Heats of State Changes
Heats of State Changes involve the conversion of a substance from one phase to another. They are measured in joules per gram; note that the temperature does not change during a state change. For instance, the amount of heat required to melt one gram of water is 333 joules per gram. To melt one mole of water (18 grams), 333 J/g * 18 g = 5994 joules, or almost 6 kilojoules.
Here are some heats involved in state changes:
- Heat of melting (the amount of heat required to convert one gram of a substance from a solid to a liquid)
- Heat of vaporization (the amount of heat required to convert one gram of a substance from a liquid to a gas)
- Heat of sublimation (the amount of heat required to convert one gram of a substance from a solid to a gas)
- Heat of solution (the amount of heat involved in creating a solution, positive or negative)
Note that the heats of melting, vaporization, and sublimation are always positive. The heat of solution can be either positive or negative. To find the opposite state change (freezing versus melting), simply multiply the heat by -1.
As a final example of state changes, consider a rainstorm. Let's say 1 centimeter of rain falls on a 5 kilometer by 5 kilometer area. How much energy is released during the condensation of this water? First, find the volume of water: 5000 meters * 5000 meters * .01 meter = 250000 cubic meters of water. Convert this to milliliters: 250000 cubic meters * (1000000 milliliters/cubic meter) = 250,000,000,000 milliliters, or 250,000,000,000 grams of water. Finally, the heat of condensation is 2260 J/g, giving an energy release of 5.65 x 1014 joules! No wonder thunderstorms can generate so much power!
Other thermodynamic values you may come across include thermal conductivity, which is a measure of how fast a substance can transfer heat, measured in watts per meter-kelvin; and heat capacity, which is a measure of the amount of heat energy needed to raise the temperature of the system under study by 1 degree Celsius, measured in joules per degree Celsius. This differs from specific heat in considering the entire system, whereas the specific heat only refers to one gram of the substance under study. Since these two concepts are typically not required for a high-school, introductory college, or AP Chemistry course, we will not discuss them further.