Lecture 2: Properties of gases

Question 1

Why do we need to know gas properties?

Answer 1

• Aerosol consists of two phases;
• • the solid or liquid particles
• • the gas, in which the particles are suspended
• The gas and the particle interactions;
• • particle motion -> particle behavior

Question 2

Kinetic Theory of Gases

Answer 2

(assumptions)

• gases contain a large number of molecules
• the molecules are small compared with the distances between them
• the molecules are rigid spheres traveling in straight lines between elastic collisions

Question 3

Primary measurements of Kinetic Theory of Gases

Answer 3

1. Temperature (T): measures the kinetic energy of the gas molecules
2. Pressure (P): comes from the force of molecules impacts on the container walls
3. Viscosity (n (eta)): represents the transfer of momentum by molecular motion.
4. Diffusivity (D): the transfer of molecular mass

kinetic theory describes these properties in terms of the properties of the molecules (number per unit volume n, mass m, diameter dm, and velocity c.)

Question 4

Definitions of IDEAL GASES

Answer 4

Macroscopic:
– gases that obey this law under all conditions are ideal gases
Microscopic:
– a gas consists of particles, which are gas molecules
– the molecules are in random motion and obey Newton’s Law of Motion
– the volume of molecules in negligibly small
– no appreciable forces act on the molecules except during the collision

(almost all gases in indoor environment can be considered ideal)

Question 5

What is the Ideal Gas Law?

Answer 5

A result of two experiments;

1. Boyle’s Law: For a given mass of gas @ constant T, the pressure is inversely proportional to the volume
2. Charles’s Law: for a given mass of gas @ constant P, the volume is directly proportional to the temperature

PV=nRT

Question 6

Describe Molecular Arithmetic Mean Speed

Answer 6

Crms molecular speed

• Mostly affected by temperature T and molecular weight MW
• one type of average molecule speed
• related to the kinetic energy KE of the gas
• (3 dimensional) Arithmetic mean speed: dividing the sum of all the molecular velocities at a given instant by the total number of molecules

Question 7

Define Mean Free Path

Answer 7

the average distance traveled by a molecule between successive collisions

Question 8

Viscosity of a gas

Answer 8

Macroscopic description:

• shear stress and velocity gradient
• Newton’s Law of Viscosity

Microscopic description:

• viscosity of a gas represents a transfer of molecular momentum from a faster moving layer to a slower moving layer
• this transfer is accomplished by the random thermal motion of molecules traveling between the layers

(depends ONLY on temperature)

Question 9

Define Diffusion

Answer 9

• the transfer of mass of one gas through another in the absence of fluid flow
• the result of the motion of the gas molecules in a concentration gradient (always down gradient)
• Described by Fick’s Second Law of Diffusion

Question 10

Properties of the Air @ Standard Conditions: 20C, 1 atm (293K, 101 kPa)

Answer 10

density: 1.2 kg/m^3 (ambient air)
viscosity: 1.8110^-5 Pa.s (N.s/m^2)
diffusivity: 2.010^-5 m^2/s
mean free path: 0.066 mm
molecular weight: 28.9 g/mole