Thermal 2

Question 1

What is Boyle’s Law?

Answer 1

for a fixed mass of gas, at a constant temperature, the pressure of a gas is inversely proportional to its volume

Question 2

What is the Pressure Law?

Answer 2

for a fixed mass of gas, at constant volume, the pressure is directly proportional to the absolute temperature

Question 3

What is Charles’ Law?

Answer 3

at constant pressure, the volume of a gas is directly proportional to its absolute temperature

Question 4

What is the ideal gas equation? (Given in formula booklet)

Answer 4

pV = nRT or pV = NkT

Question 5

What does “n” represent in the ideal gas equation?

Answer 5

the number of moles

Question 6

What is meant by absolute zero?

Answer 6

the temperature where an ideal gas has zero volume and exerts zero pressure, this is because molecular KE = 0

Question 7

What is meant by Brownian motion?

Answer 7

particles suspended in a medium follow a random walk

Question 8

What is meant by a random walk?

Answer 8

particles move in straight lines, of random length and in random directions

Question 9

What is Brownian motion of (for example smoke) particles due to?

Answer 9

due to collision with very small air molecules

Question 10

What does “N” represent in the ideal gas equation?

Answer 10

the number of molecules

Question 11

What is meant by the molar mass of a substance?

Answer 11

the mass of 6 x 10^23 particles, Mr

Question 12

How can you find the number of moles in the substance if you have its mass, and the molar mass?

Answer 12

n = mass of substance / molar mass

Question 13

What is meant by one mole of a substance?

Answer 13

1 mole of a substance is the quantity of that substance that contains 6 x 10 ^23 particles

Question 14

How can you find the number of molecules in a substance if you have its mass and molar mass?

Answer 14

N = (Avogadro constant) x (mass of substance)/(molar mass)

Question 15

What are the 8 ideal gas assumptions?

Answer 15

• all molecules of a gas are identical
• the gas contains a large number of molecules
• the molecules have negligible volume compared with the volume of the gas
• the molecules move in random motion (direction and speed)
• attraction between molecules is negligible
• collisions between a molecule and another molecule/wall are elastic
• the molecules move in a straight line between collisions
• the forces acting during collisions act for a short time relative to the time between collisions

Question 16

How do we use the assumption that all molecules of a gas are identical?

Answer 16

means the momentum change experienced by the molecules will be the same

Question 17

How do we use the assumption that the gas contains a large number of molecules?

Answer 17

we can apply stats, take average

Question 18

How do we use the assumption that molecules have negligible volume compared with the volume of the gas?

Answer 18

• means there is large spacing between gas molecules

- (also means the attraction between molecules in negligible as F ∝ 1/r^2)

Question 19

What is meant by molecules moving in random motion?

Answer 19

moving in a random direction and moving with random speed

Question 20

How do we use the assumption that molecules move in random motion?

Answer 20

motion in x, y, and z directions is equal (on average)

Question 21

How do we use the assumption that the attraction between molecules is negligible?

Answer 21

• PE = 0 for ideal gas molecules

- internal energy = ∑KE

Question 22

How do we use the assumption that collisions between molecules are elastic?

Answer 22

means kinetic energy is conserved

Question 23

Why is it we assume that molecules move in a straight line between collision?

Answer 23

as there is no “attractive” force, only force when molecules “collide”

Question 24

Why is it we assume that the forces acting during collisions act for a short time relative to the time between collisions?

Answer 24

as there is large spacing between molecules

Question 25

Explain Boyle’s Law (at the molecular level)

Answer 25

• the pressure of the gas increases when the volume reduces because:
• the molecules travel less distance between impacts
• so the impacts with the walls are more frequent

Question 26

Explain the pressure law (at the molecular level)

Answer 26

• the pressure of a gas increases as the temperature rises because:
• the molecules have a greater average speed
• so the impacts with the walls are harder and more frequent

Question 27

What is the relationship between the pressure a gas applies to the wall of its container, and the mean speed of the molecules?

Answer 27

the pressure a gas applies to the wall of its container is proportional to the mean square of the speed of the molecules

Question 28

Explain why it is a squared relationship between the pressure of a gas and the mean speed of the molecules?

Answer 28

• when mean speed doubles:
• 2x rate of collision
• 2x momentum exchange per collision

Question 29

What is the equation for the root mean square speed of the molecules?

Answer 29

Crms = √(((C1)^2 + (C2)^2 + …(CN)^2) / N)

Question 30

Using pV = NkT and pV = (1/3)Nm(Crms)^2 show that the mean molecular KE = (3/2)kT

Answer 30

• KE = (1/2)(m)(Crms)^2
• (2/3)(KE)(N) = (1/3)(N)(m)(Crms)^2
• (2/3)(KE)(N) = pV
• (2/3)(KE)(N) = NkT
• (2/3)(KE) = kT
• KE = (3/2)kT

Question 31

What does “m” represent in the equation pV = (1/3)Nm(Crms)^2 ?

Answer 31

the mass of 1 molecule