MR. SURRETTE VAN NUYS HIGH SCHOOL
THERMAL PHYSICS
Thermal physics describes the effects of kinetic energy on the molecular scale.
TEMPERATURE
Temperature is a measurement of the average kinetic energy of molecules. As molecules gain speed, the temperature increases. Conversely, as molecules slow down, the temperature decreases. This is shown in the following equation.
TEMPERATURE EQUATION
K avg
= ( 3 /
2
) k
B
T
(k
B
= Boltzmann’s constant = 1.38x10
-23 J/K)
TEMPERATURE SCALES
1.
The degree Fahrenheit ( o F) non-metric temperature scale was devised so that the freezing and boiling temperatures of water are whole numbers.
2.
The degree Celsius ( o C) scale was devised by dividing the range of temperature between the freezing and boiling temperatures of pure water into 100 equal parts.
3.
The Kelvin (K) temperature scale is an extension of the degrees Celsius scale down to absolute zero , a hypothetical temperature characterized by a complete absence of heat energy.
TEMPERATURE SCALE COMPARISONS
TEMPERATURE CONVERSION EQUATIONS
T
C
is the Celsius temperature and T is the Kelvin temperature. The size of a degree on the Kelvin scale is identical to the size of a degree on the Celsius scale.
TEMPERATURE CONVERSION EQUATIONS
Some temperature conversions are:
T
C
= T – 273
T
F
= ( 9 /
5
)T
C
+ 32
T
C
= 5 /
9
(T
F
– 32)
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CHEMISTRY

MR. SURRETTE VAN NUYS HIGH SCHOOL
Example 1. Oxygen condenses into a liquid at approximately 90 o
Kelvin (90 K). What temperature is this in degrees Fahrenheit?
1A.
(1) T
C
= T – 273
(2) T
C
= 90 – 273
(3) T
C
= - 183 o C
(4) T
F
= 9 /
5
(T
C
) + 32
(5) T
F
=
9
/
5
( - 183 ) + 32
(6) T
F
= - 297.4 o F
THE KINETIC THEORY OF GASES
The model of an IDEAL GAS is based on the following assumptions:
1. The number of molecules is large, and the average separation between them is large.
2.
The molecules obey Newton’s laws of motion, but the individual molecules move in a random fashion.
3. The molecules undergo elastic collisions with each other.
4. The forces between molecules are negligible, except during a collision.
5.
The gas under consideration is a pure gas.
6.
The gas is in thermal equilibrium with the walls of the container.
DIFFUSION OF GASES
The individual molecules of a gas move randomly and frequently collide. This means it takes time for gas molecules to spread out and fill a container or room.
DIFFUSION OF GASES
The process by which a gas undergoes collisions and spreads out in every direction is called diffusion.
GAS PRESSURE
The random motion of gas molecules and their collisions with a surface create the observable gas pressure on that surface.
STANDARD TEMPERATURE AND PRESSURE
The volume of a gas is usually measured at standard temperature and pressure (STP). Standard temperature is 0 o
C.
STANDARD TEMPERATURE AND PRESSURE
Standard pressure is 1 atmosphere (atm). At STP, 1 mole of any gas occupies a volume of 22.4 L.
AVOGADRO’S NUMBER
Equal volumes of gas at the same temperature and pressure (STP conditions) contain the same number of molecules.
AVOGADRO’S NUMBER
Single moles of any gas at standard temperature and pressure contain the same number of molecules:
N
A
= 6.022 x 10 23 / mole
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CHEMISTRY

MR. SURRETTE VAN NUYS HIGH SCHOOL
IDEAL GAS EQUATIONS
In the following ideal gas equation, T is the temperature in kelvins and n is the number of moles. R is the universal gas constant and P is pressure.
IDEAL GAS EQUATIONS
PV = nRT
(R = 8.31 J/mol.K or 0.0821 L.atm/mol.K)
Example 2. Three moles of an ideal gas are confined to a 30 liter container at a pressure of 2 atmospheres. What is the gas temperature? (R = 0.0821 L.atm/mole.K)
2A.
(1) PV = nRT
(2) T = (PV) / (nR)
(3) T = [(2 atm)(30 liters) / [(3 mol)(0.0821)]
(4) T = 243.6 K
IDEAL GAS EQUATIONS
Let P
1
, V
1
, and T
1
be initial conditions. Let P
2
, V
2
, and T
2
be final conditions. T , the absolute temperature, is in kelvins. Therefore:
(P
1
V
1
) / T
1
= (P
2
V
2
) / T
2
ROOT MEAN SQUARE
The expression for the root mean square (rms) speed shows that, at a given temperature, lighter molecules move faster on the average than heavier molecules (M = molecular weight).
ROOT MEAN SQUARE EQUATION v rms
= (3RT/M) 1/2 v rms
= (3k
B
T/

) 1/2
(k
B
= Boltzmann’s constant = 1.38 x 10 -23

= mass of one molecule)
J/K)
Example 3. If the rms velocity of a helium atom at room temperature is 1350 m/s, what is the rms velocity of an oxygen (O
2
) molecule (mass of O
2
= 32 grams, mass of He = 4 grams)?
3A.
(1) v rms
= [( 3RT )/M]
1/2
(2) v rms
O
2
is proportional to (1/32)
1/2
(3) v rms
He is proportional to (1/4)
1/2
(4) v rms
(O
2
/ He) is proportional to (32) 1/2 / (4) 1/2
(5) v rms
(O
2
/ He) = 2.8
(6) v rms
= 1350 m/s / 2.8
(7) v rms
= 477.3 m/s
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CHEMISTRY

MR. SURRETTE VAN NUYS HIGH SCHOOL
HEAT VERSUS TEMPERATURE
When two systems at different temperatures are in contact, energy will transfer between them until they reach the same temperature. This energy is called heat, or thermal energy, Q .
TEMPERATURE CHANGES
Two objects are in thermal contact if they can affect each other’s temperature. Thermal equilibrium exists when two objects in thermal contact no longer affect each other’s temperature.
THERMAL CONTACT VERSUS EQUILIBRIUM
If a carton of milk from the refrigerator is set on the kitchen counter top, the two objects are in thermal contact . After several hours, their temperatures are the same, and they are then in thermal equilibrium .
UNITS OF HEAT
One unit of heat is the calorie , defined as the amount of heat needed to raise the temperature of 1 gram of water at room temperature one degree Celsius.
UNITS OF HEAT
Another unit of heat is the joule:
SPECIFIC HEAT
The amount of heat needed to increase the temperature of a given substance varies. Every substance has a unique specific heat value, c .
1 calorie = 4.186 joules
SPECIFIC HEAT EQUATION
Q = mc

T
Q = heat [joules] m = mass [grams or kilograms] c = specific heat [joules/kg.
o C]

T = change in temperature [ o C]
LATENT HEAT
The heat needed to cause a phase change depends on the value of latent heat. The latent heat of fusion,
L f
, is used when going from solid to liquid and the latent heat of vaporization, L v
, is used when going from liquid to gas.
LATENT HEAT EQUATION
Q = mL
Q = heat [joules] m = mass [grams or kilograms]
L = latent heat [joules/kg]
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CHEMISTRY

MR. SURRETTE VAN NUYS HIGH SCHOOL
TOTAL HEAT ENERGY
Total heat energy is equal to the sum of specific heat and latent heat:
Q = Q
C
+ Q
L
Example 4. How much heat energy is required to melt a 20 gram block of silver at 20 o
C? The melting point of silver is 960 o C, its specific heat is 2.34 x 10 2 J/kg.
o C, and its heat of fusion is 8.83 x 10 4 J/kg.
4A.
(1) Q
C
= mc

T
(2) (0.020 kg )(2.34 x 10
2
J/ kg .
o
C)(960 o
C – 20 o
C)
(3) Q
C
= 4.40 x 10 3 J
(4) Q
L
= mL
(5) Q
L
= (0.020 kg)(8.83 x 10
4
J/kg)
(6) Q
L
= 1.77 x 10 3 J
(7) Q = Q
C
+ Q
L
(8) Q = ( 4.40 x 10 3 J ) + ( 1.77 x 10 3 J )
(9) Q = 6.17 x 10 3 J
CONSERVATION OF HEAT ENERGY
The heat given off by a substance in a closed environment must be equal to the heat absorbed by its environment:
Q
OUT
= Q
IN
Example 5. A 1 kg block of copper at 20 o
C is dropped into a large vessel of liquid nitrogen at 77 K.
How many kilograms of nitrogen boil away by the point the copper reaches 77 K? (The specific heat of copper is 385 J/kg.
o
C. The heat of vaporization of nitrogen is 2 x 10
5
J/kg.)
5A.
(1) T c
= T – 273
(2) T = T c
+ 273
(3) T = 20 + 273
(4) T = 293 K
(5) Q
C
= mc

T
(6) Q
C
= (1 kg)(385 J/kg.
o C)( 293 K – 77 K)
(7) Q
C
= 8.32 x 10 4 J
(8) Q
OUT
= Q
IN
(9) Q
C
= Q
L
(10) Q
L
= mL
(11) m = Q
L
/ L
(12) m = 8.32 x 10 4 J / 2 x 10
5
J/kg
(13) m = 0.42 kg
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CHEMISTRY