1. GASES

2. Properties of a Gas
Uniformly fills any container (no fixed shape)
Large distance between molecules (no fixed volume)
Mixes completely with any other gas
Exerts pressure on its surroundings
WHY?

3. Simple barometer invented by Evangelista Torricelli
The pressure exerted by the surrounding air is equal to the pressure exerted by a column of Hg 760 mm in height.
Simple barometer invented by Evangelista Torricelli

4. What is “pressure”? force/unit area
English unit = pounds per square inch
SI unit = Newton/meter2 = 1 Pascal (Pa)
1 standard atmosphere = 101,325 Pa
1 standard atmosphere = 1 atm =
760 mm Hg = 760 Torr
What’s a Newton?

5. Which diagram, a or b, shows a low pressure day?
Simple Manometer

6. Volume of a gas decreases as pressure increases
Boyle’s Law
P vs V
Volume of a gas decreases as pressure increases
at constant temperature

7. P vs V
V vs 1/P
BOYLE’S LAW DATA
Why do we care about making the graph a straight line?

8. k = a proportionality constant
Boyle’s Law
(Pressure)( Volume) = k
P V = k
P1V1 = P2V2
V  1/P
k = a proportionality constant
as long as T is held constant and no additional gas is added or removed

9. Boyle’s Law Calculations
In an automobile engine, the initial cylinder volume is L. After the piston moves up, the volume is L. The initial pressure is 1.00 atm, what is the final pressure?
P1 = 1.00 atm
P2 = ?
V1 = L
V2 = L
V1P1 = V2P2
P2 = V1P1/V2
P2 = (0.725 L)(1.00 atm)/(0.075 L)
P2 = 9.7 atm
Is this answer reasonable?

10. Volume of a gas increases as heat is added
Charles’s Law
T vs V
Volume of a gas increases as heat is added
when pressure is held constant.

11. k = a proportionality constant
Charles’s Law
(Volume) = k (Temperature)
V = k T
V1 = V2
T T2
V  T
T must be in Kelvin
k = a proportionality constant
as long as P is held constant and no additional gas is added or removed
WHY?

12. Charles’s Law Calculations
Consider a gas with a a volume of L at 35 oC and 1 atm pressure. What is the temperature (in oC) of the gas when its volume is L at 1 atm pressure?
V1 = L
V2 = L
T1 = 35 oC = 308 K
T2 = ?
V1/V2 = T1/T2
T2 = (308 K)(0.535 L)/(0.675 L)
T2 = 244 K - 273
T2 = - 29 oC

13. At constant temperature and pressure, increasing
Avogardro’s Law
n vs V
At constant temperature and pressure, increasing
the moles of a gas increases its volume.

14. k = proportionality constant
Avogadro’s Law
For a gas at constant temperature and pressure, the volume is directly proportional to the number of moles of gas.
V = k n
V  n
k = proportionality constant

15. AVOGADRO’S LAW 3 O2(g)  2 O3(g) V1 = 12.2 L V2 = ? n1 = 0.50 mol
A 12.2 L sample containing 0.50 mol of oxygen gas, O2, at a pressure of 1.00 atm and a temperature of 25 oC is converted to ozone, O3, at the same temperature and pressure, what will be the volume of the ozone?
3 O2(g)  2 O3(g)
V1 = 12.2 L
V2 = ?
n1 = 0.50 mol
n2 = 0.33 mol
V1/V2 = n1/n2
V2 = (12.2 L)(0.33 mol)/(0.50 mol)
V2 = 8.1 L
What would happen if the volume was fixed?

16. Pressure exerted by a gas increases as temperature
Gay-Lussac’s Law
P vs T
Pressure exerted by a gas increases as temperature
increases provided volume remains constant.

17. k = a proportionality constant
Gay-Lussac’s Law
The pressure of a gas is directly proportional to temperature, if the volume is held constant.
P = k T
P  T
k = a proportionality constant
as long as V is held constant and no additional gas is added or removed

18. Ideal Gas Law P V = n R T R = proportionality constant
= L atm  mol
P = pressure in atm
V = volume in liters
n = moles
T = temperature in Kelvins

19. Ideal Gas Law Calculations
A 1.5 mol sample of radon gas has a volume of 21.0 L at 33 oC. What is the pressure exerted by the gas?
P = ?
V = 21.0 L
n = 1.5 mol
T = 33 oC + 273
T = 306 K
R = L atm/mol·K
PV = nRT
P = nRT/V
P = (1.5)(0.0821)(306)
(21.0)
P = 1.8 atm

20. Ideal Gas Law Calculations
A sample of hydrogen gas, H2, has a volume of 8.56 L at a 0 oC and 1.5 atm. Calculate the number of moles of hydrogen present in this sample.
P = 1.5 atm
V = 8.56 L
R = Latm/molK
n = ?
T = O oC + 273
T = 273K
PV = nRT
n = pV/RT
n = (1.5)(8.56)
(0.0821)(273)
n = 0.57 mol

21. Standard Temperature and Pressure
“STP”
P = 1 atmosphere = kPa
= 760 torr = 760 mm Hg
T = C or 273 K
The molar volume of an ideal gas is approximately 22.4 liters at STP

22. as long as n is held constant, no additional gas is added or removed
COMBINED GAS LAW
as long as n is held constant, no additional gas is added or removed

23. IDEAL GAS 1. Molecules are infinitely small--zero molecular volume.
2. Zero attractive forces exist among the molecules.
3. Collisions between molecules are perfectly elastic (no energy loss).
What real gases would behave most like an ideal gas?

24. REAL GAS
1.The volume of the molecule is small compared to the distance between molecules.
2. Very small attractive forces exist between molecules.
What properties of a gas molecule would cause it to deviate most from an ideal gas?

25. Real Gases
Must correct ideal gas behavior when at high pressure (smaller volume) and low temperature (attractive forces become important).

26. Real Gas Equation   corrected pressure corrected volume Pideal
Videal
Luckily, this equation will never be used.