1. Calculating Energy Changes at Phase Changes

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

3. What is the melting point of this substance?
The boiling point?

4. 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)

5. 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 this 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 (22,600J) than from solid to liquid (3,340J). Why?

8. H2O changing from solid to liquid requires 3,340J/g

9. H2O changing from liquid to gas requires 22,600J/g
greater energy required to change from solid to liquid because particles are spreading out a lot more!

10. Heats of fusion & vaporization
Determined in calorimetry experiments

11. Temperature Time Q = mHv Q = mHf Q = mCliquidT Q = mCsolidT
Q = mCgasT
Time

12. 3 equations for Q 1. Q = mCT 2. Q = mHf 3. Q = mHv
figure out which to use given problem
depends on section of heating curve
look for hints in word problem

13. Q = mCT Temperature changed T ↑ T ↓ Initial temperature
Start temperature
Final temperature
Ending temperature
From __ ˚C to __ ˚C
Water

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

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