Chapter 10States of Matter

10.1 The nature of gases

Things you will learn

• You will understand the motion of gas particles according to the kinetic theory.

• You will be able to interpret gas pressure in terms of kinetic energy and absolute temperature.

Gases we’re used to

• O2

• CO2

• N2

• He• N2O• Ar

Most gases are molecular, not ionic;They’re from the group of non-metals

Things we might know about gases

• Lightweight• Compressible• Transparent• Fills a container completely, like liquids do• Disperses smells• Supports flames or is flammable

Kinetic theory of gases(atoms in all forms of matter are in motion)

1. A gas is composed of small hard spheres with insignificant volume separated by large empty spaces. No attractions or repulsions exist between particles.

Kinetic theory of gases(atoms in all forms of matter are in motion)

2. The particles in a gas move rapidly in constant random motion. Their short-term motion is in a straight line. Gases fill their containers regardless of shape. Uncontained gases disperse.

Kinetic theory of gases(atoms in all forms of matter are in motion)

3. All collisions are perfectly elastic. Kinetic energy is transferred between particles such that the total kinetic energy remains constant.

Kinetic theory of gases(atoms in all forms of matter are in motion)

1. Small hard particles2. In constant random motion3. Collisions elastic – EK remains constant

Gas pressure• Collisions of lots of very small particles on a

surface will exert a force on that surface• Pressure is the measure of that force• Pressure =force/area• When there are no particles present, there is

no force. This is called a vacuum.

Low pressure area on human being

Gas pressure• The units of pressure are called Pascals• This is Newtons (force) / meters2 (area)• A newton is about 102 grams of force• Pressure at sea level is 101.3 kPa

Equivalent terms of standard pressure

• Pressure at sea level is • 101.3 kPa• 1 atm• 760 mm Hg ; 29.9 in Hg• 14.7 lbs/in2

Gravity holds all the earth’s atmosphere near the surface, but the pressure of the atmosphere depends only on what is stacked above it, so pressure goes down as you go up in elevation (so air down lower is denser)

Barometers• Barometers are instruments which measure

pressure• Atmospheric pressure was originally measured

with a glass tube filled with mercury• Newer barometers use calibrated springy

metal diaphrams

Mercury barometer

Aneroid barometer

Lord Kelvin discovered that when you raise or lower the temperature of a gas by 1°C, you change its volume

slightly.

Working backwards, he was able to determine an absolute temperature

scale where the volume of a gas theoretically went to zero!!!

Kelvin temperature scale

• Kinetic theory has to do with the motion of particles

• Temperature is a measure of the average kinetic energy of a bunch of particles

• Kelvins are the same “size” as degrees centigrade• Kelvins are directly related to kinetic energy and

pressure of a gas• Kelvins are °C + 273!

Average kinetic energy

STP standard temperature and pressure

• Pressure - sea level pressure 101.3 kPa (we’ll use 100 kPa) 760 mm Hg 1 atm 14.7 lbs/in2

• Temperature – freezing point of water 273 K 0°C

Equivalent terms

Equivalent terms

A gas is at a pressure of 1.50 atm. What is its pressure in:

• kiloPascals

• millimeters of mercury

Express the pressure 545 mm mercury in :

• kiloPascals

• atmospheres

A cylinder of gas is at 85C and 60 atm. pressure. What is the pressure if the

temperature is raised to 185C?

Sim # 2