Transcript Slide 1
Chapter 12
Physical Characteristics of Gases
Kinetic Molecular Theory
Particles of matter are ALWAYS in motion Volume of individual particles is
zero. Collisions of particles with container walls cause pressure exerted by gas. Particles exert no forces on each other. Average kinetic energy
temperature of a gas. Kelvin
The Meaning of Temperature
(KE) avg 3
RT
2
Kelvin temperature is an index of the random motions of gas particles (higher Temp means greater motion & greater ENERGY .)
Kinetic Energy of Gas Particles At the same conditions of temperature, all gases have the same average kinetic energy.
KE
1 2
mv
2
The Nature of Gases Gases expand to fill their containers Gases are fluid – they flow
Gases have low density 1/1000 the density of the equivalent liquid or solid Gases are compressible Gases effuse and diffuse
Diffusion
Diffusion The rate : describes the mixing of gases. of diffusion is the rate of gas mixing.
Effusion Effusion : describes the movement of gas from an area of HIGHER gas concentration to an area of LOWER gas concentration.
Pressure
Is caused by the collisions of molecules with the walls of a container is equal to force/unit area SI units = Newton/meter 2 = 1 Pascal (Pa) Atmospheres (atm) is the unit most commonly used in Chemistry.
1 atm = 760 mm Hg = 760 torr = 101,325 Pa = 101.325 kPa
Ideal Gases vs Real Gases Ideal gases are imaginary gases that perfectly fit all of the assumptions of the kinetic molecular theory.
Ideal: Gases consist of tiny particles that are far apart relative to their size.
Real: Same!
Collisions between gas particles and between particles and the walls of the container are elastic collisions No kinetic energy is lost in elastic collisions Real: No perfectly elastic collisions. Some energy is lost or gained in every collision.
Ideal vs. Real Gases (continued) Gas particles are in constant, rapid, random motion. They therefore possess kinetic energy, the energy of motion Real: Yes EXCEPT when gas gets VERY cold.
At Absolute zero there is no molecular motion.
Absolute zero = O Kelvein, - 273 o C, or -459 o F
There are no forces of attraction between gas particles Real: NO!
All gases are made of protons and electrons which attract each other!
Polar Gases (H2O & NH3 can have strong attractions between molecules.
Ideal vs. Real Gases (continued) The kinetic energy of gas particles depends on temperature, not on the identity of the particle.
Real: NO. temperature.
AVERAGE kinetic energy af ALL of the gas particles depends on the temp. Can not determine the K. E. of each particle based on
Measuring Pressure The first device for measuring atmospheric pressure was developed by Evangelista Torricelli during the 17 th century.
The device was called a
“
barometer
”
Baro Meter = weight = measure
An Early Barometer The normal pressure due to the atmosphere at sea level can support a column of mercury that is 760 mm high.
The Aneroid Barometer
The Digital Barometer
Standard Temperature and Pressure
“
STP
”
P = 1 atmosphere, 760 torr T =
0 °
C, 273 Kelvins The molar volume of an ideal gas is 22.42 liters at STP
Robert Boyle ( 1627-1691)
o
Boyle was born into an aristocratic Irish family
o
Became interested in medicine and the new science of Galileo and studied chemistry.
o
A founder and an influential fellow of the Royal Society of London
o
Wrote prolifically on science, philosophy, and theology.
Boyle
’
s Law
* Pressure is inversely proportional to volume when temperature is held constant. Pressure
´
P
1
V
1 = P 2
V
2 Volume = Constant (k) (T = constant)
A Graph of Boyle
’
s Law
Why Don
’
t I Get a Constant Value for PV = k?
1. Air is not made of ideal gases 2. Real gases deviate from ideal behavior at high pressure
Jaques Charles (1746-1823)
French Physicist
Conducted the first scientific balloon flight in 1783
Charles
’
s Law The volume of a gas is directly proportional to temperature, and extrapolates to zero at zero Kelvin. (P = constant)
V
1
T
1
V
2
T
2
(
P
constant)
Converting Celsius to Kelvin Gas law problems involving temperature require that the temperature be in KELVINS!
Kelvins =
C + 273
°
C = Kelvins - 273
Joseph Louis Gay-Lussac 1778 - 1850
physicist
French chemist and Known for his studies on the physical properties of gases. In 1804 he made balloon ascensions to study magnetic forces and to observe the composition and temperature of the air at different altitudes.
Gay Lussac
’
s Law The pressure and temperature of a gas are directly related, provided that the volume remains constant.
P
1
T
1
P T
2 2
The Combined Gas Law The combined gas law expresses the relationship between pressure, volume and temperature of a fixed amount of gas.
P
1
V
1
T
1
P
2
V
2
T
2
Dalton
’
s Law of Partial Pressures For a mixture of gases in a container,
P
Total = P 1 + P 2 + P 3 + . . .
This is particularly useful in calculating the pressure of gases collected over water.
Standard Molar Volume
Equal volumes of all gases at the same temperature and pressure contain the same number of
molecules. Amedeo Avogadro
Standard Molar Volume
Remember the mole triangle??
Ideal Gas Law
P V
= n
R T
P = pressure in atm V = volume in liters T = temperature in Kelvins n = moles R = a constant = 0.0821 L atm/ mol·
K
Holds closely at P < 1 atm
What is R?
P = atm
If P V
= n
R T
and V = liters n = moles T = Kelvins
R
=
P V n T SO R
=
atm x L mol x K