Topic 6.5 Energy of Phase Changes: explain why temperature is constant during a phase change, interpret a heating curve, and calculate the energy of a phase change from the enthalpy of fusion or vaporisation.

What this topic is asking

The College Board (Topic 6.5) wants you to explain why temperature is constant during a phase change, interpret a heating curve, and calculate the energy of a phase change from the enthalpy of fusion or vaporisation. The central insight is that heat added during a phase change increases potential energy (separating particles), not kinetic energy (raising temperature).

The heating curve

Reading from cold solid: the temperature rises (heating the solid), pauses at the melting point (melting plateau), rises again (heating the liquid), pauses at the boiling point (boiling plateau), then rises as gas. The plateaus are where two phases coexist and the energy goes into the transition.

Why temperature is constant during a phase change

This is the heart of the topic. Melting and boiling are not about breaking the covalent bonds inside molecules; they are about separating molecules from one another against their intermolecular attractions. The heat input does work against those attractions (potential energy up) while the average speed of the particles (kinetic energy, temperature) holds steady.

Calculating the energy of a phase change

The energy of a phase change is the molar enthalpy times the number of moles:

The enthalpy of fusion $\Delta H_\text{fus}$ is the energy to melt one mole; the enthalpy of vaporisation $\Delta H_\text{vap}$ is the energy to boil one mole. For any substance $\Delta H_\text{vap} > \Delta H_\text{fus}$, because vaporisation separates the molecules completely whereas fusion only frees them to move past one another. A full heating-curve problem adds the $mc\Delta T$ contributions of the sloped sections to the phase-change contributions of the plateaus.

Try this

Q1. Calculate the heat to melt $3.00$ mol of ice, given $\Delta H_\text{fus} = 6.01\ \text{kJ mol}^{-1}$. [2 points]

• Cue. $q = (3.00)(6.01) = 18.0\ \text{kJ}$.

Q2. Explain why the temperature does not rise while ice is melting at $0\ ^\circ\text{C}$. [2 points]

• Cue. The heat overcomes intermolecular forces (raising potential energy), not the average kinetic energy, so the temperature stays at the melting point until all the ice has melted.