Chapter 7 hot cold packs
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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 processes? Which are chemical processes?. Hot & Cold Packs.

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Chapter 7: Hot & Cold Packs

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Chapter 7 hot cold packs

Chapter 7: Hot & Cold Packs


Introductory activity

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 processes?

  • Which are chemical processes?


Hot cold packs

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 packs1

Use

Can be done in

between

Is determined with

Effect on temperature depends on

Hot/Cold Packs

Physical change

Transfer of energy

Chemical change

System & Surroundings

Materials ability to absorb energy without noticeable temperature change

Calorimetry


Section 7 1 endothermic and exothermic

Section 7.1—Endothermic and Exothermic

Why are hot packs “hot” and cold packs feel “cold”


Endothermic exothermic

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

System & Surroundings

  • It’s very important to define the system & surroundings correctly to use the endo- and 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 surroundings1

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

Exothermic & You

You touch the beaker and it feels hot

Energy is being transferred TO YOU

You are the surroundings

When energy moves from system to surroundings, it’s exothermic


Exothermic the thermometer

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 increasing

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


Endothermic

Endothermic

  • The opposite is also true

  • If the container feels cold to you, energy is being transferred FROM YOU (the surroundings) into the system—endothermic

  • If the thermometer goes down, energy is being transferred FROM the water molecules (surroundings) into the system--endothermic


Let s practice

Let’s Practice

Example:

Identify the system and surroundings when you hold an ice cube while it melts. Is this endo- or exothermic?


Let s practice1

Let’s Practice

Example:

Identify the system and surroundings when you hold an ice cube while it melts. Is this endo- or 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

Reaction Diagrams


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