Chapter 7: Hot & Cold Packs

1. Chapter 7: Hot & Cold Packs

2. 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?

3. 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

4. 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

5. Section 7.1—Endothermic and Exothermic Why are hot packs “hot” and cold packs feel “cold”

6. 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

7. 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

8. 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.

9. 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

10. 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

11. 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

12. Let’s Practice Example: Identify the system and surroundings when you hold an ice cube while it melts. Is this endo- or exothermic?

13. 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

14. Reaction Diagrams