How are energy and changes of state related?
Energy in a body of water can be gained or lost depending on conditions. When water is heated above a certain temperature steam is generated. The increase in heat energy creates a higher level of disorder in the water molecules as they boil off and leave the liquid.
Changes of State and Free Energy
At the temperature at which a change of state occurs, the two states are in equilibrium with one another. For an ice-water system, equilibrium takes place at 0°C, so
The entropy change is positive as the solid state changes into the liquid state. If the transition went from the liquid to the solid state, the numerical value for
A similar calculation can be performed for the vaporization of liquid to gas. In this case we would use the molar heat of vaporization. This value would be 40.79 kJ/mol. The
The value is positive, again reflecting the increase in disorder going from liquid to vapor. Condensation from vapor to liquid would give a negative value for
- Calculations are shown for determining entropy changes at transition temperatures (ice → water or water → vapor and reverse).
- What precautions need to be taken in selecting a value for
- Why is temperature selection important?
- Why would the entropy of vaporization be so much larger than the entropy of fusion?