1. Do Now: Just to review before we start…
What is the melting point of this substance?
50˚C
The boiling point?
100˚C

2. Calculating Energy Changes at Phase Changes

3. There is a specific amount of energy required to melt ice and to turn liquid water into steam…
Heating curve of H2O

4. So, how do we calculate the amount of energy required during a phase change?
HF = Heat of Fusion (Q = mHF)
HV = Heat of Vaporization (Q=mHV)

5. Q = mHF
Q = mHV
Temperature IV I II
III V
Time

6. Hf = Heat of Fusion is amount energy required to change 1 gram
of pure substance from solid to liquid at its MP (meaning you aren’t changing the temperature)
Is a physical constant
Hf for water is 334J/g

7. Practice Problem: How much heat is absorbed when 10 grams of ice melts at 0oC?
Q = mHf = (10 g)(334 J/g) = 3340 J

8. HV = Heat of Vaporization
is the amount energy required to convert 1 gram
of pure substance from liquid to gas at its BP (meaning you aren’t changing the temperature)
Is a physical constant
HV for water is 2260J/g

9. Practice Problem: How much energy does it take to vaporize 10 g of water?
Q = mHv
Q = (10 g)(2260 J/g) = J

10. It takes a lot more energy to go from liquid to gas than from solid to liquid. Why?
* greater energy required to change from liquid to gas because particles are spreading farther apart!
Heating curve of H2O

11. Knowing which equation to use…hints
Q = mHf
Q = mHv
Ice
Freezing
Melting
Occurs at 0C (for H2O)
At constant temperature
Steam
Boiling
Condensation
Occurs at 100C (for H2O)
At constant temperature