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