Heating and Cooling
Curves
April 23, 2015
Chemistry CP
1
Heating/Cooling Curve Revisited
! Potential Energy: energy of position (stored energy)
! During phases changes
! Kinetic Energy: energy of motion (temperature)
! Increases as temperature increases
LecturePLUS Timberlake 99 3
Cooling Curve for Water
Heating/Cooling Curve
1. In the
heating curve
for iron,
describe the
phase change
that occurred
between
points B and
C on the
graph.
Heating/Cooling Curve
2.Explain why the
temperature
stayed constant
between points
B and C.
Heating/Cooling Curve
3. What is the
melting
temperature
of iron?
Heating Cooling Curve
4. What is the
boiling
temperature
of iron?
Calculating Heat Involved in a Phase
Change
! The energy required to melt one gram
of a substance at its melting point is
called the heat of fusion (∆ H
fusion
)
! It is different for each substance
! The units are joules or calories per
gram
! To calculate the energy to melt a
substance at its melting point we use
the following formula: Q = m ×∆H
fus
8
Ex. Problem: Calculating Heat
Involved in a Phase Change
! Calculate the energy necessary to melt 50
grams of water.
Heat of fusion (H
fusion
) of water = 334 J/g
! Q = m × H
fusion
= 50 g × 334J/g = 16,700J
9
Calculating Heat Involved in
a Phase Change
! The energy required to boil one gram
of a substance at it boiling point is
called the heat of vaporization
(∆H
vap
)
! It is different for each substance
! The units are joules or calories per
gram
! To calculate the energy to boil a
substance at its melting point we use
the following formula: Q = m x ∆H
vap
10
Ex. Problem: Calculating Heat
Involved in a Phase Change
! Calculate the energy necessary to
boil 150 g of water. Heat of
vaporization (∆H
vap
)
=
2260 J/g.
! Q = m × ∆H
vap
= 150 g × 2260 J/g
! Q = 339,000 J
11
Calculating Energy Required to Make
a Temperature and Phase Change
! How much energy is needed to raise
the temperature of 250 grams of water
from 25
o
C to it’s boiling point and then
boil it? The specific heat of water is
4.18 J/g
o
C and the heat of
vaporization is 2260 J/g.
12
Let’s Solve
Step 1: Calculate the energy necessary to raise the
temperature from 25
o
C to 100
o
C
Q = m × ΔT × Cp
Q = 250g × (100
o
C - 25
o
C) × 4.18J/g
o
C
Q = 78,375 J
Step 2 Calculate the energy necessary to boil the water
∆H
vap
= m × H
vap
∆H
vap
= 250g × 2260 J/g = 565,000 J
Step 3 Add together the results of steps 1 and 2
78,375J + 565,000J = 643,375J
13
Five Step Problem for Water
Draw a heating curve for water, going from -20ºC to 125ºC
on the axis below. Determine the heat needed to 15 g of
ice at -20ºC to 125ºC.
Five Step Problem for Water
Melting
point
Boiling
Point
Heat of
Fusion
Heat of
Vapor.
Cp (solid) Cp liquid) Cp (vapor)
0.0°C !100.0°C 334 J/g
!2260 J/
g
!2.05 J/
gºC
4.18 J/
gºC
!1.90 J/
gºC
The data below are for water (H
2
O)
1. Heat needed to raise the temperature of ice
at -20°C to 0°C.
Determine the heat needed to 15 g of ice
at -20ºC to 125ºC.
Five Step Problem for Water
Melting
point
Boiling
Point
Heat of
Fusion
Heat of
Vapor.
Cp (solid) Cp liquid) Cp (vapor)
0.0°C !100.0°C 334 J/g
!2260 J/
g
!2.05 J/
gºC
4.18 J/
gºC
!1.90 J/
gºC
The data below are for water (H
2
O)
2. Heat needed to melt ice 0°C.
Determine the heat needed to 15 g of ice
at -20ºC to 125ºC.
Five Step Problem for Water
Melting
point
Boiling
Point
Heat of
Fusion
Heat of
Vapor.
Cp (solid) Cp liquid) Cp (vapor)
0.0°C !100.0°C 334 J/g
!2260 J/
g
!2.05 J/
gºC
4.18 J/
gºC
!1.90 J/
gºC
The data below are for water (H
2
O)
3. Heat needed to raise temperature of water
from 0°C to 100°C.
Determine the heat needed to 15 g of ice
at -20ºC to 125ºC.
Five Step Problem for Water
Melting
point
Boiling
Point
Heat of
Fusion
Heat of
Vapor.
Cp (solid) Cp liquid) Cp (vapor)
0.0°C !100.0°C 334 J/g
!2260 J/
g
!2.05 J/
gºC
4.18 J/
gºC
!1.90 J/
gºC
The data below are for water (H
2
O)
4. Heat needed to vaporize water at 100°C.
Determine the heat needed to 15 g of ice
at -20ºC to 125ºC.
Five Step Problem for Water
Melting
point
Boiling
Point
Heat of
Fusion
Heat of
Vapor.
Cp (solid) Cp liquid) Cp (vapor)
0.0°C !100.0°C 334 J/g
!2260 J/
g
!2.05 J/
gºC
4.18 J/
gºC
!1.90 J/
gºC
The data below are for water (H
2
O)
5. Heat needed to raise temperature of water
from 100 to 125ºC.
Determine the heat needed to 15 g of ice
at -20ºC to 125ºC.
Five Step Problem for Water
Melting
point
Boiling
Point
Heat of
Fusion
Heat of
Vapor.
Cp (solid) Cp liquid) Cp (vapor)
0.0°C !100.0°C 334 J/g
!2260 J/
g
!2.05 J/
gºC
4.18 J/
gºC
!1.90 J/
gºC
The data below are for water (H
2
O)
TOTAL HEAT: ADD UP ALL VALUES FOR Q
Determine the heat needed to 15 g of ice
at -20ºC to 125ºC.
