temperature remains constant during a phase change (energy used to overcome IMFs) and how intermolecular forces affect the magnitude of $\Delta H_{transition}$.
6
Hess's Law: — Remember the relationship $\Delta H_{sub} = \Delta H_{fus} + \Delta H_{vap}$ and apply it for related processes like deposition ($\Delta H_{dep} = -\Delta H_{sub}$).