Phase Changes and Heat
| Absorbs heat | ||||
|---|---|---|---|---|
| Melting | Boiling | |||
| Solid | ←→ | Liquid | ←→ | Gas |
| Freezing | Condensing | |||
| ΔHfusion | ΔHvaporization | |||
| Releases heat | ||||
To calculate the energy of a change from one set of conditions (physical state AND temperature) to a second physical state, it is necessary to calculate and add together the energy changes associated with all of the individual steps. The individual steps can involve only a change in temperature of a single phase OR a change of phase at a single temperature.
For example, to calculate the energy change associated with conversion of 75.0 grams of water from ice at -23oC to steam at 172oC, the following steps must be performed.
| q1 = s m ΔT = (0.48 cal/g Co) * (75.0 g) * (23 Co) | = | 828 cal |
| q2 = m ΔHfus. = (75.0 g) * (80 cal/g) | = | 6000 cal |
| q3 = s m ΔT = (1.0 cal/g Co) * (75.0 g) * (100Co) | = | 7500 cal |
| q4 = m ΔHvap. = (75.0 g) * (540 cal/g) | = | 40500 cal |
| q5 = s m ΔT = (0.48 cal/g Co) * (75.0 g) * (72Co) | = | 2592 cal |
| qtotal | = | 57420 cal |
Phase Diagrams
Shown below is the phase diagram for a hypothetical substance. In this diagram, the three phases (solid, liquid, and gas) are labeled. The triple point is the point on the diagram where all three of these phases can coexist, and occurs (for this substance) at a temperature of 20o and a pressure of 0.40.
The line between the solid and liquid phases gives the temperatures where this compound is expected to melt (which is near 20o and only changes slightly with temperature), and the curve between the liquid and gas phases gives the boiling points (which does changes significantly).
