Chapter 12Chapter 12Physical Physical

Characteristics of Characteristics of GasesGases

Kinetic Molecular Kinetic Molecular TheoryTheory

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 Kelvin temperature of a gas.

The Meaning of TemperatureThe Meaning of Temperature

 Kelvin temperature is an index of the random motions of gas particles (higher Temp means greater motion & greater ENERGY.)

(KE)32avg RT

Kinetic Energy of Gas Kinetic Energy of Gas ParticlesParticles

At the same conditions of temperature, all gases have the same average kinetic energy.

2

2

1mvKE

The Nature of GasesThe 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: describes the mixing of gases. The rate of diffusion is the rate of gas mixing.

DiffusionDiffusion

EffusionEffusionEffusion: describes the movement of gas from an area of HIGHER gas concentration to an area of LOWER gas concentration.

PressurePressure  Is caused by the collisions of molecules

with the walls of a container  is equal to force/unit area  SI units = Newton/meter2 = 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 Ideal Gases vs Real Real GasesGases

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.

  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: Same!

  Real: No perfectly elastic collisions. Some

energy is lost or gained in every collision.

Ideal vs. Ideal vs. RealReal Gases Gases (continued)

  Gas particles are in constant, rapid, random motion. They therefore possess kinetic energy, the energy of motion

  There are no forces of attraction between gas particles

  Real: Yes EXCEPT when gas gets VERY cold. At Absolute zero there is no molecular motion. Absolute zero = O Kelvein, - 273 oC, or -459 oF

  Real: NO! All gases are made of protons and

electrons which attract each other!

Polar Gases (H2O & NH3 can have strong attractions between molecules.

  The kinetic energy of gas particles depends on temperature, not on the identity of the particle.

  Real: NO. AVERAGE kinetic energy af ALL of the gas particles depends on the temp. Can not determine the K. E. of each particle based on temperature.

Ideal vs. Ideal vs. RealReal Gases Gases (continued)

Measuring Measuring PressurePressure

The first device for measuring atmospheric pressure was developed by Evangelista Torricelli during the 17th century.The device was called a “barometer”

  Baro = weight   Meter = measure

An Early An Early BarometerBarometer

The normal pressure due to the atmosphere at sea level can support a column of mercury that is 760 mm high.

The Aneroid BarometerThe Aneroid Barometer

The Digital BarometerThe Digital Barometer

Standard Temperature and Standard Temperature and PressurePressure

““STPSTP””

P = 1 atmosphere, 760 torr T = C, 273 Kelvins The molar volume of an ideal

gas is 22.42 liters at STP

Robert BoyleRobert Boyle((1627-1691)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.

BoyleBoyle’’s Laws Law**

Pressure Volume = Constant (k) P1V1 = P2V2 (T = constant)

Pressure is inversely proportional to volume when temperature is held constant.

A Graph of BoyleA Graph of Boyle’’s s LawLaw

Why Don’t I Get a Constant Value for PV = k?

1. Air is not made

of ideal gases2. Real gases deviate from ideal behavior at high pressure

Jaques Charles (1746-1823Jaques Charles (1746-1823)

French Physicist Conducted the

first scientific balloon flight in 1783

CharlesCharles’’s Laws Law

 The volume of a gas is directly proportional to temperature, and extrapolates to zero at zero Kelvin.

(P = constant)

VT

VT

P1

1

2

2 ( constant)

Converting Celsius to KelvinConverting Celsius to Kelvin

Gas law problems involving temperature require that the temperature be in KELVINS!

Kelvins = C + 273

°C = Kelvins - 273

Joseph Louis Gay-LussacJoseph Louis Gay-Lussac1778 - 18501778 - 1850

French chemist and physicist 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 LussacGay Lussac’’s Laws Law

The pressure and temperature of a gas are directly related, provided that the volume remains constant.

2

2

1

1

T

P

T

P

The Combined Gas LawThe Combined Gas Law

The combined gas law expresses the relationship between pressure, volume and temperature of a fixed amount of gas.

2

22

1

11

T

VP

T

VP

DaltonDalton’’s Law of Partial Pressuress Law of Partial Pressures

For a mixture of gases in a container,

PTotal = P1 + P2 + P3 + . . . This is particularly useful in calculating the pressure of gases collected over water.

Standard Molar Standard Molar VolumeVolume

Equal volumes of all gases at the same temperature and pressure contain the same number of molecules.

- Amedeo Avogadro

Standard Molar VolumeStandard Molar Volume

Remember the mole triangle??

Ideal Gas LawIdeal Gas Law

PV = nRT  P = pressure in atm

•  V = volume in liters  T = temperature in Kelvins  

n = moles •  R = a constant

= 0.0821 L atm/ mol·

Holds closely at P < 1 atm

What is R?

If PV = nRT and

P = atm V = liters   n = moles T = Kelvins

R = PV SO R = atm x L

nT mol x K

End of Slide Show