Chapter 12.1

Characteristics of Gases

1. Describe the general properties of gases.2. Define pressure, give the SI unit for

pressure, and convert between standard units of pressure.

3. Relate the kinetic-molecular theory to the properties of an ideal gas.

Objectives:

Kinetic-Molecular Theory (KMT)Based on the idea that particles of matter

are always in motion.Provides a model of an ideal gas

Imaginary gas that perfectly fits all the assumptions of the kinetic-molecular theory.

KMT is based on the following 5 assumptions:

1. Gases consist of large numbers of particles that are far apart relative to their size.o Volume of gas particles occupy a volume

usually about a thousand times greater than liquid and solid particles.

•·       

2. Collisions between gas particles and between particles and container walls are elastic collisions.o Elastic collisions are ones in which there is

no net loss of kinetic energy.

3. Gas particles are in continuous, rapid, random motion. They therefore possess kinetic energy.o Kinetic energy is the energy of motion.

4. There are no forces of attraction or repulsion between gas particles.o Think of gas particles as small billiard balls,

when they collide , they do not stick together, they bounce apart immediately.

5. The average kinetic energy of gas particles depends on the temperature.

Higher the temperature, the faster the particles move

Kinetic-molecular theory and the nature of gases

1. Expansion: Gases do not have definite shape nor volume The expand to fill container and take it’s

shape and volume

Kinetic-molecular

theoryGas particles move rapidly in all directions (assumption 3) without significant attraction or repulsion between them ( assumption 4)

2. Fluidity Ability to flow Liquids and gases are considered fluids

because they both flow

Kinetic-molecular

theory

3. Low Density Density of a gas is 1/1000th that of liquid or solids.

Kinetic-molecular

theoryParticles are so much farther apart in a gaseous state (assumption 1).

Animation

4. Compressibility Crowding particles closer together.

Kinetic-molecular

theoryGas particles are initially far apart (assumption 1), allowing the particles to be crowded together.

5. Diffusion Spontaneous mixing of the particles of two substances

caused by their random motion.

Kinetic-molecular

theoryRandom and continuous motion of the molecules (assumption 3) carries them throughout the available space.

6. Effusion Process by which gas particles pass through a tiny

opening.

Animation of Effusion

http://www.chem.iastate.edu/group/Greenbowe/sections/projectfolder/flashfiles/gaslaw/micro_effusion.html

Source:

PressureTo describe a gas fully, there are four

measurable quantitiesVolumeTemperatureNumber of moleculesPressure

Pressure and ForcePressure – defined as the force per unit

area on a surfacePressure = force

area

Use pressure to explain why it is harder to cut this pineapple with the butter knife.

How does pressure help us skate?

How do snow shoes work?

Pressure = force

area

N

cm2Units:

H0w is a magician able to survive laying on a bed of nails?

Atmospheric Pressure :

Blanket of air surrounding earth, exerting pressure

Barometer: Device used to measure atmospheric pressure.

At OoC, atmosphere can support 760 mm column of mercury.

Units of Pressure: 1 atmosphere or 1 atm

1 atm = 760 mm Hg

millimeters mercury or mm Hgtorr (in honor of EvangelistaTorricelli)Pascal, Pa - pressure exerted by a force of one Newton acting on an area of one square meter.

Conversions1 torr = 1 mm Hg1 atm = 760

torr1 atm = 1.013 25 x 105 Pa1 atm = 101.3 25 kPa

Standard Temperature and Pressure

Scientists have agreed on standard conditions of :1 atm pressure and O0C

Also known as STP

Sample Problem 1The average atmospheric pressure in Denver, Colorado, is 0.830 atm. Express this pressure (a) in mm Hg and (b) in kPa.

0.830 atm

=760 mm Hg1 atm

x 631 mm Hg

(a)

0.830 atm

=101.325 kPa1 atm

x 84.1 kPa(b)