CHAPTER 18

CHAPTER 18

GASES

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


• 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


• 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


• 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.


• Standard Pressure - 101.325 kPa

• Standard Temp. - 0 oC

• STP - Standard temp. & press.

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

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.

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

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.

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

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


• 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 present @ the same temp.– Press. exerted by an indiv. gas in a mixture is

its Partial Pressure.


• Air contains ~ 78% nitrogen

78% of press. is due to nitrogen– partial press. of N in air @ std. conditions is

78% x 101.325 = 79 kPa


• 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.

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 vol. @ 0 oC.• Examples?

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

CHARLES’ LAW

• CHARLES’ LAW - The vol. of a quantity of gas @ 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 273.16 K

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

T1 T2

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

T1 T2

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.


• Diffusion - random scattering of gas molecs.– as gas molecs. diffuse, they become more

evenly distributed throughout the room or container


• All gases do not diffuse @ 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.


• @ the same temp:

V1 = M2

V2 M1

relative rates of diffusion of 2 gases vary inversely w/ the square root of their molecular masses


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

Gas Density

• Usually expressed in g/dm3

• May calculate density of a gas @ any temp. & press.– A decr. in temp. will decr. vol. & incr. density

– D2 = D1 x T1 x P2

T2 P1

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


• @ 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.


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.


• For most common gases, ideal gas laws are accurate to 1% @ normal lab temps. & press.

assume these gases have ideal gas properties

• He approaches ideal behavior closer than any other


• 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.


• 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.

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CHAPTER 18