The Properties Of Gases

Question 1

What is the ideal gas equation?

Answer 1

pV = nRT

Question 2

When does a real gas behave more like an ideal gas?

Answer 2

As its pressure is reduced towards 0, and at high temperature.

Question 3

What is the relationship between pressure and volume? (Boyle’s Law)

Answer 3

They are inversely proportional to each other at constant temperature.

Question 4

What is Charle’s Law?

Answer 4

At constant pressure, volume is proportional to temperature

Question 5

What is Dalton’s Law?

Answer 5

The pressure exerted by a mixture of ideal gases is the sum of the pressures that each gas would exert if it were alone in the container at the same temperature.

Question 6

Why is the potential energy of an ideal gas 0?

Answer 6

As the only contribution to the total energy for an ideal gas comes from the kinetics of the molecules - as molecular interactions aren’t considered.

Potential energy = attractions + repulsions

Total energy = kinetics + potential

Question 7

What is the critical point of a compound?

Answer 7

Critical point is where the substance is in 1 phase, no gas or liquid.

This is called supercritical.

Question 8

How do attractions affect potential energy?

Answer 8

Attraction forces reduce the potential energy of a gas.

Question 9

What does Pc, Vc, Tc and SCF stand for?

Answer 9

The pressure, molar volume, and temperature at the critical point.

SCF = Supercritical fluid - The dense fluid obtained by compressing a gas when its temp is higher than its critical temperature, not a true liquid as has no surface that separates from vapour phase. It is also too dense to be a gas.

Question 10

What is the difference between vapour and gas?

Answer 10

A vapour is the gaseous phase of a substance below its critical temperature which can be liquefied by compression.

A gas is the gaseous phase of a substance above its critical temperature that cannot be liquefied by compression alone.

Question 11

What is the compression factor?

Answer 11

Z = (Vm / Vm˚)

Where Vm is the molar volume of a gas, and Vm˚ is the molar volume of an IDEAL gas under the same conditions.

Question 12

What does it mean when Z (compression factor) = 1?

Answer 12

When Z = 1, the gas is ideal.

Hence, deviations from 1 are a measure of how far a real gas is from behaving ideal.

Question 13

What does it mean if Z < 1?

Answer 13

The molar volume is smaller than that of an ideal gas, and so attractive interactions are dominant.

Question 14

What does it mean if Z > 1?

Answer 14

The molar volume is greater than that expected of an ideal gas, and so repulsive forces are dominant.

Question 15

What is the viral equation of state?

Answer 15

Z = (pVm / RT) = 1 + (B/Vm) + (C/Vm2) + …

As Vm = V / n

Under most conditions, C/Vm2 «< B/Vm and so is neglected.

Question 16

When is C/Vm2 considerable in the viral equation of state?

Answer 16

C/Vm2 becomes large when under highly compressed gases.

Question 17

What does the van der Waals equation of state consider?

Answer 17

Repulsive interactions: this implies molecules cannot come closer than a certain distance and therefore the actual volume is reduced to V - nb. b is an empirical constant.

Attractive interactions: These reduce the pressure exerted by the gas as attractions slow molecules down, they strike the walls less frequently and with less force.

Question 18

What are the attractive forces of a gas proportional to?

Answer 18

These attractive forces are proportional to concentration, n/v.

Reduction in pressure = ∂ (n/v)^2

Where ∂ (alpha) is the constant of proportionality.

Question 19

When can attractive interactions be ignored in the van der Waals equation of state?

Answer 19

They are neglected at high temperatures, as amount of energy input reduces effect of attractive forces

Question 20

When can repulsive interactions be ignored in the van der Waals equation of state?

Answer 20

At low pressures; the molar volume is so large that V-nb is effectively V.

Question 21

What are the 3 critical constants that are related to the van der Waals coefficients?

Answer 21

Vc = 3b

Tc = 8a / 27Rb

Pc = a / 27b^2

Question 22

When does the van der Waals equation of state become the ideal gas equation?

Answer 22

When at high temperature and low pressure. This is when both attractive and repulsive interactions can be ignored.

Question 23

What is joules in terms of Si units?

Answer 23

Well F=ma, hence N = Kg m s^-2

And Joules = Nm

So, J = Kg m^2 s^-2

Question 24

What is the conversion from atm to Pa?

Answer 24

1atm = 101325Pa

Question 25

What can be said about the total pressure of a mix of gases (real and ideal)?

Answer 25

That Pj = Xj P

Where Pj is the partial pressure of a gas

Xj is the mole fraction of gas j. (nj / ntotal)

And P is the total pressure