01/24 Ideal Gases vs Real Gases

Question 1

what are the 2 assumptions of an Ideal gas

Answer 1

1.) we assume that the molecules are point particles that have no size and do not take up space

2.) we assume that there are no interactions between the molecules (attractive or repulsive forces)

Question 2

what are the two assumptions of a real gas

Answer 2

1.) the molecules have a definite size and take up space
2.) there are interactions between molecules (attraction + repulsion)

Question 3

what are the conditions for an ideal gas

Answer 3

1.) high temperatures
2.) low densities
- low density means small # of moles or a large volume for the molecules to move around in

Question 4

what are the conditions for a real gas

Answer 4

1.) low temperatures
2.) high densities, the molecules have less room to move around and interact more

Question 5

why can we neglect the size contribution of the gas molecules in a low density box?

Answer 5

there is a small number of moles relative to the size of the box, therefore the molecules do not take up much space relative to the size of the total volume and we can neglect their size and treat them as point particles

Question 6

at high temperatures, why do we assume it is an ideal gas and ignore the interactions between molecules

Answer 6

at high temperatures, the kinetic energy increases substantially relative to the potential energy

The potential energy is related to the interactions between the molecules

we can ignore the contribution of the potential energy (ignore the interactions) and say that Etotal= KE

Question 7

at low temperatures, why do we assume it is a real gas and cannot ignore the interactions between molecules

Answer 7

at low temperatures, both the KE and PE are low and must both be accounted for

this means that the interactions between the molecules (which effect the PE) matter more in their contribution to the total energy

Question 8

what does the modification V-nb mean?

Answer 8

it tells us the actual amount of free volume available

V is the volume of the container
n is the number of moles of gas
b is the parameter that accounts for the size of the molecules (species dependent)

Question 9

what does the modification an^2/ V^2 mean, why is it subtracted

Answer 9

a accounts for the attraction between the molecules, it is subtracted because the attraction brings the molecules in closer and lowers the pressure

Question 10

between methane and ethane, which would have a larger b

Answer 10

b accounts for the gas’s size, ethane would have a larger b

Question 11

between methane and water, which would have a larger a

Answer 11

a accounts for attraction, since water is polar it would have a larger attraction and a parameter

Question 12

if we get the same value when using both van der waals and the ideal gas law, what does it mean

Answer 12

it means the gas was probably behaving as if it was ideal

Question 13

if the pressure of the real gas is lower than the pressure of the actual gas, what is true about the attractive forces

Answer 13

the attractive forces must be significant