TOPIC: ENERGY Do Now:. All physical & chemical changes are accompanied by change in energy The...
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Transcript of 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