Energy and Heat15.1 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 1oC

1 food calorie = 1 Cal = 1kilocal = 1000 cal

– Joule: (J) • SI unit of heat

– 1 cal = 4.184 J

How do we measure change in heat of

different materials and different masses?

Specific Heat

• Specific Heat (J/g●oC) = the energy required to raise the temperature of 1 gram of ANY substance 1oC– What’s the difference between specific heat

and a calorie??

Equation for Specific Heat

•c = specific heat•q = heat•m = mass

•ΔT = temperature change (Tfinal– Tinitial)

Tcmq * *

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 25.0oC to 80.0oC. 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

•Exothermic •Endothermic

Heat change > 0 Positive q

Heat change < 0 Negative q

heatheat

www.youtube.com/watch?v=yTzcoyzPQE0

•Exothermic •Endothermic

Heat change > 0 Positive q

Positive H

Heat change < 0 Negative q

Negative H

heatheat

For systems at constant pressure, the heat content is the same as a property called enthalpy (H) of the system.

Demo: (Draw Reaction Coordinate)

Exothermic Endothermic

Course of the reaction

Potential Energy

Course of the reaction

Potential Energy

Quick quiz: Reaction energy diagrams.

PE PE

PEEndothermic

Exothermic

Exothermic

Quick Quiz 2…

1. Label the following as endothermic or exothermic:

1. Melting Ice2. Adding salt to a beaker of water

made the beaker cold3. All combustion reactions4. Negative “q” value

2. Any questions or concerns?