1. Gases
Chapter 8

2. 5.1

3. Physical Characteristics of Gases
Gases assume the volume and shape of their containers.
Gases are the most compressible state of matter.
Gases will mix evenly and completely when confined to the same container.
Gases have much lower densities than liquids and solids.
5.1

4. Force Pressure = Area Units of Pressure 1 pascal (Pa) = 1 N/m2
Barometer
Pressure =
Units of Pressure
1 pascal (Pa) = 1 N/m2
1 atm = 760 mmHg = 760 torr
1 atm = 101,325 Pa
5.2

5. 10 miles 0.2 atm 4 miles 0.5 atm Sea level 1 atm
5.2

6. 5.2

7. As P (h) increases
V decreases
5.3

8. Boyle’s Law
P a 1/V
P x V = constant
P1 x V1 = P2 x V2
5.3

9. P1 x V1 = P2 x V2 P1 = 726 mmHg P2 = ? V1 = 946 mL V2 = 154 mL P1 x V1
A sample of chlorine gas occupies a volume of 946 mL at a pressure of 726 mmHg. What is the pressure of the gas (in mmHg) if the volume is reduced at constant temperature to 154 mL?
P1 x V1 = P2 x V2
P1 = 726 mmHg
P2 = ?
V1 = 946 mL
V2 = 154 mL
P1 x V1 V2
726 mmHg x 946 mL
154 mL =
P2 =
= 4460 mmHg
5.3

10. As T increases
V increases
5.3

11. Variation of gas volume with temperature
at constant pressure.
Charles’ Law
V a T
V = constant x T
V1/T1 = V2/T2
Temperature must be
in Kelvin
T (K) = t (0C)
5.3

12. V1/T1 = V2/T2 V1 = 3.20 L V2 = 1.54 L T1 = 398.15 K T2 = ? V2 x T1 V1
A sample of carbon monoxide gas occupies 3.20 L at 125 0C. At what temperature will the gas occupy a volume of 1.54 L if the pressure remains constant?
V1/T1 = V2/T2
V1 = 3.20 L
V2 = 1.54 L
T1 = K
T2 = ?
V2 x T1 V1
1.54 L x K
3.20 L =
T2 =
= 192 K
5.3

13. Avogadro’s Law V a number of moles (n) V = constant x n V1/n1 = V2/n2
Constant temperature
Constant pressure
V = constant x n
V1/n1 = V2/n2
5.3

14. 4NH3 + 5O2 4NO + 6H2O 1 mole NH3 1 mole NO At constant T and P
Ammonia burns in oxygen to form nitric oxide (NO) and water vapor. How many volumes of NO are obtained from one volume of ammonia at the same temperature and pressure?
4NH3 + 5O NO + 6H2O
1 mole NH mole NO
At constant T and P
1 volume NH volume NO
5.3

15. Ideal Gas Equation 1 Boyle’s law: V a (at constant n and T) P
Charles’ law: V a T (at constant n and P)
Avogadro’s law: V a n (at constant P and T)
V a nT P
V = constant x = R nT P
R is the gas constant
PV = nRT
5.4

16. PV = nRT PV (1 atm)(22.414L) R = = nT (1 mol)(273.15 K)
The conditions 0 0C and 1 atm are called standard temperature and pressure (STP).
Experiments show that at STP, 1 mole of an ideal gas occupies L.
PV = nRT
R = PV nT =
(1 atm)(22.414L)
(1 mol)( K)
R = L • atm / (mol • K)
5.4

17. PV = nRT nRT V = P 1.37 mol x 0.0821 x 273.15 K V = 1 atm V = 30.6 L
What is the volume (in liters) occupied by 49.8 g of HCl at STP?
T = 0 0C = K
P = 1 atm
PV = nRT
n = 49.8 g x
1 mol HCl
36.45 g HCl
= 1.37 mol
V =
nRT P
V =
1 atm
1.37 mol x x K
L•atm
mol•K
V = 30.6 L
5.4

18. PV = nRT n, V and R are constant nR V = P T = constant P1 T1 P2 T2 =
Argon is an inert gas used in light bulbs to retard the vaporization of the filament. A certain light bulb containing argon at 1.20 atm and 18 0C is heated to 85 0C at constant volume. What is the final pressure of argon in the light bulb (in atm)?
PV = nRT
n, V and R are constant nR V = P T
= constant
P1 = 1.20 atm
T1 = 291 K
P2 = ?
T2 = 358 K P1 T1 P2 T2 =
P2 = P1 x T2 T1
= 1.20 atm x
358 K
291 K
= 1.48 atm
5.4

19. C6H12O6 (s) + 6O2 (g) 6CO2 (g) + 6H2O (l)
Gas Stoichiometry
What is the volume of CO2 produced at 370 C and 1.00 atm when 5.60 g of glucose are used up in the reaction:
C6H12O6 (s) + 6O2 (g) CO2 (g) + 6H2O (l)
g C6H12O mol C6H12O mol CO V CO2
1 mol C6H12O6
180 g C6H12O6 x
6 mol CO2
1 mol C6H12O6 x
5.60 g C6H12O6
= mol CO2
0.187 mol x x 310 K
L•atm
mol•K
1.00 atm =
nRT P
V =
= 4.76 L
5.5

20. Dalton’s Law of Partial Pressures
V and T are constant P1 P2
Ptotal = P1 + P2
5.6

21. Gas Laws
Dalton’s law of partial pressures: the total pressure, PT, of a mixture of gases is the sum of the partial pressures of each individual gas
Problem: to a tank containing N2 at 2.0 atm and O2 at 1.0 atm we add an unknown quantity of CO2 until the total pressure in the tank is 4.6 atm. What is the partial pressure of CO2?
Solution:

22. Kinetic Molecular Theory of Gases
A gas is composed of molecules that are separated from each other by distances far greater than their own dimensions. The molecules can be considered to be points; that is, they possess mass but have negligible volume.
Gas molecules are in constant motion in random directions. Collisions among molecules are perfectly elastic.
Gas molecules exert neither attractive nor repulsive forces on one another.
The average kinetic energy of the molecules is proportional to the temperature of the gas in kelvins. Any two gases at the same temperature will have the same average kinetic energy
5.7

23. Kinetic Molecular Theory
Ideal gas: the six assumptions of the KMT give us an idealized picture of the particles of a gas and their interactions with one another
Real gases
their atoms or molecules do occupy some volume
there are forces of attraction between their atoms or molecules
In reality, no gases are ideal
at pressures above 1 to 2 atm and temperatures well above their boiling points, most real gases behave in much the same way as predicted by the KMT

24. Intermolecular Forces
The strength of attractive forces between molecules determines whether any sample of matter is a gas, liquid, or solid
the forces of attraction between molecules of most gases are so small that they can be ignored
when T decreases or P increases or both, the forces of attraction become important to the point that they cause condensation (gases to liquids) and ultimately solidification (liquids to solids)
in order to understand the properties of liquids and solids, we must look at the nature of these intermolecular forces of attraction

25. Intermolecular Forces
Three types of intermolecular forces
their origin is electrostatic; that is, the attraction between positive and negative dipoles
the strengths of covalent bonds are also shown for comparison

26. London Dispersion Forces
London dispersion forces are the attraction between temporary induced dipoles

27. London Dispersion Forces
LDF exist between all atoms and molecules
only forces for nonpolar particles
they range in strength from 0.01 to 2 kcal/mol depending on mass, size, and shape
in general, their strength increases as the mass and number of electrons in a molecule increases
even though these forces are very weak, they contribute significantly to the attractive forces between large molecules because they act over large surface areas

28. Dipole-Dipole Interactions
Dipole-dipole interactions; the electrostatic attraction between positive and negative dipoles
butane is a nonpolar molecule; the only interactions between butane molecules are London forces
acetone is a polar molecule; its molecules are held together in the liquid state by dipole-dipole interactions

29. Hydrogen Bonds
Hydrogen bond: a force of attraction between the partial positive charge on a hydrogen and the partial negative charge on a nearby O or N

30. Hydrogen Bonds
the strength of hydrogen bonds ranges from 2 to 10 kcal/mol
that in liquid water is 5.0 kcal/mol
by comparison, the strength of an O-H covalent bond in a water molecule is 119 kcal/mol
the relatively high boiling point of water is due to hydrogen bonding between water molecules
hydrogen bonds are not restricted to water; they form whenever there is are O-H or N-H groups

31. Liquids
as pressure increases attractions between molecules become more important
in liquids, there is very little space between molecules - liquids are difficult to compress
the density of liquids is much greater than that of gases because the same mass occupies a much smaller volume
the position of molecules in a liquid is random and there is irregular space between them into which other molecules can slide; this causes liquids to be fluid

32. Surface Tension Surface tension:
molecules in the interior of a liquid have equal intermolecular forces in all directions
molecules at the liquid-gas interface experience a greater attraction toward the interior of the liquid than toward the gas phase above it
therefore, there is preferential pull of molecules on the surface toward the interior of the liquid
this preferential pull crowds the molecules on the surface, and creates a thin elastic skin-like layer

33. Surface Tension

34. Evaporation/Condensation
Liquids evaporate
in a liquid there is a distribution of kinetic energies (KE) among its molecules
if a molecule at the surface is moving slowly (has a low KE), it cannot escape from the liquid because of the attractions of neighboring molecules
if, however, it is moving more rapidly (has a high KE) and moving upward, it can escape the liquid and enter the gaseous space above it

35. Evaporation/Condensation
if the container is open, molecules escape
if the container is closed, molecules remain in the air space above the liquid
Equilibrium and vapor pressure
vapor pressure is a function of temperature
vapor pressure increases with temperature until it equals the atmospheric pressure
boiling point: the temperature at which the vapor pressure of a liquid equals the atmospheric pressure

36. Evaporation/Condensation
normal boiling point: the temperature at which the vapor pressure of a liquid equals atmospheric pressure

37. Boiling Point
Table 5.3
boiling points of covalent compounds depend primarily on two factors: (1) the nature and strength of intermolecular forces and (2) molecular size and shape

38. Boiling Points Intermolecular forces
consider CH4 (MW 16, bp -164°C) and H2O (MW 18, bp 100°C). The difference in boiling points between them is due to the greater strength of hydrogen bonding in water compared with the much weaker London dispersion forces in methane
consider methane, CH4 (MW 16, bp -164°C), and hexane C6H14 (MW 86, bp 69°C). Because of its larger surface area, London dispersion forces are stronger between hexane molecules than between methane molecules

39. Boiling Points Molecular shape
when molecules are similar in every way except shape, the strength of London forces determines boiling point
both have the same molecular weight
2,2-dimethylpropane is roughly spherical while pentane is a linear molecule
pentane has the higher boiling point because it has the larger surface area

40. Solids
when liquids are cooled, their molecules come so close together and attractive forces between them become so strong that random motion stops and a solid is formed
crystallization (solidification): formation of a solid from a liquid

41. Types of Solids

42. Phase Changes
Phase: any part of a system that looks uniform throughout
examples: solid water (ice), liquid water, and gaseous water (steam)
Phase change: a change from one physical state (gas, liquid, or solid) to another

43. Phase Changes
The heating curve of ice

44. Phase Changes
Calculation of energy required to heat 1.0 gram of solid water from -20°C to 120°C

45. Phase Changes
All phase changes for any substance can be shown on a phase diagram

46. Chapter 5 Gases, Liquids, and Solids
End
Chapter 5