Chemistry You Need to Know

Chemistry You Need to Know

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 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: Hesss Law Hot/Cold Packs Use

Transfer Transferof of energy energy between System System&& Surroundings Surroundings Is determined with

Calorimetry Calorimetry Physical Physical change change Can be done in Effect on temperature depends on Chemical Chemical

change change Materials Materialsability abilityto to absorb absorbenergy energy without withoutnoticeable noticeable temperature

temperature change change Section 7.1Endothermic and Exothermic Why are hot packs hot and cold packs feel cold Endothermic & Exothermic When the system absorbs energy from the surroundings, its an endothermic process

When the system releases energy to the surroundings, its an exothermic process System & Surroundings Its 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 waterbut for this picture, its best to represent water as one thing since its the surroundings and focus on the molecules reacting as the system. 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, its 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 (its not actually reacting), energy is being transferred to the surroundings Exothermic shows an increase in the temperature within the container

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 Lets Practice

Example: Identify the system and surroundings when you hold an ice cube while it melts. Is this endoor exothermic? Lets Practice Example: 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 youso energy is leaving you (surroundings) When energy goes from surroundings to system its endothermic

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