1. Calculating Energy Changes at Phase Changes

2. The heating rate is 150 J/min
If substance takes 4 minutes to melt, how much heat energy was used to melt it?
150J/min x 4min = 600J

3. Heat of Fusion
= amount energy required to change 1 gram of pure substance from solid to liquid at its mp
Heat of Fusion = Hf = physical constant
Hf for water = 334 J/g
(Table B)

4. How much heat is absorbed when 10 grams of ice melts at 0oC?
Heat absorbed =
mass of substance x heat of fusion of substance
Q = mHf = (10 g)(334 J/g) = 3340 J
Where does that energy go?
Particles must overcome forces of attraction to move farther apart

6. Heat of Vaporization
= amount energy required to convert 1 gram of pure substance from liquid to gas at its bp
Heat of vaporization = Hv = physical constant
Hv for water = 2260 J/g

7. How much energy does it take to vaporize 10 g of water?
Q = mHv
Q = (10 g)(2260 J/g) = J
It takes a lot more energy to go from liquid to gas than from solid to liquid. Why?

8. The particles are spreading out a lot more!

9. Calculating Energy Q = mHf Q = mHv
figure out which to use for given problem
Look for hints in problem

10. Q = mHv
Q =
mHf
Temperature
Time

11. Q = mHf
Ice
Freezing
Melting
At 0C (for H2O)
At constant temperature

12. Q = mHv Steam Boiling Condensation At 100C (for H2O)
At constant temperature