1. Plan for Mon, 6 Oct 08
New schedule and practice problems for Ch 5, 6, 10
Exp 1 post-lab
Typo in Exp 2 pre-lab
Lecture
Pressure (5.1)
Gas laws of Boyle, Charles, and Avogadro (5.2)
The ideal gas law ( 5.3)
Exam 1 returned

2. Pressure Pressure is force per unit area:
In SI units, pressure is expressed in newtons per square meter, N/m2, the pascal (Pa).
1 Pa = 1 N/m2

3. Pressure Example
What is the pressure that one penny exerts on a table? The penny has a radius of 9.3 mm, and weighs 2.5 g.
We need to find the force that the penny exerts, and the area over which it exerts that force.
Force due to the penny:
F = (mass) x (accel. due to gravity)
= (2.5 x 10-3 kg) x (9.81 m/s2)
= 2.5 x 10-2 kg m/s2
= 2.5 x 10-2 N

4. Pressure Example (cont)
Cross-sectional area of the penny:
A = p(radius)2
= p(9.3 x 10-3 m)2
= 2.7 x 10-4 m2
Therefore the pressure due to the penny is:
The pascal is actually a pretty puny unit, so more often the kilopascal (kPa) is used.

5. How do we measure the force of a gas?

6. Barometer Closed Open to atmosphere Vacuum 1 atm 1 atm 1 atm 1 atm
Petrucci pg. 177

7. Making a Mercury Barometer
A dish is filled with mercury.
A long glass tube – closed at one end – is also filled with mercury.

8. Making a Mercury Barometer
Air bubbles are removed from the mercury in the tube.
Mercury is added until the tube is completely filled.

9. Making a Mercury Barometer
Vacuum
The tube is carefully upended and placed in the dish of mercury, so that none of the Hg in the tube falls out.
Some portion of the mercury will then flow out of the tube, until the pressure of the Hg is equal to the atmospheric pressure on the Hg in the dish.

10. Making a Mercury Barometer
Measuring the height of the Hg column will tell you what the atmospheric pressure is.
Units: “mm Hg” (milimeters of mercury)
Also known as “torr” after Evangelista Torricelli, inventer of the barometer.

11. Manometers Pgas = PHg = h Pgas = Patm – h Pgas = Patm + h
Manometers operate on the same principle as barometers, but they measure the pressure of an isolated gas sample rather than the whole atmosphere.
Manometers
Pgas = PHg = h
Pgas = Patm – h
Pgas = Patm + h h h h
Note: Patm is obtained from a barometer.

12. Empirical Gas Laws
Gases are relatively easy to measure and observe in a laboratory.
This made the physical properties of gases a popular object of study in the 17th, 18th, and 19th centuries.
Boyle, Charles, and Avogadro (yes, that Avogadro) determined fundamental connections between P, V, T and n for gases.

13. Boyle’s Law (c. 1650) or… PV = constant
Boyle studied the connection between P and V of gases.
or…
PV = constant
E&G pg. 179

14. A gas for which PV = const is called “ideal.”
Boyle’s Law (cont)
A gas for which PV = const is called “ideal.”

15. Cooling down the blue balloon decreases its volume dramatically.
Charles’s Law (c. 1800)
Charles studied the connection between T and V of gases.
Cooling down the blue balloon decreases its volume dramatically.
Liquid nitrogen, T = 77 K
Blue balloon is placed in the liquid N2…

16. Same plot, except temperatures are given in Kelvin instead of Celcius.
Charles’s Law
The volume of an ideal gas is directly proportional to its temperature measured in Kelvin
V = bT
T = t
V = bt + a
Do derivation t
Vol vs. temp data for several gases. When these lines are extrapolated, they all intersect at oC.
Same plot, except temperatures are given in Kelvin instead of Celcius.

17. Avogadro’s Law
V = an
Volumes of gases that react do so in small whole number ratios: 2 vol H2 + 1 vol O2 = 2 vol H2O
Gay-Lussac found that when gases measured at the same T and P are allowed to react, the volumes of gaseous reactants and products are in small whole-number ratios.
Avogadro proposed that at the same T and P, each of the identical flasks contains the same number of molecules. Notice how the combining ratio: 2 volumes H2 to 1 volume O2 to 2 volumes H2O leads to a result in which all the atoms present initially are accounted for in the product.
Avogadro said this was so because the same volume of two different gases at the same T and P will have the same number of particles.

18. Empirical Gas Law Summary
Boyle’s Law:
Charles’s Law:
Avogadro’s Law:
Since V is directly proportional to 1/P, T, and n, V must also be directly proportional to the product:
add proportionality constant

19. Ideal Gas Law Common R values: P = pressure V = volume
n = number of moles
R = “gas constant”
T = temperature in Kelvin
CHE161
CHE162

20. What is Ideality?
Molecules of an ideal gas do not attract or repel one another
The volume of an ideal gas molecule is negligible with respect to the container
Essentially, an ideal gas is a collection of non-interacting point particles.
Is this a reasonable approximation?
Under what conditions would you expect ideality to fail?
high P  molecules get too close, start interacting
low T  same thing
Over moderate T and P ranges, gases are fairly dilute, so the ideal gas law is a reasonable approximation.
DON’T FORGET YOU MUST NEVER USE CELCIUS TEMPRATURES! KELVIN ONLY!!