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

2. Topic: Calculating Energy Changes at Phase Changes (Hv and Hf)

3. It takes energy to heat stuff up!
for pure substance in single phase - can calculate how much E needed using:
Q = mCT
Q = energy in Joules
m = mass in grams
C = specific heat capacity
T = change in temperature = Tf - Ti
on other hand, when something cools down, energy is released!

4. Temperature Time IV I II III V Q = mCliquidT Q = mCgasT
Q = mCsolidT
Temperature IV I II
III V
C = specific heat capacity
(amount heat required to raise temp of 1g of pure substance by 1C)
C is a physical constant
unique for every pure substance
CAN YOU FIND THE SPECIFIC HEAT OF WATER ON YOUR REFERENCE TABLE?
Time

5. Why can’t I use Q = mCT for II and IV??
Temperature IV I II
III V
Because T = 0, temp isn’t changing!!!!
Time

6. 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)
We use one of these two constants instead of specific heat and delta T
Q = mCT

7. 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
Check out Reference Table B, what is the heat of fusion for water?
The Equation Q = mHf

8. 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 during phase change (s → l)

9. 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
Check out Reference Table B, what is the heat of vaporization for water?
The Equation Q = mHv

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

11. H2O changing from liquid to gas requires 22,600J/g
It takes a lot more energy to go from liquid to gas than from solid to liquid. Why?
H2O changing from liquid to gas requires 22,600J/g
H2O changing from solid to liquid requires 3,340J/g
* greater energy required to change from liquid to gas because particles are spreading farther apart!

12. Temperature Time IV I II III V Q = mHV Q = mHF Q = mCsolidT
Q = mCLIQUIDT
Q = mCgasT
Q = mHV
Q = mHF
Temperature IV I II
III V
Time

13. Heating curve of H2O

14. 3 equations for Q 1. Q = mCT 2. Q = mHf 3. Q = mHv
figure out which to use
depends on section of heating curve
look for hints in word problem

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

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

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

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