1. Energy Changes & Phase Changes
Heating & Cooling Curves

2. It takes energy to heat stuff up!
If 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!

3. Q = mCT C = specific heat capacity C is a physical constant
(amount heat required to raise temp of 1g of pure substance by 1C)
C is a physical constant
unique for every pure substance
CH2O = 4.18 J/gС

4. Heat Flow: hot to cold
Environment
System –
1 phase

5. What about phase changes?
sometimes more than one phase of substance is present
when melting ice: both liquid water & ice are present at same time
temperature is constant: T=0
even though ice water is absorbing heat

6. All chemical & physical changes are accompanied by energy changes
Phase changes are physical changes:
sometimes energy is absorbed
sometimes energy is released
Energy change for given phase change can be measured/calculated

7. What are 6 possible phase changes and their names?

8. Potential Energy Energy of relative position
Molecules are always attracted to one another
put energy into system to pull molecules apart from one another
the farther apart they are, the higher their PE

9. Melting Ice Ice water on hot plate: ice is melting
ice absorbing heat from hot plate and using to change phase
temperature of ice-water mix is constant
heat energy from hot plate going into phase change (PE) of system
heat energy not going to KE of particles
Remember, temperature is …

10. Identify a phase change as endothermic or exothermic
Gas
Condensation
Freezing
Deposition
Fusion
Vaporization
Sublimation PE
Liquid
Solid

11. Heating & Cooling Curves
way to investigate energy changes
Measure temperature as function of time at constant heating or cooling rate

12. Temperature Time I II III IV V Liquid & Gas Gas Solid & Liquid Liquid
Boil pt.
K.E.
K.E.
K.E.↔
K.E.↔
P.E.↔
K.E.
Temperature
P.E.
P.E.
Melt pt.
P.E.↔
P.E.↔
Time

13. Melting & Boiling Points
Plateaus = Phase changes = PE changes
Notice if 2 phases are present, temperature is constant
Melting point, Boiling point

14. What happens to temperature as heat is added at boiling point?
Nothing, until only 1 phase is present!

15. To analyze a heating/cooling curve:
Does curve go uphill or downhill?
Label phases present in each region
Describe what happens to KE in each region
Describe what happens to PE in each region
Locate melting point and boiling point

16. Heating curve of H2O