GassesKinetic molecular theory

Pressure

Introduction• Earth’s atmosphere is a gaseous solution

composed of mostly nitrogen and oxygen• The atmosphere is important for life on earth– Provides a medium for many chemical reactions

with waste products– Shields the Earth from harmful radiation– Retains heat on earth’s surface

• The behavior of gas particles contributes to their important properties

States of Matter• Atoms/Molecules in all states of matter have

attractive forces– Strongest in solids, weakest in gasses– Affected by changes in temperature

• Kinetic energy (KE) = energy of motion• Temperature = a measurement of kinetic energy

Solids Liquids GasesDefinite shape X

Definite volume X X

Takes the shape of its container X X

Cannot be compressed X X

Kinetic Molecular Theory• Gasses consist of tiny particles• The particles are so small (compared between the

distances between them) that the individual particles have nearly no volume

• The particles are in constant random motion, colliding with the walls of the container. These collisions cause the pressure exerted by the gas.

• The particles are assumed not to attract or repel each other.

• The average kinetic energy of the gas particles is proportional to the Kelvin temperature of the gas.

Pressure• Gas fills any container uniformly– It is evenly spread out

• Gasses exert pressure on their surroundings– Example: when you blow up a balloon, the gasses

inside push against the sides and keep it firm.• A Barometer measures atmospheric pressure– Invented by Evengelista Torricelli in 1643

• Atmospheric pressure results from the air being pulled by gravity– Varies with altitude– Changes with weather conditions

Units of Pressure• The units for pressure are based on the height of

the mercury column (in millimeters) that the gas pressure can support– mm Hg (millimeters of mercury) is also called torr

• Standard atmospheres (abbreviated atm) are also used.

1 standard atm = 1.000 atm = 760.0 mm Hg = 760.0 torr• The SI unit for pressure is the pascal (Pa)

1 atm = 101,325 Pa• Engineers use pounds per square inch (psi)

1.000 atm = 14.69 psi

Pressure Unit ConversionsConvert the following to atmospheres:

1. 109.2 kPa (1 kPa = 1000 Pa)2. 781 torr3. 15.2 mm Hg

Convert the following into units of mm Hg:4. 9.75 psi5. 121.4 kPa6. 1.14 atm

Convert the following into kilopascals (kPa)7. 105,390 Pa8. 764 mm Hg9. 1.29 atm10. 697 torr