Class #104: Energy and Phase Change

AIM: Students will understand that changes in states
of matter involve changes in energy.

Class #104

2

Melting
(Fusion)

Vaporization

Changes of State - Adding Heat

​3

Condensation

Deposition

Changes of State - Removing Heat

4

5

Heat

● Heat is a quantity of energy
● We cannot measure heat directly

●

We can only measure heat when
it is being TRANSFERRED from
one (hotter) object to another
(cooler) object

●

Units for measuring heat:

Joules (J)

Kilojoules (kJ)

6

Gas

Solid

Liquid

How can we describe the difference in particle motion in

different states of matter?

Temperature

● Temperature is a measure of the average kinetic energy of a

substance’s particles

○ kinetic energy is the energy of MOTION

● Temperature tells us how fast the particles in a substance

are moving “on average”

Celsius (℃)

Kelvin (K)

●

Units for measuring temperature:

7

Temperature

● When we talk about a change in temperature (△T) we are

talking about a change in the average kinetic energy

(△KE) of the particles in a object/substance

△ = “change in”

● When the temperature of a substance is going DOWN, on

average it’s particles are starting to move _____________

● When the temperature of a substance is going UP, on average

it’s particles are starting to move _____________

slower

faster

8

Celsius

Kelvin

10

Temperature (℃)

Time

-20

0

100

120

solid/liquid

liquid/gas

solid only

liquid only

gas only

Heating Curve

Heat is added at a constant rate

11

Temperature (℃)

Time

-20

0

100

120

liquid/solid

gas/liquid

solid only

liquid only

gas only

Cooling Curve

Heat is lost at a constant rate

12

Energy Changes

Temperature (℃)

Time

-20

0

100

120

Temperature Change

KE increases

KE increases

KE increases

NO

CHANGE

in KE

NO

CHANGE

in KE

Energy Changes

Temperature (℃)

Time

-20

0

100

120

PE increases

PE increases

NO PE
CHANGE

NO PE
CHANGE

NO PE
CHANGE

Phase Change

Energy Changes

Temperature (℃)

Time

-20

0

100

120

Temperature Change
Phase Change

KE decreases

PE decreases

PE decreases

KE decreases

KE decreases

For Heat problems:

● Temperature changes when there is a change in Kinetic

Energy

● Temperature change: △T

○ Final temperature - initial temperature

● Temperature does NOT change during a phase change

13

Physical Constants for Water

Phase change: melting

Phase change: boiling/vaporizing

Change in temperature

●

Energy requirements for change in phase or in temperature

●

Every substance has its own unique energy requirements!

●

These constants are ONLY for WATER

14

Specific Heat Capacity

●

The amount of energy needed to raise the TEMPERATURE of a
specific amount of a substance 1 Kelvin or 1℃

●

The specific heat capacity of water:

○

4.18 J of heat energy is LOST when 1 gram of water is COOLED
by one degree Kelvin

○

4.18 J of heat energy must be added to raise the temperature
of 1 gram of water by one degree Kelvin

15

Change of Temperature Problems

Changing the Temperature

●

How many degrees (K or
℃) is the temperature
changing?

●

How much water do
you have by mass in
grams (g)?

●

How much heat energy
per gram (J/g) is needed
to change temperature?

Heat

needed

Specific

Heat

Capacity

x

=Mass of

x Number of

water

degrees

Change of Temperature Problems

16

A pot containing 250 g water at 20℃ is heated until the water
reaches a temperature of 80℃ . How much heat energy is
required?

mC

q = △T

m =

C =

=

△T Final temp - initial temp

17

A pot containing 250 g water at 20℃ is heated until the water
reaches a temperature of 80℃ . How much heat energy is
required?

mC

q = △T

m =

C =

=

△T

250 g

4.18 J/g•K

Final temp - initial temp = 80℃ - 20℃ = 60℃

17

A pot containing 250 g water at 20℃ is heated until the water
reaches a temperature of 80℃ . How much heat energy is
required?

mC

q = △T

m =

C =

=

△T

250 g

4.18 J/g•K

Final temp - initial temp = 80℃ - 20℃= 60℃

250 g

4.18 J/g•K

60℃

x

=

x

17

A beaker containing 250 g water at 80℃ is placed in the
refrigerator until the water cools to a temperature of 20℃ .
What is the change in heat energy in the water?

mC

q = △T

m =

C =

=

△T Final temp - initial temp

18

A beaker containing 250 g water at 80℃ is placed in the
refrigerator until the water cools to a temperature of 20℃ .
What is the change in heat energy in the water?

mC

q = △T

m =

C =

=

△T

250 g

4.18 J/g•K

Final temp - initial temp = 20℃ - 80℃= - 60℃

18

The temperature of a sample of water changes from 10℃ to 20℃ when the
sample absorbs 418 J of heat energy. What is the mass of the sample?

mC

q = △T

m =

C =

=

△T Final temp - initial temp

q =

19

The temperature of a sample of water changes from 10℃ to 20℃ when the
sample absorbs 418 J of heat energy. What is the mass of the sample?

mC

q = △T

m =

C =

=

△T

x

4.18 J/g•K

Final temp - initial temp = 20℃ - 10℃

= 10℃

418 J

q =

19

As a 15.1 g sample of a metal absorbs 48.75 J of heat, its temperature rises
25 K. What is the specific heat capacity of the metal?

mC

q = △T

m =

C =

=

△T

q =

20

As a 15.1 g sample of a metal absorbs 48.75 J of heat, its temperature rises
25 K. What is the specific heat capacity of the metal?

mC

q = △T

m =

C =

=

△T

15.1

x

25 K

48.75 J

q =

20