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

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

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

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.

KINETIC THEORY OF GASES 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 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.

Dalton’s Law of 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

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.

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

Diffusion & Graham’s Law 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.

Diffusion & Graham’s Law @ the same temp: V1 = M2 V2 M1 \ relative rates of diffusion of 2 gases vary inversely w/ the square root of their molecular masses

Diffusion & Graham’s Law 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

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.

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.

Deviations of Real Gases 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

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.

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.