1. Honors Chemistry
Chapter 5
Gases

2. 5.1 Gases Temperature vs. Intermolecular attraction Atomic Gases
Noble Gases, H2, N2, O2, F2, Cl2
Molecular Gases
Usually light molecules with weak attraction forces
Eg: HCl, CO2, NH3, H2S, NO2
Ionic Compounds
Strong forces; not normally gases

3. 5.2 Pressure Force per unit area P = F/A
N/m2 unit defined as Pascal (Pa)
Standard air pressure = kPa
Also called 1 atmosphere (atm)
Measured by unequal mercury levels
Manometers and barometers
Common unit called mmHg (or Torr)
Standard air pressure = 760 mmHg

4. 5.2 Dimensional Analysis Convert 75.0 kPa to mmHg
75.0 kPa mmHg x = 563 mmHg kPa
Try this one
Convert 1.25 atm to kPa

5. 5.3 Boyle’s Law Pressure is inversely proportional to volume
Hold temperature and amount of gas constant
V a 1/P
V = k x (1/P)
PV = k
Best used with changing conditions
P1V1 = P2V2

6. 5.3 Boyle’s Law Problems
A 175 mL sample of methane is stored at 125 kPa. What pressure is needed to compress the gas to a volume of 50.0 mL?
P1V1 = P2V2
(125 kPa) (175 mL) = P2 (50.0 mL)
P2 = 438 kPa
Try this one
A sample of argon occupies 476 mL at 650 Torr. Find the volume at 975 Torr.

7. 5.3 Charles’ Law Also credited to Gay-Lussac
Volume is directly proportional to temperature
Hold pressure and amount of gas constant
V a T
V = kT
Linear relationship
Must use Kelvins!
V V = --- T T2

8. 5.3 Charles’ Law Problems
A 5.00 L helium balloon is heated from 20oC to 75oC. Find its new volume.
V1/T1 = V2/T2
5.00 L V = K K
V2 = 5.94 L
Try this one
A 670 mL sample of chlorine is stored at 50oC. At what temperature will its volume be 450 mL?

9. 5.3 More Gas Laws Another form of Charles’ Law Avogadro’s Law
Pressure is directly proportional to temperature
P = kT
P1/T1 = P2/T2
Avogadro’s Law
Volume is directly proportional to the amount of gas present
V a n
Volume relationships in chemical reactions

10. 5.3 Avogadro’s Law Problems
How many liters of hydrogen are needed to completely react with 1 liter of oxygen?
2 H2 + O2  2 H2O
2 mol hydrogen react with 1 mol oxygen
V a n, so….
2 L hydrogen react with 1 L oxygen
Try this one
How many liters of ammonia are formed when 1 L of hydrogen reacts with excess nitrogen?

11. 5.4 The Ideal Gas Equation Ideal Gas
No intermolecular attraction forces
Particles have no volume
Combine Boyle’s, Charles, and Avogadro’s Laws
PV = nRT
STP = 1 atm, 273 K
Molar volume of a gas = L at STP
R = atm L / mol K

12. 5.4 Ideal Gas Equation Problems
A sample of fluorine occupies 3.65 L at 45oC and 2.50 atm. How many moles of fluorine are present?
PV = nRT
(2.50 atm)(3.65 L) = n (0.0821)(318 K)
n = mol
Try this one
A mol sample of propane occupies 2.15 L. If the temperature is 28oC, find the pressure.

13. 5.4 Gas Density Since n = m/M…. PV = (m/M) RT MPV = mRT
Divide by V to get density (m/V)
MP = rRT
Gas density expressed in g/L

14. 5.4 Gas Density Problems Find the density of nitrous oxide at STP.
First, find molecular mass of N2O
MP = rRT
(44.0 g/mol)(1.00 atm) = r (0.0821)(273 K)
r = 1.96 g/L
Try this one….
A gas is found to have a density of 2.54 g/L at 15oC and 1.50 atm. Find its molecular mass.

15. 5.5 Gas Stoichiometry Mass-Mass problems (review)
Volume-Volume problems
Volume is proportional to moles, so….
Mol relationship from reaction can be used directly
No conversions needed!

16. 5.5 Volume-Volume Problem
2 H2 + O2  2 H2O
If 3.25 L of oxygen react, how many liters of water vapor are formed?
3.25 L O L H2O x = 6.50 L H2O L O2
Volume-Volume is just Avogadro’s Law!

17. 5.5 Mass-Volume Problems Key step – get to moles!
Mass conversion – use molecular mass
Volume conversion – use gas equation
Need to know temperature and pressure conditions

18. 5.5 Mass-Volume Problems
25.0 g of sodium react with excess water at STP. How many liters of hydrogen are produced?
2 Na + 2 H2O  2 NaOH + H2
25.0 g Na 1 mol Na 1 mol H x x = mol g Na 2 mol Na
Now use ideal gas equation to get volume

19. 5.5 Mass-Volume Problems PV = nRT
(1.00 atm) V = (0.543 mol)(0.0821)(273 K)
V = 12.2 L
Try this one
Potassium chlorate decomposes into potassium chloride and oxygen gas. How many grams of KClO3 are needed to produce 5.00 L of oxygen at atm and 18oC?
Hint: This one is backwards!

20. 5.6 Dalton’s Law
Partial pressure – the pressure of an individual gas in a mixture of gases
Total pressure of a mixture equals the sum of the partial pressures of each gas
Pt = P1 + P2 + P
Partial pressure is proportional to the mol fraction (X1 = n1 / nt)
P1 = X1 Pt

21. 5.6 Dalton’s Law
2.00 mol He is mixed with 1.00 mol Ar. Find the partial pressure of each at 1.75 atm pressure.
XHe = 2.00 mol / 3.00 mol = 0.667
XAr = 1.00 mol / 3.00 mol = 0.333
PHe = (0.667) (1.75 atm) = 1.17 atm
PAr = (0.333) (1.75 atm) = atm
Try this...
Find the partial pressure of oxygen in air if it makes up 21% of the Earth’s atmosphere by volume. (Note: The volume gives you the mole ratio because of Avogadro’s law.)

22. 5.7 Kinetic Molecular Theory
Explains gas behavior in terms of molecular motion
Energy
Work done by a moving object
Measured in SI unit Joule (J)
Kinetic energy
Energy due to motion
K = ½ mv2
KMT is a simplification of reality (ideal gas)

23. 5.7 Kinetic Molecular Theory
Gas molecules are separated by great distances
They can be treated as “point masses”
Gas molecules are in constant random motion
Frequent elastic collisions (no energy lost)
No attractive or repulsive forces
Average K is proportional to Temperature

24. 5.7 Distribution of Molecular Speeds
Maxwell-Boltzmann Distribution
Molecular speeds distributed around average
Peak velocity depends on temperature and on molec. mass
Root Mean Square Speed
_____ vrms = √3RT/M
Rate of diffusion

25. 5.8 Deviations from Ideal Behavior
We made approximations!
Point masses
No intermolecular forces
These approximations become bad at...
High pressure
Low temperature
Liquefaction
van der Waals Equation
(P + an2/V2) (V – nb) = nRT