Short on time? This page is the recap. For explanations and worked examples, use the full study guide.
Recap
- Melting, vaporization and sublimation need energy to overcome attractions between particles. The system’s energy rises: endothermic.
- Freezing, condensation and deposition release energy as attractions form. The system’s energy falls: exothermic.
- During a phase change of a pure substance, the temperature stays constant. The energy changes the particles’ potential energy, not their average kinetic energy.
- ΔH_vap is much larger than ΔH_fus because boiling separates the particles completely; melting only loosens them. Stronger intermolecular forces mean a larger ΔH_vap.
Key relationships
| Relationship | Symbols and units | Use it to |
|---|---|---|
| q = n × ΔH | q in kJ, n in mol, ΔH in kJ mol⁻¹ | find the energy for a phase change at constant temperature |
| n = m / M | m in g, M in g mol⁻¹ | turn a mass into moles before using ΔH |
| ΔH_freezing = −ΔH_fus; ΔH_cond = −ΔH_vap | same size, opposite sign | find the energy released by the reverse change |
| q = mcΔT | m in g, c in J g⁻¹ °C⁻¹ | the sloping parts of a heating curve (one phase only) |
Water: ΔH_fus = 6.01 kJ mol⁻¹ (0 °C), ΔH_vap = 40.7 kJ mol⁻¹ (100 °C).
Heating curve: flat section = phase change (its temperature is the melting or boiling point; its length is the energy). Sloping section = one phase warming (a steeper slope means a smaller heat capacity).
Assumptions behind the numbers
- The substance is pure, so it has a single, sharp melting and boiling point.
- The pressure is constant, so q equals the enthalpy change.
- ΔH values apply at the stated temperature; they change slightly at other temperatures.
- In multi-step problems, no energy is lost to the container or the air.
Mistakes to avoid
- Multiplying grams by ΔH. Convert to moles first.
- Adding J and kJ. Convert every step to the same unit.
- Skipping a phase change in a multi-step problem. Split at every melting and boiling point.
- Using one c value for all phases. Ice, liquid water and steam each have their own.
- Wrong sign for freezing or condensing: they are exothermic, so q is negative.
- Saying particles speed up while melting. The temperature is constant, so their average kinetic energy is too.
Quick self-check
- Energy needed to melt 2.00 mol of ice at 0 °C? (+12.0 kJ)
- Energy change when 0.500 mol of steam condenses at 100 °C? (−20.4 kJ, i.e. 20.4 kJ released)
- Energy change when 9.01 g of water freezes at 0 °C? (−3.01 kJ)
- On a heating curve, what does the length of the higher flat section give you? (The energy to vaporize the whole sample; divide by the moles to get ΔH_vap.)
Next: practice questions.