Chapter 7: Hot & Cold Packs
Introductory Activity
How many things can you think of in
everday life that either give off heat or
absorb heat?
Which of these things are physical
Which are chemical processes?
Hot & Cold Packs
This chapter will introduce the chemistry
needed to understand how Hot & Cold
Packs work
Section 7.1: Endothermic & Exothermic
Section 7.2: Calorimetry and heat capacity
Section 7.3: Changes in State
Section 7.4: Heat of a Chemical Reaction
Section 7.5: Hess’s Law
Hot/Cold Packs
Transfer of energy
System &
Is determined with
Can be done in
Effect on
depends on
Materials ability to
absorb energy
without noticeable
Section 7.1—Endothermic and
Why are hot packs “hot” and cold packs feel “cold”
Endothermic & Exothermic
When the system absorbs energy from the
surroundings, it’s an endothermic process
When the system releases energy to the
surroundings, it’s an exothermic process
System & Surroundings
It’s very important to define the system &
surroundings correctly to use the endoand exothermic definitions!
Most people define the system too broadly
They include everything in the beaker or
container as the system
However, the system is only the molecules
undergoing the change
System & Surroundings
The system is only made of the
molecules undergoing the change
The water molecules & the container…
Your hand and the air…
Even the thermometer…
They are all the surroundings
Note that water is made up of water
molecules—not a solid chunk of water…but
for this picture, it’s best to represent water as
one thing since it’s the surroundings and focus
on the molecules reacting as the system.
Exothermic & You
You touch the beaker and it feels
Energy is being transferred TO YOU
You are the surroundings
When energy moves from system to
surroundings, it’s exothermic
Exothermic & the Thermometer
The temperature (measured by the
thermometer) is related to the
average kinetic energy of the
molecules in the container
The majority of the molecules in a
solution are water
If the temperature is increasing, the
energy of the water molecules is
Since water is the surrounding (it’s
not actually reacting), energy is being
transferred to the surroundings
Exothermic shows an increase in the temperature within the container
The opposite is also true
If the container feels cold to you, energy is
being transferred FROM YOU (the
surroundings) into the system—
If the thermometer goes down, energy is
being transferred FROM the water
molecules (surroundings) into the system-endothermic
Let’s Practice
Identify the system
and surroundings
when you hold an
ice cube while it
melts. Is this endoor exothermic?
Let’s Practice
Identify the system
and surroundings
when you hold an
ice cube while it
melts. Is this endoor exothermic?
System: Water molecules in the form of ice
Surroundings: You and the air
It feels cold to you…so energy is leaving you (surroundings)
When energy goes from surroundings to system it’s endothermic
Reaction Diagrams

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