Topic: Heating/Cooling Curves
(energy/phase changes)
Do Now: Water boils at _____torr,
____1atm, _______mm Hg,
______kPa

Review:
Name the phase change
S  L Melting (fusion)
L  G
G  L
L  S
S  G
G  S
Boiling (vaporization) condensation
Freezing (solidification) sublimation deposition

REVIEW: All Phase changes accompanied by energy changes
• Phase changes are physical changes:
– sometimes energy is absorbed (endothermic)
– sometimes energy is released (exothermic)
• Energy change for given phase change can be measured/calculated (we’re going to learn how to do this shortly)

• As you go from S  L  G, you are increasing the distance between molecules, so you are increasing their
PE
(click)
!
Endothermic
Sublimation Gas Deposition
PE
Vaporization
(boiling)
Liquid
Condensation
Melting
(fusion)
Solid Freezing
(solidification)
Exothermic
We can see these energy changes by looking at a heat/cooling curve – next slide

ENDO
ENDO
B
MELTING
D

E
Vaporization
(boiling)
C

B
EXO
FREEZING
E

D Condensation

• Which phase changes absorb energy
(endothermic)?
Melting, Boiling (vaporization)
• Which phase changes release energy
(exothermic)?
Freezing
,
Condensation

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

• is temp at which liquid & solid phases of substance coexist at equilibrium
• higher the mp, the stronger the IMF
• Lets check it out: http://www.harcourtschool.com/activity/hot plate/index.html

• Temp at which liquid is converted to crystalline solid
• How does fp compare to mp?
They ’ re the same: for H
2
O = 0˚C

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

Melting point/Freezing
Point
Boiling Point
B
MELTING
D

E
Vaporization
(boiling)
C

B
FREEZING
E

D Condensation

Melting & Boiling Points (start of the phase change)
• P lateaus = P hase changes = P E changes
– always given as temp (˚C, K)
• WHY IS IT CALLED A POINT?

• More than one phase is present
– Melting : solid and liquid
– Boiling: liquid and gas
(lets check out a tiger animation)

– There is no change in temp!!!!!
***NOTE: if 2 phases are present, temp is constant

QUICK QUESTION:
What happens to temperature as heat is added to boiling water?
• Nothing, temp doesn't ’ t change b/c 2 phases are present
–  temp will change when only 1 phase is present!

PAGE 13 answer 3-8
7. When is the kinetic energy of the sample the greatest?
GAS phase
8. Given a constant heating rate of 150 Joules per minute, how many joules of heat must the sample absorb to melt all of the sample?
= 1350 Joules 150 J 9 minutes minute
To vaporize all of the sample?
150 J 30 minutes minute
= 4500 Joules
9 30

• Energy of relative position
• This position is important b/c molecules are always attracted to one another
• How close they are to one another affects how much energy is needed to be put into
system to pull molecules apart from one
another
• the farther apart, the higher their PE
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