1. Heating Curves and Thermodynamics
Chemistry
Chapter 15

2. Phase Change Overview What are the names of the phase changes? Gas
Solid
Liquid
Energy Absorbed
Energy Released
Endothermic
Exothermic a b c d e f
ENDOTHERMIC
Phase changes
EXOTHERMIC
Phase Chages a.
Sublimation d.
Deposition b. e.
Condensation
Fusion c.
Vaporization f.
Solidification

3. Heating Curve What happens when we keep adding energy to a solid ?
Solid becomes a liquid then the liquid
becomes a gas
As this energy is
added, KE increases
and so does temp.
BUT, does temp uniformly increase
over time?
NO!

4. Heating Curve If we heat a solid, it’s temperature increases steadily
until we finally reach the temperature at which the solid begins to melt
It begins to go through a phase change: solid  liquid
This phase change is called
fusion

5. Solid/Liquid Phase Transition
As the solid begins to melt, something unusual happens.
As we continue to add heat to the solid the temperature stops rising as the solid continues to melt.
But why does this happen?
Solid
Melting Solid

6. Solid/Liquid Phase Transition
Solids exist in a rigid, closely packed, highly
structured pattern
Liquids however
have no such rigid
structure.
As we reach the solid’s melting point there is just enough energy to begin overcoming the intermolecular forces between molecules holding them together in the solid state
Molecules begin to separate
The farther apart molecules become, the more PE they have

7. Heat of Fusion Added additional heat energy goes into
separating more and more molecules
As molecules move from solid to liquid the PE
increases. But since the temperature doesn’t rise
the KE remains constant.
The energy necessary to melt 1 GRAM of a solid is called the …
It is unique for every substance.
For water, the heat of fusion = 334 Joules/gram
HEAT OF FUSION (Hf)

8. Solid/Liquid Phase transition
Remember: heat ALWAYS flows from hot to cold
Until the last piece of solid melts the temperature the
solid/liquid mixture remains constant
Any excess heat in the liquid immediately flows
back into the colder solid
Once there’s no solid left all additional heat added
begins to increase the temperature of the liquid. H C

9. Solid/Liquid Phase transition
When heat is added what happens to Kinetic Energy, Temperature,
& Potential Energy ?
Only a single solid (s) phase is present
KE (T) increases
PE is Constant
Dual L/S phase is present
KE (T) is Constant
PE increase
Only a single liquid (l) phase is present
PE is constant
Solid
solid & liquid
Liquid
Heating Curve

10. Liquid/Vapor Phase Transition
B.P.
Any additional heat added to the liquid
increases temperature and
vapor pressure.
Once VP = Patm the liquid boils
As in the transition from solid
to liquid, two phases are now present and the temperature of the boiling water remains constant
Solid
solid & liquid
Liquid

11. Heat of Vaporization Molecules in the liquid phase
form a close but loosely
organized structure
Molecules in the gas
phase have no structure
and are widely separated (PE increases)
Separating these molecules this much takes lots of energy.
This energy is called the HEAT OF VAPORIZATION (Hv)
Liquid
S & L
Solid
Gas
L & G

12. Heat of Vaporization For water, Hv = 2,260 Joules/g Note Hv > Hf
As long as the liquid is boiling, temperature and
kinetic energy will be constant
Potential energy
will increase as
Molecules move further apart in the gas phase
Liquid &
gas
Solid &
liquid
M.P
B.P.
The Heating Curve

13. Summary (b.p.) T const. / KE const. /PE inc ( 2 phases )
T inc. / KE inc / PE Const.
( 1 phase )
T const. / KE const. /PE inc
( 2 phases )
Hf = J/g
(m.p.)
(b.p.)
Hv = 2,260 J/g

14. Heating Curve of Water

15. Heating Curve A heating curve
illustrates the changes of state as a solid is heated.
uses sloped lines to show an increase in temperature.
uses plateaus (flat lines) to indicate a change of state.
Copyright © by Pearson Education, Inc.
Publishing as Benjamin Cummings

17. Heating curve of Water… can you label the important temperatures, phases and phase changes?

18. Heating curve of Water… can you label the important temperatures, phases and phase changes?
Ice
Solid

19. Heating curve of Water… can you label the important temperatures, phases and phase changes?
Ice
Solid

20. Heating Curve of Water… can you label the important temperatures, phases and phase changes?
Melting / Freezing
Ice
Solid

21. Heating curve of Water… can you label the important temperatures, phases and phase changes?
Liquid Water
Liquid
Melting / Freezing
Ice
Solid

22. Heating curve of Water… can you label the important temperatures, phases and phase changes?
100
Liquid Water
Liquid
Melting / Freezing
Ice
Solid

23. Heating curve of Water… can you label the important temperatures, phases and phase changes?
100
Boiling / Condensation
Liquid Water
Liquid
Melting / Freezing
Ice
Solid

24. Heating curve of Water… can you label the important temperatures, phases and phase changes?
Vapor
100
Boiling / Condensation
Gas
Liquid Water
Liquid
Melting / Freezing
Ice
Solid

25. How energy moves from one place to another:
Heat (q): The movement of energy from one thing to another through the motion of molecules (thermal energy).
Heat spontaneously moves from hot things to cold. This is why a hot pan can burn you and you can’t burn a hot pan – the energy goes only from the pan to you because it’s hotter.
Heat and temperature are NOT the same thing: Heat is the transfer of energy, temperature is a measure of the kinetic energy of the object once the energy has finished transferring.
Example: If I give you five dollars, the money is what I give you (heat) and your net value is what goes up after you get the money (temperature). Your wealth is not a five dollar bill, but the five dollar bill does affect it.

26. System: Whatever we’re studying.
In chemistry, heat is a lot more useful than work because chemistry generally deals with the movement of atoms and molecules a lot more often than it deals with the movement of large machines.  
Since heat (energy that’s being transferred through thermal motion) is more important than work (for our purposes), how do we measure it? More definitions of use:
System: Whatever we’re studying.
This can be practically anything. If we are studying what happens when we heat a pan on the stove, the pan will be the system we are studying.
If a system gets energy added to it, the amount of energy it has after the change is positive. Because of this, an endothermic process is any process in which energy is absorbed by the system we’re talking about.
If a system has energy taken away from it, the amount of energy it has after the change is negative. Because of this, an exothermic process is any process in which energy is given off by the system we’re talking about. 

27. Potential Energy Diagrams

28. Potential Energy Diagrams
The amount of
energy in the
products is greater
than the energy
in the reactants….
so this process is
ENDOTHERMIC.
This means the
reaction
ABSORBS energy
The

29. Potential Energy Diagrams
The

30. Potential Energy Diagrams
The amount of
energy in the
products is less
than the energy
in the reactants….
so this process is
EXOTHERMIC.
This means the
reaction
GIVES OFF energy
The