1. CHAPTER 18
GASES

2. KINETIC THEORY OF GASES
A given amt. of gas will occupy the entire volume of its container.
Changes in temp. have a greater effect on the vol. of a gas than on a liquid or solid

3. KINETIC THEORY OF GASES
Gas particles are in constant random motion.
not held in fixed position by attractive forces
size of gas molec. is insignificant in comparison w/ the dist. betw. molecs.
\ we assume gas particles have no effect on ea. other

4. KINETIC THEORY OF GASES
Gas particles are treated as Point Masses
considered to have no vol. or diameter
Ideal Gas - imaginary gas composed of molecs. w/ mass but no vol. and no mutual attraction betw. particles

5. KINETIC THEORY OF GASES
Vol. of gas, # of gas particles, press. of gas, & temp. of gas are variables that depend on ea. other.
The # of particles in a vol. of gas depends on the press. & temp. of the gas
\ it’s necessary to give temp. & press. of gas along w/ vol. when discussing quantity of a gas.

6. KINETIC THEORY OF GASES
Standard Pressure kPa
Standard Temp. - 0 oC
STP - Standard temp. & press.

7. BOYLE’S LAW Gas press. depends on 2 factors:
1. # of molecs. per unit volume
2. Avg. kinetic energy of the molecs
- temp.
A change in either will change the press. of a gas

8. BOYLE’S LAW If the # of molecs. in a constant vol. incr., press. incr.
If # of molecs. & vol. remain constant, but K.E. of molecs. incr., press incr.
If temp. & # of molecs. remain constant, but vol. is decr., press. is incr.

9. BOYLE’S LAW What happens when volume is decr. by half?
press. doubles
same # of molecs. in 1/2 the volume
molecs. hit the wall of container twice as often & w/ same force per collision
constant temp., press. varies inversely as vol.
the product of press. & vol. is constant

10. BOYLE’S LAW
BOYLE’S LAW - If the amt. & temp. of a gas remains constant, the press. exerted by the gas varies inversely as the vol.
PV = k
k - constant - takes into account # of molecs. & temp.
Press. varies directly w/ # of molecs.

11. APPLYING BOYLE’S LAW Not all experiments can be carried out @ STP
In order to compare vols., we adjust them to standard conditions
V1P1 = V2P2
V1, P1 - original conditions
P2, V2 - new conditions

12. Dalton’s Law of Partial Pressure
Gas is often obtained by bubbling it through water
collecting gas over water or by water displacement
gases collected must be practically insoluble in water
Water vapor will be present in the gas

13. Dalton’s Law of Partial Pressure
Dalton’s Law of Partial Pressure - The total pressure in a container is the sum of the partial pressures of the gases in the container
ea. gas exerts the same press. it would if it alone were the same temp.
Press. exerted by an indiv. gas in a mixture is its Partial Pressure.

14. Dalton’s Law of Partial Pressure
Air contains ~ 78% nitrogen
\ 78% of press. is due to nitrogen
partial press. of N in std. conditions is
78% x = 79 kPa

15. Dalton’s Law of Partial Pressure
If gas is collected over water, the press. in the container = the sum of the partial press. of the gas & the water vapor
\ to find the press. of the gas alone (dry gas), subtract the water vapor press. for that temp.

16. CHARLES’ LAW Jacque Charles found a relationship betw. vol. & temp.
For ea. C o incr. in temp., the vol. of a gas is incr. by 1/273 of its 0 oC.
Examples?

17. CHARLES’ LAW Suggests that @ -273 oC (0 K) a gas will have no volume
Not true - all gases liquefy before this temp.
relationship holds true only for gases

18. CHARLES’ LAW
CHARLES’ LAW - The vol. of a quantity of constant press. varies directly w/ the kelvin temp.
experimental info led to formation of the Kelvin Scale K = oC + 273
Zero pt. of Kelvin scale is absolute zero
triple pt. of water is K

19. APPLYING CHARLES’ LAW
For a direct proportion, the quotient is constant
V/T = k
Temperature must be in Kelvin
If temp. goes up, vol. goes up
V1 = V2
T T2

20. COMBINED GAS LAW
Usually need to correct for both temp. & press. of a gas
Can do this by applying Boyle’s Law, then taking new vol. & putting it into Charles’ Law
Can also be done in one step
Temp. must be in Kelvin
P1 V1 = P2 V2
T T2

21. Diffusion & Graham’s Law
Gas molecs. travel in straight lines betw. collisions
If NH3 is opened in back of room, can soon be detected in front of room.
Molecs. travel from back to front of room in straight lines betw. collisions
collide w/ air molecs.

22. Diffusion & Graham’s Law
Diffusion - random scattering of gas molecs.
as gas molecs. diffuse, they become more evenly distributed throughout the room or container

23. Diffusion & Graham’s Law
All gases do not the same rate
rate varies w/ velocity
@ same temp. molecs. w/ lower mass diffuse faster than molecs. w/ larger mass bec. they travel faster.
They also pass thru a sm. hole - effuse - more rapidly than higher mass molecs.

24. Diffusion & Graham’s Law
@ the same temp:
V1 = M2
V M1
\ relative rates of diffusion of 2 gases vary inversely w/ the square root of their molecular masses

25. Diffusion & Graham’s Law
Graham’s Law - the relative which 2 gases under identical conditions of temp. & press. will diffuse vary inversely as the square roots of the molecular masses of the gases.

26. Gas Density Usually expressed in g/dm3
May calculate density of a any temp. & press.
A decr. in temp. will decr. vol. & incr. density
D2 = D1 x T1 x P2
T P1

27. Deviations of Real Gases
@ low press., real gases behave like ideal gases
molecs. are far apart - vol. molecs. occupy is small compared to total vol.
vol. is mostly empty space

28. Deviations of Real Gases
@ higher press., real gas molecs. are forced closer together.
molecs. begin to occupy a significant portion of total vol.
If molecs. have slowed down enough, van der Waals forces will have an effect.

29. Deviations of Real Gases
\ Assumption that there’s no attractive forces betw. gas molecs. is not always true.
If gas molecs. are polar, gas behaves significantly diff. than an ideal gas would
weak forces will cause some diff.

30. Deviations of Real Gases
For most common gases, ideal gas laws are accurate to normal lab temps. & press.
\ assume these gases have ideal gas properties
He approaches ideal behavior closer than any other

31. Deviations of Real Gases
A property of real gases which depends upon the attractive forces betw. molecs.
Joule-Thomson Effect - If a highly compressed gas is allowed to escape through a sm. opening, its temp. decr.
In order to expand, the molecs. must do work to overcome attractive forces betw. molecs.
this energy comes from their kinetic energy
\ as K.E. decr., temp. decr.

32. Deviations of Real Gases
This can be seen when spraying an aerosol can.
As product & propellant are released through nozzel, can & contents become cooler
Adiabatic System - a syst. completely insulated so no heat exchange can take place w/ surroundings.