6.5 - Ideal Gases

Question 1

What is absolute zero and what is it defined as?

Answer 1

• -273°C (or 0K)
• The temperature at which the molecules in a substance have 0 kinetic energy

Question 2

Has absolute zero been achieved here?

Answer 2

No, at the moment it is purely theoretical as the amount of work necessary to remove heat from a substance increases the colder you try to go.
Reaching it is purely theoretical.

Question 3

What are the assumptions of an ideal gas?

Answer 3

The volume of each molecule is considered to be negligible.

Question 4

What is an ideal gas?

Answer 4

A gas that follows the gas laws perfectly - the only interactions happening in a fixed container of an ideal gas are perfectly elastic collisions of the molecules.

Question 5

What is Boyle’s law?

Answer 5

P ∝ 1/V
(if temp of ideal gas is constant)

Question 6

What is the relationship between the pressure and volume for a fixed mass of gas at constant temp? (equation)

Answer 6

P₁V₁ = P₂V₂

initial pressure (Pa) and volume (m³) final pressure and volume

Question 7

What does Boyle’s law look like as a graph?

Answer 7

Reciprocal function in the positive axis
y = 1/x

Question 8

What happens to a Boyle’s Law graph if the temperature increases?

Answer 8

The graph becomes further from the origin.

Question 9

What is Charle’s Law?

Answer 9

V ∝ T
(if the pressure of an ideal gas is constant)

Question 10

What is the relationship between the volume and temperature for a fixed mass of gas at constant pressure? (equation)

Answer 10

V₁/T₁ = v₂/T₂

initial volume (m³) / initial temp (K) = final volume / final temp

Question 11

What does Charles’s Law graph look like (in °C)?

Answer 11

Positive straight line with an x intercept at -273°C.

Question 12

What is the relationship between pressure and temp?

Answer 12

If the volume is constant, P ∝ T.
P₁/T₁ = P₂/T₂

Question 13

What is an equation that shows the relationships between pressure, volume and temp?

Answer 13

pV ∝ T

Question 14

What is pressure defined as?

Answer 14

The frequency of collisions of the gas molecules per unit area of a container.

Question 15

If the temperature of a gas is increased, how does this effect the molecules in a fixed container and what does it conclude (assuming this is an ideal gas)? (long worded question - 5 points)

Answer 15

• The the gas gets hotter, the molecules move faster
• The molecules collide with the surface of the walls more frequently
• Each collision applies a force across the surface area of the walls
• The faster the molecules hit the walls, the greater force on them (F= Δmv)
• Therefore pressure also increases

Question 16

If the volume of a container with gas decreases and T stays constant, how is pressure effected? (Consider Boyle’s law)

Answer 16

• Smaller surface area = more collisions
  = Therefore creating more pressure

Question 17

What are the 2 ways of writing the ideal gas equation?

Answer 17

pV = nRT
p = pressure
V = volume
n = number of moles (mol)
R = Molar gas constant = 8.31Jkg⁻¹mol⁻¹

or
pV = NkT
N = number of molecules
k = Boltzmann constant = 1.38 x 10⁻²³JK⁻¹
T = temp (K)

Question 18

What are the 4 rules for a gas to be an ideal gas?

Answer 18

• Collisions which are elastic
• Can’t be liquified
• Has no interactions between the molecules (except during collisions)
• Obeys the ideal gas laws (Boyle’s, Charles’, Pressure Laws)

Question 19

What is a mole?

Answer 19

A unit used to measure the amount of substance.
( 1 mole = 12g of carbon-12 = 6.02x10²³ atoms)

Question 20

What is the work done when a volume of gas changes at constant pressure defined as?

Answer 20

W = pΔV
W = work done (J)
p = external pressure
V = volume of gas

Question 21

What is an example of ‘work done by a volume of gas’?

Answer 21

In a steam engine where expanding steam (the volume of gas) pushes a piston to turn the engine.

Question 22

What is Avogadro’s Constant and what is it defined as?

Answer 22

6.02 x 10²³
The number of atoms of carbon-12 in 12g of carbon-12.

Question 23

What is one mole of any element equal to?

Answer 23

The relative atomic mass (in grams).

Question 24

What is the molar mass?

Answer 24

The mass of a substance in grams, in 1 mole
(unit = g mol⁻¹)

Question 25

What is the equation to find the number of moles?

Answer 25

Number of moles = mass (g) / molar mass (g mol⁻¹)

Question 26

What is the Boltzmann constant?

Answer 26

k = R / Na = 1.38 x 10⁻²³ JK⁻¹
R = molar gas constant (8.31)
Na = Avagadro’s constant

Question 27

Why is the Boltzmann constant so small?

Answer 27

The increase in kinetic energy of a molecule is very small for every incremental increase in temperature.

Question 28

What is the definition of the Boltzmann Constant?

Answer 28

The relation of the average kinetic energy of gas particles and the temperature of the gas.

Question 29

What is the difference between molar and molecular mass?

Answer 29

Molar mass - the mass of the mole of a substance (g/mol)
molecular mass - the mass of a single molecule and is expressed in atomic mass units

Question 30

What does adiabatic mean?

Answer 30

No heat transfer in the system and Energy is only transferred as work done