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ENERGY
Potential Energy
Energy of position
Kinetic Energy
Energy of motion KE=1/2 mv2
All atoms are in motion, therefore all have KE.
[except at absolute zero…0K …–273C]
Law of Conservation of Energy
State:“Energy can be converted from one form to another, but can neither be created nor destroyed”
WORK“Force over a distance”
nd
22 2
2
2
2
3
F=ma [force = mass x acceleration] . . .Newton's 2 law
meterNewtons = kilograms x sec
sec sec
force x distance = Joules [also N-m]sec
WorkPower = watts
time sec
kg mN kg m
kg mWork
kg m
State Function
“state function is a PROPERTY of the system that changes INDEPENDENTLY of its pathway”.
ENERGY is a state function! The energy in a system is NOT dependent on the path the system took to reach that state.
Example of State function
The energy difference of a boulder between the top of a mountain and the bottom is fixed.
It does not matter if a boulder rolls down the side or is carried down by a helicopter, the potential energy difference possessed by the boulder is the same regardless of the path down the mountain.
Other State Functions
Density is a state function density does not depend on how much matter
you have.
Temperature is a state function The temperature of a piece of aluminum is
independent of the amount of aluminum.
Pressure is independent of the of the container size.
NOT State Functions“dependent”
Heat and work are not state functions
The more mass you have, the more heat you have
The more mass you have the more work can be done.
Temperature vs. Heat
Temperature is “Average heat” Heat is a measure of molecules in
motion. Heat may be defined as:
“the flow of energy due to a temperature difference”
The flow of energy called heat is the way in which “thermal energy” is transferred from a hot object to a colder object.
System vs. Surrounding
Exothermic = negative change in energy
Endothermic = positive change in energy
EXOTHERMIC ENDOTHERMIC
Measuring Energy Change
calorie (lower case –c-) is defined as the amount of energy (heat) required to raise the temperature of one gram of water by one degree Celsius.
One Calorie (capital –C- ) is one kilocalorie …. 1000 calories.
The SI unit for heat is the Joule (J). 1 cal = 4.18 Joules
Conversion from Joule to calories and calories to Joules How many calories is 700 Joules?
calorie 700.0J x 167.5
4.18Jcalories calories
4.18J = 3400.0 calories x 813.4 Joules
calJoules
How many Joules is 3400.0 calories?
Specific Heat Capacity “The amount of heat required to change
the temperature of one gram of a substance by one degree Celsius.
1 calThe Specific heat of water is 1.000
1g x 1 C
1 cal 4.18 J 1 cal 4.18 Ji.e. or or
T T
cal
g C
org C g C g g
Sp Heat of other substancesunits = J/g-C
Water . . . . . 4.18 Aluminum . . 0.89 Iron . . . . . . 0..45 Gold . . . . . . . 0.13 How much heat is required to raise
the temperature 100.0 g of each of these from 25C to 100C?
ENERGY(kiloJoules) = s x m ΔT ΔT = 75C grams = m = 100.0g
Water:kJ= 4.18J x 100 x 75 = 31,350J (31.4kJ)
Aluminum:kJ = 0.89 x 100 x 75 =6675J (6.7kJ)
Iron:kJ = 0.45 x 100 x 75 = 3375J (3.4kJ)
Gold:kJ = 0.13 x 100 x 75 = 975J (0.975kJ)
Laws of Thermodynamics(3)
1st Law of Thermodynamics states:“ The energy of the universe is constant”
2nd Law of Thermodynamics states:“The universe is proceeding from
order to disorder . . .entropy (S) is increasing”
3rd Law of Thermodynamics states:“The entropy (S) of a perfect crystal
at absolute zero is zero . . . i.e. perfect order!
ENTHALPY
Enthalpy is an energy function Enthalpy is a measure of heat
produced or absorbed in a chemical reaction.
At constant pressure enthalpy is: Hp
Hp = heat
Calculation When one mole of methane is combusted,
the measured heat is 890kJ; How much heat is produced by burning 11.8 g CH4?
4
44 4
mol CH -890kJJoules 11.8g CH x x = -654.6kJ
16.042 g CH mol CH
Why minus?
Hess’s Law
“… in going from a particular set of reactants to a particular set of products, the change in enthalpy (heat) H is the SAME whether the reactions takes place in one step or a series of steps.
