Energy and Heat 15.1 and 15.2 (pgs. 516 – 528)

Main Idea… Energy can change form and flow, but it is always conserved The enthalpy change for a reaction is the enthalpy of the products minus the enthalpy of the reactants

Energy Transformations Energy is the ability to do work or produce heat –Two types: potential and kinetic energy –Chemical Potential Energy: stored within a substance due to its composition Example: Gasoline  used to power cars

Thermochemistry and the Universe Although energy can be in different forms it cannot be destroyed or created: Law of conservation of energy –All energy can be accounted for as work, heat or stored energy Example – my poor car

Questions so far….?

Heat Heat (q) = energy that is in the process of flowing from a warmer object to a cooler object

Common units of Heat: –Calorie: the amount of energy (heat) needed to raise the temperature of 1g of water 1 o C 1 food calorie = 1 Cal = 1kilocal = 1000 cal –Joule: (J) SI unit of heat –1 cal = J

How do we measure change in heat of different materials and different masses?

Specific Heat Specific Heat (J/g● o C) = the energy required to raise the temperature of 1 gram of ANY substance 1 o C –What’s the difference between specific heat and a calorie??

Equation for Specific Heat c = specific heat q = heat m = mass ΔT = temperature change (T final – T initial )

Practice Problems Will the specific heat of 50. g of a substance be the same as, or greater than, the specific heat of 10. g of the same substance?

Practice Problems Using calories, calculate how much heat 32.0g of water absorbs when it is heated from 25.0 o C to 80.0 o C. How many joules is this?

Calorimetry Study of energy changes in physical and chemical processes Uses a calorimeter to measure the amount of heat absorbed or released

Thermochemistry and the Universe Thermochemistry: the study of the heat changes that accompany chemical reactions and phase changes

Exothermic and Endothermic Processes The system is the part of the universe on which we want to focus our attention The surroundings include everything else in the universe When looking at the universe, we have two major parts:

Exothermic and Endothermic Processes When a process gives-off heat, it is considered exothermic Energy flows out of the system Processes that absorb energy from the surroundings are endothermic Energy flows into the system

Exothermic and Endothermic Processes ExothermicEndothermic Heat change > 0 Positive q Heat change < 0 Negative q heat

ExothermicEndothermic Heat change > 0 Positive q Positive H Heat change < 0 Negative q Negative H heat For systems at constant pressure, the heat content is the same as a property called enthalpy (H) of the system.

Demo: (Draw Reaction Coordinate) ExothermicEndothermic Course of the reaction Potential Energy Course of the reaction Potential Energy

Quick quiz: Reaction energy diagrams. PE Endothermic Exothermic

Quick Quiz 2… 1.Label the following as endothermic or exothermic: 1.Melting Ice 2.Adding salt to a beaker of water made the beaker cold 3.All combustion reactions 4.Negative “q” value 2.Any questions or concerns?