Gases and the Kinetic-Molecular Theory
Kinetic Molecular Theory of Gases
1) Gases are made up of tiny molecules.
2) Gas molecules are always in constant motion.
3) The forces of attraction between gas molecules
are negligible.
4) Gas molecules undergo elastic collisions.
5) The average kinetic energy of gas molecules is
proportional to the Kelvin temperature of the
gas.
An Overview of the Physical States of Matter
The Distinction of Gases from Liquids and Solids
1. Gas volume changes greatly with pressure.
2. Gas volume changes greatly with temperature.
3. Gases have relatively low viscosity.
4. Most gases have relatively low densities
under normal conditions.
5. Gases are miscible.
The three states of matter.
Gases can be measured and described with four
properties or variables.
A)
B)
C)
D)
Pressure (P)
Volume (V)
Temperature (T)
Amount of Gas (n)
Gases at STP (Standard Temperature Pressure) is
equivalent to 22.4 L at 1 atm and 273.15K.
Volume = Liter (L)
Temperature = Kelvin (K)
K = OC + 273.15
Amount of Gas (n)
n= given mass/molar mass of gas
Pressure is defined as the force pushing over a
certain area.
FORCE
A
R
E
A
P = Force / Area
Effect of atmospheric pressure on objects
at the Earth’s surface.
A mercury barometer.
Two types of
manometer
closed-end
open-end
Common Units of Pressure
Unit
Atmospheric Pressure
Pascal (Pa);
1.01325 x 105 Pa;
kilopascal (kPa)
101.325 kPa
atmosphere(atm)
millimeters of
mercury(Hg)
torr
pounds per square
inch (psi or lb/in2)
bar
1 atm*
760 mm Hg*
760 torr*
14.7 psi or lb/in2
1.01325 bar
Scientific Field
SI unit; physics, chemistry
chemistry
chemistry, medicine, biology
chemistry
engineering
meteorology, chemistry,
physics
*This is an exact quantity; in calculations, we use as many significant figures
as necessary.
A container of gas measures the pressure to
be 104.9 kPa. Convert this pressure to psi.
Solution: Use dimensional analysis
104.9 kPa x 14.7 psi
1
101.3 kPa
= 15.2 psi
The safety disk in a scuba tank will blow at
a pressure of approximately 25000 kPa.
Convert this pressure to mm Hg.
25 000 kPa x 760 mm Hg = 187515.42 mm Hg
101.325 kPa
= 187515 mm Hg
Converting Units of Pressure
PROBLEM: A geochemist heats a limestone (CaCO3) sample and
collects the CO2 released in an evacuated flask attached to
a closed-end manometer. After the system comes to room
temperature, Dh = 291.4 mm Hg. Calculate the CO2
pressure in torrs, atmospheres, and kilopascals.
PLAN: Construct conversion factors to find the other units of
pressure.
SOLUTION: 291.4 mmHg
760 torr
= 291.4 torr
760 mmHg
291.4 torr
1 atm
760 torr
0.3834 atm
= 0.3834 atm
101.325 kPa
1 atm
= 38.85 kPa
Exercises
1) A container of gas measures the pressure to be
104.9 kPa. Convert this pressure to psi.
2) Atmospheric pressure is reported as 30.1 inHg.
Convert this to torr.
3) Many pneumatic tools operate at an air
pressure of 90 psi. What is the equivalent
pressure in kilopascals (kPa)?