1. Section 7.1—Endothermic and Exothermic
How do they make hot packs feel “hot” and cold packs feel “cold”?

2. 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
So: Endo (heat goes into – absorbs into)
Exo (heat exits or releases) 

3. 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

4. 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

5. 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

6. Endothermic
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

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

8. 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

9. HOW TO ID AN ENDOTHERMIC RXN from an EXOTHERMIC RXN:
There are many ways chemists show the heat of a reaction. Please be familiar with all 3 of these! The first way is GRAPHICAL!

10. Other Ways to ID Endo Vs. Exo
Another clue for the endo vs. exo
When heat is on the LEFT or REACTANT side it is ENDO:
6.01kJ + H2O(s)  H2O(l)
When heat is on the RIGHT or PRODUCT side it is EXO:
H2O(l)  H2O(s) kJ

11. Other Ways to ID Endo Vs. Exo
Yet another way to Endo and Exo!
H2O(s)  H2O(l) ΔH = kJ ENDO
H2O(l)  H2O(s) ΔH = kJ EXO
The + is used to indicate that energy is ADDED to the system for endo!
The – is used to indicate that energy is RELEASED or leaving the system for exo!