Energy of Phase Changes — AP Chemistry
1. Core Concepts of Phase Change Energy ★☆☆☆☆ ⏱ 2 min
Energy of phase changes (also called latent heat) refers to the heat energy absorbed or released when a pure substance undergoes a transition between solid, liquid, or gaseous phases, without changing temperature. Unlike heating that increases molecular kinetic energy (and thus temperature), phase change energy goes entirely into altering the potential energy of intermolecular forces between molecules.
All phase changes are classified as endothermic or exothermic, based on the direction of heat flow:
- **Endothermic**: Absorb energy from surroundings (melting, vaporization, sublimation)
- **Exothermic**: Release energy to surroundings (freezing, condensation, deposition)
2. Molar Enthalpies and Phase Change Calculations ★★☆☆☆ ⏱ 4 min
The most commonly tested molar enthalpies are:
- $\Delta H_{\text{fus}}$: Molar enthalpy of fusion (solid → liquid melting)
- $\Delta H_{\text{vap}}$: Molar enthalpy of vaporization (liquid → gas vaporization)
- $\Delta H_{\text{sub}}$: Molar enthalpy of sublimation (solid → gas sublimation)
For the reverse transition (e.g., liquid → solid freezing), the enthalpy change is the negative of the forward value: $\Delta H_{\text{freezing}} = -\Delta H_{\text{fus}}$. The core formula for calculating total heat energy $q$ for a phase change is:
q = n \Delta H_{\text{phase}}
Where $n$ is moles of substance, and $\Delta H_{\text{phase}}$ is the molar enthalpy of the phase change occurring. If given mass instead of moles, convert to moles via $n = \frac{m}{M}$, where $m$ is mass in grams and $M$ is molar mass in g/mol. $\Delta H_{\text{vap}}$ is almost always much larger than $\Delta H_{\text{fus}}$ because vaporization requires breaking all intermolecular interactions, while melting only loosens them.
Exam tip: Always confirm the direction of the phase change before assigning the sign of ΔH. AP exam questions often give you ΔHvap as a positive value and ask for the heat released during condensation, so you must add the negative sign explicitly to get the correct answer.
3. Heating and Cooling Curve Analysis ★★★☆☆ ⏱ 5 min
Heating (or cooling) curves plot the temperature of a substance versus the total heat added to the substance as it is heated from solid to gas (or cooled from gas to solid for cooling curves). The curve has two distinct region types:
- **Sloped regions**: Only one phase is present, heat added changes temperature. Use $q = mc\Delta T$.
- **Flat (horizontal) regions**: Temperature is constant, corresponds to a phase change. Use $q = n\Delta H$.
For a heating curve starting from a low-temperature solid, the order of regions is: (1) heat solid to melting point, (2) melt solid to liquid, (3) heat liquid to boiling point, (4) vaporize liquid to gas, (5) heat gas to final temperature. A common AP exam question asks you to calculate the total heat required to heat a substance from an initial cold temperature to a final hot temperature, which requires adding the $q$ from every region in sequence.
Exam tip: When calculating total heat for a multi-step heating process, always add the q from every single region in order—never skip a region even if it seems small. AP exam questions often award partial credit for correctly calculating each region's q, so write out every term separately.
4. Hess's Law for Phase Change Enthalpy ★★☆☆☆ ⏱ 3 min
Hess's law (the total enthalpy change for a process is independent of the path taken) applies to phase changes just as it does to chemical reactions. For example, sublimation (solid → gas) can occur either directly, or via an indirect path: solid → liquid (fusion) then liquid → gas (vaporization). The enthalpy of sublimation is therefore the sum of the enthalpies of fusion and vaporization at the same temperature and pressure:
\Delta H_{\text{sub}} = \Delta H_{\text{fus}} + \Delta H_{\text{vap}}
This relationship is often tested when you are given two of the three values and asked to calculate the third, or when you need to find the enthalpy of a reverse transition like deposition (gas → solid), which equals $-\Delta H_{\text{sub}}$.
Exam tip: Always check that your phase change equations add up correctly, canceling out any intermediate phases, just like you do for chemical reactions in Hess's law problems. A common mistake is reversing one of the enthalpies when it is not needed.
Common Pitfalls
Why: Students confuse $q = mc\Delta T$ (which uses mass) with $q = n\Delta H$ (which uses moles), so they plug mass directly in.
Why: Students focus so much on the phase change regions that they skip the steps where the substance is heated between phase changes.
Why: Reference tables always report ΔHfus, ΔHvap as positive values for the forward endothermic process, so students forget to reverse the sign for the reverse process.
Why: Students assume all energy change changes temperature, so they use the wrong formula for constant-temperature phase changes.
Why: Students think sublimation is directly solid to gas so it only requires vaporization energy, missing the fusion component.