General Chemistry

1. How do Gases
Behave?
Chem Unit 12

2. Main Ideas
For a fixed amount of gas, a change in
one variable – pressure, temperature,
or volume – affects the other two.
The ideal gas law relates the number of
particles to pressure, temperature, and
volume.
When gases react, the coefficients in
the balanced chemical equation
represent both molar amounts and
relative amounts.

3. The Gas Laws
12.1

4. The Gas Laws: Objectives
State the relationship among
pressure, temperature and volume of
a constant amount of gas.
Apply the gas laws to problems
involving the pressure, temperature,
and volume of a constant amount of
gas.

5. Pressure Units
Pressure Units:
1 atm = 1 atmosphere
1 atm = 760 Torr (short for Toricelli)
1 atm = 760 mm Hg
1 atm = 101,325 Pa (short for Pascal)
1 atm = 101 kPa (kilopascal)

6. Boyles Law
Robert Boyle (1629-1691), an Irish
chemist, described this relationship
between pressure and the volume of a
gas.
How are pressure and volume
related?
As volume goes down, pressure
goes up. Inverse relationship
Example:

7. Boyles Law
Boyle’s law states that the volume of a
fixed amount of gas held at a constant
temperature varies inversely with the
pressure.
Formula: P1V1 = P2V2

8. Practice Problem #1
A diver blows a 0.75 L air bubble 10 m
under water. As it rises to the surface,
the pressure goes from 2.25 atm to
1.03 atm. What will be the volume of
air in the bubble at the surface?
1.6 L

9. Charles Law
Jacques Charles (1746-1823), a French
physicist, studied the relationship
between volume and temperature.
How are temperature and volume
related?
As temperature goes up, volume
goes up. Direct relationship
Example:

10. Charles Law
Charles’s law states that the volume of a given
amount of gas is directly proportional to its
Kelvin temperature at constant pressure.
Formula:
Temperature in Kelvin
A temperature of 0 K corresponds to 0 ml,
and doubling the temperature doubles the
volume. Zero on the Kelvin scale is also
known as absolute zero.
This is the lowest possible theoretical
temperature.
V1
T1
=
V2
T2

11. Practice Problem #2
A helium balloon in a closed car
occupies a volume of 2.32 L at 40.0°C.
If the car is parked on a hot day and
the temperature inside rises to 75.0°C,
what is the new volume of the balloon,
assuming the pressure remains
constant?
2.58 L

12. Lussac’s Law
Joseph Lussac (1778-1850), found that a
direct pressure of a fixed amount of gas
varies directly with the Kelvin temperature
when the volume remains constant.
How are temperature and pressure
related?
As temperature goes up, pressure goes
up. Direct relationship
Example:

13. Lussac’s Law
Lussac’s Law states that the pressure of a
fixed amount of gas varies directly with the
Kelvin temperature when the volume
remains constant.
Formula: P1
T1
=
P2
T2

14. Practice Problem #3
The pressure of the oxygen gas inside
a canister is 5.00 atm at 25.0°C. The
canister is located at a camp high on
Mount Everest. If the temperature
there falls to -10.0°C, what is the new
pressure inside the canister?
4.41 L

15. Combined Gas Law
The Combined Gas Law states the
relationship among pressure, temperature,
and volume of a fixed amount of gas. The
relationships are the same as the other
laws but combined into one mathematical
statement.
Formula: P1V1
T1
=
P2V2
T2

16. Practice Problem #4
A gas at 110kPa and 30.0°C fills a
flexible container with an initial volume
of 2.00 L. If the temperature is raised
to 80.0°C and the pressure increases
to 440 kPa, what is the new volume?
0.58 L

17. Homework
CALM: 5 questions
P443 #1,2; P446 #4-6
P448 #8-10; P450 #11-13
P451 #14-18

18. The Ideal Gas
Law
12.2

19. The Ideal Gas Law: Objectives
Relate number of particles and
volume using Avogadro’s principle.
Relate the amount of gas present to
its pressure, temperature, and
volume using the ideal gas law.
Compare the properties of real and
ideal gases.

20. Avogadro’s Principle
Avogadro’s Principle states that equal
volumes of gases at the same temperature
and pressure contain equal numbers of
particles.
The size of the molecules do not matter;
therefore the identity of the gas does not
matter.
Example:

21. Molar Volume
Molar Volume of a gas is the volume that 1
mol occupies at 0.00° C and 1 atm
pressure.
STP: The conditions of 0.00°C and 1.00
atm are known as standard temperature
and pressure.
1 mol of gas at STP = 22.4 L

22. Practice Problem #5
How many moles are in a sample of
gas that has a volume of 3.72 L at
STP?
1.66 L

23. Practice Problem #6
The main component of natural gas
used for home heating and cooking is
methane (CH4). Calculate the volume
that 2.00 Kg of methane gas will
occupy at STP.
2.80x103 L

24. Ideal Gas Law
Avogadro’s principle, Boyle’s law, Charles’s
law and Lussac’s law can all be combined
into a single mathematical statement that
describes the relationship among pressure,
volume, temperature and number of moles
of a gas.
Constant
PV
nRT
=

25. Ideal Gas Law
Since ideal gases react the same no
matter their identity, every gas has the
same constant when using the Ideal Gas
Law.
 0.08206 (L atm)/(mol K)

26. Practice Problem #7
Calculate the number of moles of
ammonia gas (NH3) contained in a 3.0
L vessel at 3.00 x 102 K with a
pressure of 1.50 atm.
0.18 mol

27. Real vs. Ideal Gases
Ideal gases follow the assumptions of the
kinetic molecular theory (KMT).
Assumptions:
1.An ideal gas is one whose particles do
not take up space.
Gas molecules do not have volume; their
movement creates volume.

28. Real vs. Ideal Gases
Ideal gases follow the assumptions of the
kinetic molecular theory (KMT).
Assumptions:
2.Ideal gases do not experience
intermolecular attractive forces.
Gas molecules are too far apart to attract
or repel each other.

29. Real vs. Ideal Gases
Ideal gases follow the assumptions of the
kinetic molecular theory (KMT).
Assumptions:
3.Ideal gas particles are in constant,
random motion and collide with each
other and the walls of the container.
Collisions of the molecules are elastic
and cause pressure.

30. Real vs. Ideal Gases
In reality, no gas is truly ideal.
Most gases behave like ideal
gases at a wide range of
temperatures and pressures.
Under the right conditions,
calculations made using the ideal
gas law closely approximate
experimental measurements.

31. Real vs. Ideal Gases
When do real gases not behave as ideal
gases?
1. Low temperatures
Gas molecules do not have the kinetic
energy they usually do and do not move
as fast. Because they are moving
slowly, attractive forces can change the
way they behave.

32. Real vs. Ideal Gases
When do real gases not behave as ideal
gases?
2. High pressures
Gas molecules are crowded and their
volume becomes significant to the
overall volume of the container.

33. Gas
Stoichiometry
13.3

34. Gas Stoichiometry: Objectives
Determine volume ratios for gaseous
reactants and products by using
coefficients from chemical equations.
Apply gas laws to calculate amounts
of gaseous reactants and products in
a chemical reaction.

35. Gas Stoichiometry
Stoichiometry of reactions involving gases
often give pressure, volume and/or
temperature in order to find moles.
The “core” process of the stoichiometry
remains the same.

36. Practice Problem #8
What volume of oxygen gas is needed
for the complete combustion of 4.00L
of propane gas (C3H8)? Assume that
pressure and temperature remain
constant.
20.0 L O2; because moles and
volume are directly related, volume
can be used with the mole ratios.

37. Practice Problem #9
Ammonia is synthesized from hydrogen
and nitrogen.
N2(g) + 3H2(g)  2NH3(g)
If 5.00 L of nitrogen reacts completely
with hydrogen at a pressure of 3.00 atm
and a temperature of 298 K, how much
ammonia, in grams, is produced?
21.0 g NH3

38. Practice Problem #10
What volume of H2O(g) measured at STP
is produced by the combustion of 5.73 g
of natural gas (CH4) according to the
following equation?
CH4(g) + 2O2(g)  CO2(g)+2H2O(g)
16 L

39. Practice Problem #11
Calcium hydride combines with water
according to the equation
CaH2(s) + 2H2O(l) 2H2(g) + Ca(OH)2(s)
Beginning with 84.0 g of CaH2 and 36.0 g
of H2O, what volume of H2 will be
produced at 273 K and a pressure of
1609 torr?

40. Practice Problem #12
Ammonia is synthesized from hydrogen
and nitrogen.
N2(g) + 3H2(g) -> 2NH3(g)
You have 15.0g of N2 and 5.0g of H2 at
STP. How many grams of NH3 can be
produced and what is the mass of the
excess reactant?

41. Accumulating
Content
12.4

42. Accumulating Content: Objectives
Apply knowledge and skills from
previous units to content learned in
this unit.

43. Accumulating Content
Paper
Write a one page paper (single spaced) about
how chemistry and gases are related.
Ideas:
Medical: hyperbaric chamber, ozone,
anesthetic gases
Ecological: greenhouse gases, gas pollution
History: Haber, chemical warfare, pneumatic
chemists