THERMOCHEMISTRY PRE-LAB LECTURE
THERMODYNAMICS • Describes the flow of energy between a system and its surroundings
• Systems• Open: exchanges matter and energy with surroundings• Closed: exchanges energy but not matter
• State: the conditions that describe a system
ENERGY CHANGES• Energy (E) is the foundation of thermodynamics• Breaking a bond consumes/uses up energy• Forming a bond releases/gives off energy
ENERGETICS OF PHASE CHANGES
MOLECULAR ENERGY OF MOTION: KE
• A system can exchange energy with its surroundings by the flow of heat (q)
• Any change in temperature is accompanied by a heat transfer• Experiments show that a change in an object’s temperature depend on 4
factors:• 1) Amount of heat transferred • 2) Direction in which heat is transferred• 3) Amount of material • 4) Identity of material
• Molar Heat Capacity: The amount of heat needed to raise the tempeature of 1 mol of substance by 1 kelvin.
MEASURING HEAT FLOW: CALORIMETERS
• A device that measures heat flow is a calorimeter
• Can measure temperature change of the calorimeter to understand heat flow
• A “coffee cup” calorimeter may be used for measuring heat involving solutions
EXOTHERMIC CHEMICAL REACTION
• Exothermic reactions transfer energy from the system to the surroundings
• Energy is released• Causes the reaction mixture and its
surroundings to become hotter • Examples: combustion (burning),
oxidation reactions (rusting of iron with oxygen), neutralization reactions (between acids and bases)
• Uses: hand warmers, self-eating cans, rocket fuel, automobiles
ENDOTHERMIC CHEMICAL REACTION• Reactions take in
energy from the surroundings, causing the reaction mixture and its surroundings to get colder
• Energy is absorbed/used up
• Examples: Baking bread, cooking an egg, producing sugar by photosynthesis
• Uses: sports injury cold packs
HESS’S LAW
INTERPRETING STANDARD ENTHALPY OF FORMATION
• Defined as the change in enthalpy when one mole of a substance in a standard state is formed from its pure elements