Section 13.1
Describing the Properties of Gases
Objectives
1.
2.
3.
4.
5.
6.
7.
8.
To learn about atmospheric pressure and how barometers work
To learn the units of pressure
To understand how the pressure and volume of a gas are related
To do calculations involving Boyle’s Law
To learn about absolute zero
To understand how the volume and temperature of a gas are related
To do calculations involving Charles’s Law
To understand how the volume and number of moles of a gas are
related
9. To do calculations involving Avogadro’s Law
10. To understand how the temperature and pressure of a gas are related.
11. To do calculations involving Gay-Lussac’s Law.
Section 13.1
Describing the Properties of Gases
Units involved in Gas Laws (memorize these)
Volume
• liters (L)
• milliliters (mL)
• centimeters cubed (cm3)
• decimeters cubed (dm3)
Pressure
• atmospheres (atm)
• Pascals (Pa)
• kilopascals (kPa)
• pounds per square inch (psi)
• millimeters of mercury (mmHg)
• Torricellis (torr)
Temperature
• degrees Celsius (°C)
• Kelvins (K)
– used primarily
Standard Temperature &
Pressure (STP)
• 0°C and 1 atmosphere
Section 13.1
Describing the Properties of Gases
•Pressure – the force a gas exerts (per unit area) on its
surroundings
•Gas pressure is caused by gas molecules colliding with
the surfaces of the surrounding substances.
Measuring Pressure
• Barometer – device that
measures atmospheric
pressure
– The weight of the air
pushing down on the open
dish of mercury supports a
column of mercury in a
closed tube
Section 13.1
Describing the Properties of Gases
A. Pressure
Atmospheric Pressure
– Changing weather conditions
Section 13.1
Describing the Properties of Gases
A. Pressure
Atmospheric Pressure
– Changing altitude
Section 13.1
Describing the Properties of Gases
A. Pressure
Units of Pressure
1 standard atmosphere of air pressure
= 1.000 atm
= 14.69 lbs / in2 (psi)
= 760.0 mm Hg
= 760.0 torr
= 101,325 Pa
= 101.325 kPa
Section 13.1
Describing the Properties of Gases
A. Pressure
Measuring Pressure
• A manometer
measures the
pressure of a gas
trapped in a
container.
• What would the
manometer look
like if the gas
pressure inside
were equal to
atmospheric
pressure?
Section 13.1
Describing the Properties of Gases
B. Pressure and Volume: Boyle’s Law
(when temperature (T) and amount of gas (n) are constant!)
• Robert Boyle’s experiment
– a certain amount of gas
is trapped in a J-tube
– as more mercury is
added, the trapped gas
is compressed into a
smaller volume
• the same number of
gas molecules in a
smaller volume = more
frequent collisions
• Higher pressure
Section 13.1
Describing the Properties of Gases
B. Pressure and Volume: Boyle’s Law
Section 13.1
Describing the Properties of Gases
B. Pressure and Volume: Boyle’s Law
• Graphing Boyle’s results
– as volume of the gas
increases the
pressure on the gas
decreases
• more space = fewer
collisions
– as the volume of the
gas decreases,
pressure on the gas
increases
• smaller space =
more collisions
Section 13.1
Describing the Properties of Gases
B. Pressure and Volume: Boyle’s Law
• This graph has the shape of half of a hyperbola
• Volume and pressure are inversely proportional.
– If one increases the other decreases.
more space=few collisions
little space=more collisions
Section 13.1
Describing the Properties of Gases
B. Pressure and Volume: Boyle’s Law
Another way of stating Boyle’s Law is
P1V1 = P2V2
(constant temperature and amount of gas (moles))
Section 13.1
Describing the Properties of Gases
C. Volume and Temperature: Charles’s Law
• When the pressure of a gas is kept constant
– As the temperature increases, gas molecules travel
faster.
– They collide with each other with much more force
– The collision force them to bounce off each other a
greater distance
– The gas molecules spread out a lot
• fill more volume
Section 13.1
Describing the Properties of Gases
C. Volume and Temperature: Charles’s Law
(when pressure and amount of gas (moles) are constant!)
• Graphing data for several gases
– as temperature increases,
volume increases
• Temperature and Volume are
directly proportional to one
another
– if one increases, the other
also increases
• T must be converted to Kelvins
Section 13.1
Describing the Properties of Gases
C. Volume and Temperature: Charles’s Law
• It is easier to write an equation for the relationship if we
make the lines intersect the origin of the graph (0 and 0).
– so we invented a new temperature scale (Kelvins)
• 0 Kelvins is called absolute zero
• At absolute zero, a gas has no volume (theoretically)
Section 13.1
Describing the Properties of Gases
C. Volume and Temperature: Charles’s Law
• Volume and temperature are directly proportional.
– If one increases the other increases.
• Another way of stating Charles’s Law is
V1 V 2
=
T1
T2
• constant pressure (P) and amount of gas (n)
• CAUTION: temperature must be expressed in Kelvins
(absolute temperature)
• if not, convert it to Kelvins
• Ktemp = °Ctemp + 273
Section 13.1
Describing the Properties of Gases
D. Volume and Moles: Avogadro’s Law
(when temperature and pressure are constant!)
Section 13.1
Describing the Properties of Gases
D. Volume and Moles: Avogadro’s Law
• Volume of a gas is directly proportional to number of moles
(amount) of gas
– If one increases the other increases.
– at a constant temperature and pressure
• Another way of stating Avogadro’s Law is
V1
V2
=
n1
n2
(constant temperature and pressure)
Section 13.1
Describing the Properties of Gases
E. Temperature and Pressure: Gay-Lussac’s Law
(when volume and amount of gas (moles) are constant!)
Temperature and Pressure are directly proportional to one another.
• As the temperature increases, gas molecules travel faster.
• They collide with each other and the walls of the container more
often, and with greater force
– more frequent & violent collisions = more pressure
P1
P2
=
T1
T2
Section 13.1
Describing the Properties of Gases
Summary of Measurable Gas Properties
• Pressure inversely proportional to Volume
– Boyle’s Law
• Volume directly proportional to Absolute Temperature
– Charles’s Law
• Volume directly proportional to Moles of gas
– Avogadro’s Law
• Pressure directly proportional to Absolute Temperature
– Gay-Lussac’s Law