topic: energy do now:. all physical & chemical changes are accompanied by change in energy the...

TOPIC: ENERGYDo Now:

All physical & chemical changes are accompanied by change in energy

The chemistry of energy changes is known as Thermochemistry!

Stability and Energy

• If energy is high, stability is low

• If energy is low, stability is high

Energy: Ability to do WorkThe SI unit for an energy

measurement is called the Joule (J)

EXAMPLE: 1 Joule = amount of energy required to lift a golf ball 1 meter

Other Energy Units: calorie, Calorie, BTU’s

1 calorie = 4.18 Joules

1 Calorie = 1000 calories = 1 kilocalorie

NOTE: When your body breaks down food, these reactions give off heat – which is measured in calories (That’s why your food is labelled in calories)

Law of Conservation of Energy

• Energy is neither created nor destroyed in ordinary chemical or physical change, rather it can be converted from one form to another

Energy before = Energy after

- potential to kinetic - radiant to electric- electric to heat - chemical to kinetic- chemical to electrical

(Not a complete list!)

Energy

Mechanical – large enough to see

Kinetic Potential

Non-mechanical – too small to see

Chemical Heat Light Electrical Nuclear

Kinetic Energy (KE) – energy of motion

• KE = ½ x Mass x Velocity2 = ½ mV2

• KE depends on how heavy and how fast

Kinetic Molecular Theory: the atoms and molecules making up substances are in constant motion

Potential Energy (PE): energy of position; stored energy of matter

EXAMPLESstaplerRubberband

• When Potential energy is released from matter it becomes kinetic energy

Energy in Chemistry=

chemical energy

heat energy

Chemical Energy

• energy stored in bonds

• Bonds are stable• You need energy to break bonds• Energy is released when bonds are formed

EXOTHERMIC If the overall energy in a chemical

reaction is negative (meaning energy is released) = the rxn is

exothermicenergy (heat) is given off =

exothermic EXO - energy leaves system (exits)

Environment

System

Energy

Exothermic RxnTemperature of environment

Temperature of system

ENDOTHERMIC If the overall energy in a chemical

reaction is positive (meaning energy is absorbed) = the rxn is

endothermic

energy (heat) is absorbed = endothermic

Endo - Energy enters system (enter)

Environment

System

Energy

Temperature of environment

Temperature of system

ENDOTHERMIC

Energy of Universe is conserved

Universe

EnvironmentEnvironment

System

Energy

Energy can move between the system and the environment

Calorimeter: an insulated devise used for measuring the amount of heat absorbed or released during a chemical or physical change

“universe” is contained in Styrofoam cup

“environment” is water****

“system” is whatever put in water

Energy lost = Energy gained

Difficult to monitor “system”Easy to monitor “environment” (water)

Energy lost/gained by environment = Energy gained/lost by system

Calorimetry

10 grams of NaOH is dissolved in 100 g of water & the temperature of the water increases from 22C to 30C

• was dissolving process endothermic or exothermic

• how do you know?

Exothermic – temperature of environment ↑

Heat EnergyHeat: energy that is in the process of

flowing from warmer object to a cooler object

Symbol for heat energy = Q or q

The amount of heat required to raise the temp. of 1 gram of water 10C = a calorie

The amount of heat transferred depends on 3

things

Temperature changeMass of substanceSpecific Heat of substance

Specific Heat• The amount of heat required

to raise the temp of any given substance by 10C

• Symbol = c

• Specific heat = a physical constant

• unique for each pure substance

Found in Table B

Calculating Heat Transferred

Q = mCT

Simple system: •pure substance in single phase •calculate heat gained or lost using:

Q = amount of heat transferredm = mass of substanceC = specific heat capacity of the substance.T = temperature change = Tfinal – Tinitial

Calorimetry problem from before

10 grams of NaOH is dissolved in 100 g of water & the temperature of the water increases from 22C to 30C

• We already said it’s exothermic – now we can actually calculate the amount of heat energy released

Energy lost by NaOH (system) = Energy gained by

water (environment)

Environment

Energy

• What’s happening when NaOH dissolves?

Add H2O

NaOH close together, not interacting

Na+ and OH- ions pulled apart & interacting with H2O

Q = mCTQ = energy (joules)M = mass (grams)C = specific heat capacity (Table B)T = temperature change = Tf - Ti

Q = ?M = Well, temp change was for water, so want mass of water

m = 100 gC = specific heat capacity (Table B) = 4.18 J/gKT = temperature of water increased from 22C to 30C

30C -22C = 8C = T

Q = mCT

• Q = (100 g)(4.18 J/gC)(8C)

• Q = 3344 Joules