Section 7.1—Endothermic and Exothermic How do they make hot packs feel “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 So: Endo (heat goes into – absorbs into) Exo (heat exits or releases) 

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

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

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

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!

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) + 6.01kJ

Other Ways to ID Endo Vs. Exo Yet another way to Endo and Exo! H2O(s)  H2O(l) ΔH = +6.01kJ ENDO H2O(l)  H2O(s) ΔH = -6.01kJ 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!