Chapter 6
Energy
Thermodynamics
1
Big Ideas



The laws of thermodynamics describe the
essential role of energy and explain and
predict the direction of changes in matter
Any bond or intermolecular attraction that
can be formed can be broken. These two
processes are in a dynamic competition,
sensitive to initial conditions and external
perturbations
Energy is neither created nor destroyed, but
only transformed from one form to another.
2
Essential Questions


Energy is transferred between systems either
through heat transfer or through one system
doing work on the other system.
When two systems are in contact with each
other and are otherwise isolated, the energy
that comes out of one system is equal to the
energy that goes into the other system. The
combined energy of the two systems remains
fixed. Energy transfer can occur through
either heat exchange or work.
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1
Energy is...

The ability to do work.
– Conserved.
– made of heat, q, and work, w.
– a state function.
• independent of the path, or how you get from point A
to B.
Work is a force acting over a distance.
Heat is energy transferred between
objects because of temperature difference.
 Practice with heat and work


4
The universe

is divided into two halves.
– the system and the surroundings.
• The system is the part you are concerned with.
– The surroundings are the rest.
Exothermic reactions release heat
energy to the surroundings.
 Endothermic reactions absorb heat
energy from the surroundings.

6
Potential energy
5
Heat
2
Potential energy
7
Heat
Direction
Every energy measurement has three
parts.
1. A unit ( Joules of calories).
2. A number how many.
3. and a sign to tell direction.

EX) negative heat energy – exothermic -504 kJ
EX) positive heat energy – endothermic +46 J
8
Surroundings
System
E <0
Means - ΔH
exothermic
Heat Energy
9
3
Surroundings
System
E >0
Means + ΔH,
endothermic
Heat Energy
10
Big Ideas



The laws of thermodynamics describe the
essential role of energy and explain and
predict the direction of changes in matter
Any bond or intermolecular attraction that
can be formed can be broken. These two
processes are in a dynamic competition,
sensitive to initial conditions and external
perturbations
Energy is neither created nor destroyed, but
only transformed from one form to another.
11
Essential Questions


Energy is transferred between systems either
through heat transfer or through one system
doing work on the other system.
When two systems are in contact with each
other and are otherwise isolated, the energy
that comes out of one system is equal to the
energy that goes into the other system. The
combined energy of the two systems remains
fixed. Energy transfer can occur through
either heat exchange or work.
12
4
Same rules for heat and work
Heat given off is negative.
 Heat absorbed is positive.
 Work done by system on surroundings
is negative.
 Work done on system by surroundings
is positive.
 Thermodynamics- The study of energy
and the changes it undergoes.

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First Law of Thermodynamics
The energy of the universe is constant.
 Law of conservation of energy.
 q = heat
 w = work
 E = q + w
 Take the systems point of view to
decide signs.

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What is work?
Work is a force acting over a distance.
w= F x d
 P = F/ area
 d = V/area
 w= (P x area) x  (V/area)= -PV
 Work can be calculated by multiplying
pressure by the change in volume at
constant pressure.
 units of liter - atm L-atm


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5
Work needs a sign

If the volume of a gas increases, the system
has done work on the surroundings.
– work is negative, - w
• The system is losing energy
• The system is using energy

If the volume of a gas decreases, the
surroundings has done work on the system.
– work is positive, + w
• The system is gaining energy
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Work needs a sign

w = - PV
– Volume is expanding: work is negative.
• +V
– Volume is contracting: work is positive.
• -V

1 L atm = 101.325 J
17
Formulas

Work of the system
w = - PV

Heat of the system

Total Internal Energy Change
q= mst
E = Ef - Ei
– Aka Energy Change of the System
E = q+w
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6
Examples
What amount of work is done when 15
L of gas is expanded to 25 L at 2.4 atm
pressure?
 If 2.36 KJ of heat are absorbed by the
gas above. what is the change in
energy?
 How much heat would it take to change
the gas without changing the internal
energy of the gas?

19
Work and Heat Activity
http://college.cengage.com/che
mistry/discipline/shared/fae/ge
neral/index.html?layer=act&src=
qtiwf_t017.3.xml
20
Calorimetry
Measuring heat.
Use a calorimeter.
 Two kinds
 Constant pressure calorimeter (called a
coffee cup calorimeter)
 heat capacity for a material, C is
calculated
 C= heat absorbed/ T = H/ T
 specific heat capacity = C/mass


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7
Calorimetry
molar heat capacity = C/moles
heat = specific heat x m x T
 heat = molar heat x moles x T
 Make the units work and you’ve done
the problem right.
 A coffee cup calorimeter measures H.
 An insulated cup, full of water.
 The specific heat of water is 1 cal/gºC
 Heat of reaction= H = sh x mass x T


22
Examples
The specific heat of graphite is 0.71
J/gºC. Calculate the energy needed to
raise the temperature of 75 kg of
graphite from 294 K to 348 K.
 A 46.2 g sample of copper is heated to
95.4ºC and then placed in a calorimeter
containing 75.0 g of water at 19.6ºC. The
final temperature of both the water and
the copper is 21.8ºC. What is the specific
heat of copper?

23
Calorimetry
Constant volume calorimeter is called a
bomb calorimeter.
 Material is put in a container with pure
oxygen. Wires are used to start the
combustion. The container is put into a
container of water.
 The heat capacity of the calorimeter is
known and tested.
 Since V = 0, PV = 0, E = q

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8
Bomb Calorimeter

thermometer

stirrer

full of water

ignition wire

Steel bomb

sample
25
Properties
intensive properties not related to the
amount of substance.
 density, specific heat, temperature.
 Extensive property - does depend on
the amount of stuff.
 Heat capacity, mass, heat from a
reaction.

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