Properties of Gases• Kinetic-Molecular Theory• Based on particle motion

1. Gas particles are tiny compared to the great distances between them

◦ O2 molecules travel 20,000 times diameter between collisions (6’ diameter – 22.7 miles)

2. Travel constantly, at extremely high velocity in all directions at many different speeds

◦ Collisions constantly change their speed and direction

Properties of Gases3. Particles do not interact with

each other or the walls of their container except during momentary collisions.

◦ Chemical, gravitational, electrical forces can be ignored. Many collisions - 5,000,000,000/second at 0C.

4. Collisions between particles are elastic

◦ Net kinetic energy unchanged5. Average kinetic energy

directly proportional to temp of gas in K

◦ As temp rises, particles move faster, collide more

Physical Properties of Gases Diffusion – gases fill

entire volume of container over time

Effusion – gas particles pass thru tiny opening into an evacuated space

◦ Graham’s law of effusion

◦ R = 1√Molar mass

Physical Properties of Gases Permeability

◦ Gas can pass thru another porous substance

Why balloons go flat and your lungs work

Fluidity◦ Can flow and take shape

of container Why there is wind

Compressibility and expansibility

Physical Properties of Gases Pressure – caused by the

force of billions of collisions on a boundary

◦ No boundary – no pressure◦ There is gas in space – but

there is no boundary Measure – force per unit

area◦ Inside a balloon – many

billions of collisions per second

◦ Pressure always perpendicular to the surface

Physical Properties of Gases Measures of pressure

◦ PSI – force (lbs) per unit area (in2)

◦ Millimeters of mercury or torr

How high atmospheric pressure causes mercury to rise in a column

◦ Atmosphere – 1 Atm = normal atmospheric pressure at sea level at latitude 45

2.0= double, 0.5 = ½

Physical Properties of Gases Measures of pressure

◦ SI unit – Pascal Force of one Newton acting on one

square meter Conversion factors 1atm 1atm 760 torr 1 atm

760 torr 14.7 psi 14.7psi 101,325 Pa

Physical Properties of Gases Pressure, Volume and

Temperature◦ Pressure up, volume down

(inverse) temp same◦ Temp up, volume up—down,

volume down◦ Volume same, temp increase –

pressure increase Temp down – pressure down PRESSURE

Gas Laws and Formulas Boyle’s Law – Pressure and

Volumes◦ The volume of a gas is inversely

realted to the pressure if the temperature is held constant.

PV = k P = pressure, V = volume, k = a constant

So, P1V1 = k = P2V2 For any value of P, V will relate such that k

remains constant and vice versa

Gas Laws and Formulas Charles’ Law– Temperature

and Volumes◦ When the pressure on a sample

of a dry gas is held constant, the Kelvin temperature and the volume are directly related.

V = k T T = Temperature V = volume, k = a

constant So, V1 = k = V2

◦ T1 T2

◦ As V increases or decreases, T must do the same so that k will remain constant

Gas Laws and Formulas Gay-Lussac’s Law Temperature and

Pressures◦ Pressure is directly proportional to

temperature in kelvins for a fixed mass of gas held in a constant volume.

P = k T T = Temperature P = Pressure, k = a constant

So, P1 = k = P2

◦ T1 T2

◦ As P increases or decreases, T must do the same so that k will remain constant

Gas Laws and Formulas Combined Gas Law

◦ Boyle’s, Charles, and Gay-Lussac’s require something to be constant.

Boyle’s – temperature Charles – pressure Gay-Lussac – mass and volume

◦ What to do when things aren’t constant?

Combined gas law Combination equation taking into account all

three variables PV = k T

Gas Laws and Formulas Dalton’s Law of Partial Pressures

◦ Gases are seldom pure, there are always contaminants

◦ The total pressure of a mixture of gases equals the sum of the partial pressures

Ptotal = P1+P2+P3+P…….. Gas collection over water introduces water vapor Pressure will equal gas collected plus vapor

pressure of water at temperature collected Since it is collected over water and displaces the

water under 1atm, total pressure is always atmospheric pressure at your location (see barometer)