Unit 4
A.11-A.14
In which you will learn about:
•Charles’ Law
•Gay-Lussac’s Law
1
Where did A.10 go?
• A.10 will be a volume-temperature lab activity
and/or pressure-temperature lab activity
• RANDOM BUT IMPORTANT:
– STP = standard temperature and pressure (0°C,
1.00 atm)
2
A.11 Temperature-Volume Behavior of
Gases
• This relationship was observed in Activity 2 in
our lab when balloons were placed in a warm
water bath and an ice water bath.
– Increasing or decreasing the temperature
produces changes in its volume
– Direct relationship, if T increases, so does V and
the same is true for a decrease.
3
History of this Relationship
• In the 1780s, French chemists (and hot-air
balloonists) Jacques Charles and Joseph GayLussac studied the changes in gas volume
cause by temperature changes at constant
pressure
– Graphs of their data are linear, thus confirming
the direct relationship
4
Absolute Zero
• Different gases and different sample sizes
produced different lines, BUT they all
extended to the same point (absolute zero)
– Lord Kelvin used this work of Charles and GayLussac to establish a simple mathematical
relationship fore temperature-volume known as
Charles’ Law
– The Kelvin temperature scale is based off this law
5
Charles’ Law
• Doubling the Kelvin temperature of a gas
sample doubles its volume at constant
pressure and constant amount of gas.
– Lowering temp by half, reduces volume by half,
etc.
• Charles’ Law:
6
Kelvin Temperature
• For Charles’ Law (and all gas laws involving
temperature), temperature must be in Kelvins
• K = °C + 273
• Note that it is just K, not °K
7
A.12 Predicting Gas Behavior:
Temperature-Volume
(Homework Questions)
1) What would happen to a balloon’s volume, originally at
20°C, if you took the balloon outdoors in a temperature of
40°C? Assume that pressure is constant and the balloon
does not allow any gas to escape.
2) In planning to administer a gaseous anesthetic to a
patient...
a)
b)
Why must the anesthesiologist take into account the fact that
during surgery the gaseous anesthetic is used both at room
temperature (18°C) and at the patient’s body temperature
(37°C)?
What problems might arise if the anesthesiologist did not
allow for the patient’s higher body temperature?
8
A.12 Cont’d
3) An air bubble trapped in bread dough at room
temperature (291 K) has a volume of 1.0 mL.
The bread bakes in the oven at 623 K (350°C):
a) Predict whether the air-bubble volume will
increase or decrease as the bread bakes. Explain
your prediction, using KMT.
b) Calculate the new volume of the air bubble,
using Charles’ law.
9
A.12 (Last One)
4) You buy a 3.0-L helium balloon in a mall and
place it in a car sitting in hot summer sunlight.
The temperature in the air-conditioned mall was
22°C, and the temperature inside the closed car is
45°C:
a) What will you observe happening to the balloon as it
sits in the warm car?
b) What will be the new volume of the balloon?
c) Sketch two illustrations that depict helium atoms in
the balloon, one when the balloon was in the mall
and one when it is in the car.
d) Explain your illustrations, using KMT.
10
A.13 Temperature-Pressure Behavior
of Gases
• Picture a closed cylinder of gas, such as a
deep-sea scuba tank where volume is
constant.
– What would happen to the average kinetic energy
of gas molecules in the cylinder if you were to
heat the tank?
– How would this affect the gas pressure?
11
Answers to “Rhetorical” Questions on
Slide 11
• Raising the Kelvin temperature of the gas at
constant volume causes an increase in gas
pressure (direct relationship)
– Increasing temp increases the kinetic energy of
the molecules
– Because the molecules are moving faster, the
number of collisions increases and therefore,
pressure increases
• Gay-Lussac’s Law:
12
A.14 Using Gas Relationships
(More Homework Questions)
Solve the following problems, using appropriate gas
relationships:
5) a) If the Kelvin temperature of a gas sample held
in a steel tank increases to three times its original
value, predict what will happen to the pressure of
the gas. Will it increase or decrease? By what
factor do you expect the gas pressure to change?
b) Draw two molecular models that represent the
movement of gas molecules inside the tank at the
two temperatures.
c) Explain your models, using KMT.
13
A.14 Cont’d
6) A gas sample at a constant pressure shrinks to onefourth its initial volume. What must have happened to
its temperature? Did it increase or decrease? By what
factor did the Kelvin temperature of the gas change?
7) Explain why automobile owners in severe northern
climates often add air to their tires in the wintertime
and release some air from the same tires in the
summertime.
8) Use KMT and gas laws to explain why a weather
balloon expands in size as it rises from Earth’s surface.
9) Why does the label on an aerosol container caution
you not to dispose of the container in a fire?
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