Class #104: Energy and Phase Change

Chemistry Class #104

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

Multiple Select

Which changes of state involve ADDING heat? (Choose 2!)

[HINT: look at your notes!]

• Freezing

• Fusion (Melting)

• Condensation

• Vaporization (Evaporation)

Multiple Select

Which changes of state involve REMOVING heat? (Choose 2!)

• Condensation

• Evaporation

• Sublimation

• Deposition

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

Multiple Select

Which units can be used to measure heat energy? (Choose TWO)

• degrees Celsius

  $$\left(\degree C\right)$$

• Joules (J)

• Kilojoules (kJ)

• Kelvin (K)

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

Multiple Choice

The energy involved in the movement of particles in a substance is

• potential energy

• kinetic energy

• both potential and kinetic energy

Multiple Choice

Temperature is defined as the

• total amount of kinetic energy in a substance

• total amount of potential energy in a substance

• average kinetic energy of the particles in a substance

• average potential energy of the particles in a substance

Multiple Choice

Use your notes OR Table T for this question!

A piece of pizza straight out of the oven has a temperature of 425 $$\degree C$$ . What is its temperature in Kelvin?

• 525K

• 698K

• 152

• 273K

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

Multiple Choice

When the temperature of a substance is increasing, the potential energy of the substance

• increases

• decreases

• stays the same

Multiple Choice

When the temperature of a substance is increasing, the kinetic energy of the substance

• increases

• decreases

• stays the same

Multiple Choice

When a substance is changing from a solid to a liquid

• the temperature increases

• the temperature decreases

• the temperature stays the same

Multiple Choice

When a substance is changing from a gas to a liquid

• the potential energy increases

• the potential energy decreases

• the potential energy stays the same

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

Multiple Choice

Use Table T or your notes for this question!

What is the symbol in the formula for Specific Heat Capacity?

• $$\Delta T$$

• m

• C

• q

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

Multiple Choice

Is the temperature changing in this problem

• Yes

• No

Multiple Choice

Looking at Table T, which equation is needed for this problem?

• $$q=mHf$$

• $$q=mC\Delta T$$

• $$q=mH\upsilon$$

Multiple Choice

The mass in this problem is

• $$20\circ C$$

• $$80\circ C$$

• 250g

Multiple Choice

The change in temperature is

• $$20\degree C-80\degree C=-60\degree C$$

• $$80\degree C-20\degree C\ =60\degree C$$

Multiple Choice

What is C in the formula and where can you find it?

• C is the volume and it is in the question.

• C is the temperature and it is on Table T.

• C is the Specific Heat Capacity and it is on Table B.

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

Fill in the Blanks

How much heat energy is required? Make sure to add the correct UNIT for heat!!

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

Fill in the Blanks

What is $\Delta T$ for this problem? (You cannot enter the unit for this problem - just enter the number value WITH the correct sign!)

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

Fill in the Blanks

What is the change in heat energy for this problem. Make sure to add the correct unit for heat!

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

Multiple Choice

What is the UNKNOWN in this problem? What are you supposed to find?

• The unknown is q, the amount of heat energy.

• The unknown is $$\Delta T$$ , the temperature change

• The unknown is C, the Specific Heat Capacity of water

• The unknown is m, the mass of the water.

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

Fill in the Blanks

Use a piece of paper and a calculator to work out this problem. Record your answer below. Make sure to include the correct unit!!

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

Multiple Choice

What is the unknown in this problem? What is it asking you to find?

• The mass, m, of the sample.

• The change in heat, $$\Delta T$$

• The Specific Heat Capacity, C

• The heat energy used - q

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

Fill in the Blanks

Enter your answer below. The decimal point is given for you. Make SURE to add the correct unit! You can find the unit on Table B. You can't type the dot $\left(\cdot\right)$ in the unit. Type x instead!