11.8 THE GAS LAWS – ABSOLUTE TEMPERATURE AND CHARLES’ LAW
ABSOLUTE TEMPERATURE
The Kelvin Cycle and Absolute Zero
In 1848, Lord Kelvin interpreted that at -273.15 °C, the motion of particles would theoretically
stop. Therefore at this temperature, the kinetic energy would be zero, and hence the volume
would also be zero. We know however, that real gas molecules do have volume.
What’s the Deal?
Lord Kelvin was able to come up with this from studying a variety of gases. He graphed their
volumes at changing temperatures and noticed that all the gases intercepted at a particular
point. The volume of gases would all be zero at approx. -273.15 °C
The Kelvin temperature scale (unit symbol: K) starts at 0 K (also called absolute zero). It is
equivalent to -273.15 °C. Each unit on the Kelvin scale is the same as a unit the Celsius scale.
Formula for Converting Celsius to Kelvin (not necessary to use GRASS)
T = t + 273.15 (T = temperature in Kelvin, t = temperature in degrees Celsius)
CHARLES’ LAW
Charles’ Law states that the volume of a gas is directly proportional to
its temperature (pressure and amount of gas must remain constant).
Consider a balloon filled balloon with air. According to the Kinetic
Molecular Theory, if the temperature is increased (and the pressure inside
the balloon stays constant) the volume will
.
Conversely, if the temperature of the balloon decreases then the volume
will
.
Since these changes are proportional, they can be expressed
mathematically as:
V = k (constant value) or V = kT
T
Mathematically, V  T
a differs depending on the gas and acts as the proportionality constant. (T)emperature is measured in
Kelvins. In this relationship, if you double the temperature, you double the volume.
We can also express Charles’ law by initial
conditions and final conditions.
V1 = V2
T1 T2
Table 1: Volume and Temperature of Hydrogen Gas
V
Figure 1: Graph of Charles’ Law Relationship
/T (K)
Volume
(mL)
Temp.
(oC)
Temp.
(K)
25
-23
250
0.1
30
27
300
0.1
35
77
350
0.1
40
127.5
400.5
0.1
45
177
450
0.1
Real- World Application: Popping Popcorn
Each popcorn kernels’ shell is strong and contains
a small quantity of water. When the kernel is
heated, the water becomes water vapour and
expands the shell, causing the shell to burst.
Example Problem 1:
A sample of carbon dioxide is placed in a piston. The initial temperature of this gas is 35 C and
it occupies a volume of 2.2 L. Calculate the temperature at which it will occupy 4.4 L. The
pressure and the amount of gas remain constant.
G: T1 = 35°C + 273 = 308 K
V1 = 2.2 L
V2 = 4.4 L
A: V1 = V2
T1 T2
R: T2 = ?
Rearrange the Equation:
S:
S:
Homework: Read 547-552 and Do Questions #1, 2ac, 3-5, 7