Thermodynamics

Question 1

One Mole

Answer 1

One mole of a substance has the same number of particles as 12 grams of carbon-12 atoms.

Question 2

Boyle’s Law

Answer 2

The pressure exerted by a fixed mass of gas is inversely proportional to its volume, provided the temperature of the gas remains constant.

Question 3

Charles’ Law

Answer 3

The volume occupied by a gas at constant pressure is directly proportional to its thermodynamic, absolute (i.e. in K) temperature.

Question 4

Pressure Law

Answer 4

The pressure exerted by a fixed mass of gas in a fixed volume is directly proportional to its thermodynamic, absolute (i.e. in K) temperature.

Question 5

Brownian motion and the 3 conclusions from it

Answer 5

Smoke/pollen particles suspended in air/water have random, unpredictable movement. This suggests that the visible particles are constantly bombarded by particles that are:

1. Too small to be resolved.
2. Considerably lighter than the visible particles.
3. Travelling much faster, with a random distribution of velocities.

Question 6

Explanation of Pressure of Gases

Answer 6

When a particle collides elastically with a wall, the wall exerts a force on the particle that reverses its momentum. By Newton’s third law, the particle exerts an equal an opposite force on the wall. The sum of all these forces over the area of the wall is equal to the pressure.

Question 7

5 Ideal Gas Assumptions

Answer 7

1. There are no forces between molecules (except those in collisions)
2. The particles can be considered as point particles (occupy negligible volume)
3. All collisions are perfectly elastic
4. The duration of each collision is negligible compared to the time between collisions.
5. Molecues have a random distribution of velocities

Question 8

What happens to an ideal gas at low temperatures/high pressures? (2 points)

Answer 8

1. The gas particles get much closer together so their finite volume becomes significant.
2. As they are closer together the intermolecular forces cannot be ignored - they could cause a change in state.

Question 9

What happens to an ideal gas at very high temperatures?

Answer 9

Collisions are no longer elastic as energy can be used to excite electrons to higher energy levels or to weaken bonds within other molecules (or to create plasma).

Question 10

3 Applications of Boltzmann Factor and what the corresponding threshold energy represents.

Answer 10

1. Rate of reaction (Arrenhius equation) - E_a is activation energy. Catalysts lower E_a and thus increase rate.
2. Current in semiconductors - E_g is the band gap energy (energy between valence and conduction bands). Doping lowers E_g (by either extending the bands or introducing an intermediate energy level).
3. Creep of polymers - Q is the activation energy, i.e. the energy needed to activate the diffusion of atoms/vacancies within crystals or the movement of dislocations.

Question 11

Internal Energy

Answer 11

The sum of the random distribution of kinetic and potential energies in an ensemble.

Question 12

First Law of Thermodynamics (in words)

Answer 12

The increase in internal energy of a system is equal to the work done on the system plus the heat supplied to the system.

Question 13

Isovolumetric (meaning + effect on QWU for increase in p)

Answer 13

Constant volume
dW=0
For increase in p:
T increases therefore dU>0
Therefore dQ>0

Question 14

Isobaric (meaning + effect on QWU for increase in V)

Answer 14

Constant pressure
For increase in V:
dW<0 (done by gas)
T increases therefore dU>0
Therefore dQ>0

Question 15

Isothermal (meaning + effect on QWU for increase in V)

Answer 15

Constant temperature
dU=0
For increase in V:
dW<0 (done by gas)
Therefore dQ>0

Question 16

Adiabatic (meaning + effect on QWU for increase in V)

Answer 16

No heat transfer with surroundings.
dQ=0
For increase in V:
dW<0 (done by gas)
Therefore dU<0

Question 17

What is a thermodynamic cycle?

Answer 17

A thermodynamic process which starts and ends at the same values of p and V.

Question 18

Heat engine definition (heat pump is opposite of heat engine)

Answer 18

A device that is supplied with thermal energy and converts some of this energy into useful work.

Question 19

Carnot Cycle

Answer 19

The most efficient thermodynamic cycle for a heat engine (consists of two isothermal and two adiabatic processes)

Question 20

Microstates and Macrostates (optional)

Answer 20

Microstates are every different possible arrangement of energy in a system. Macrostates are the more general results of a number of different microstates. The macrostates are things we observe, such as pressure or temperature.

Question 21

Entropy definition

Answer 21

Entropy is a measure of the number of ways of distributing energy (or particles) (i.e. the number of microstates) in a system (i.e. amongst the available macrostates).

Question 22

Second Law of Thermodynamics

Answer 22

The entropy of the universe tends to a maximum.

Question 23

Two cases where the entropy change of the universe can be 0

Answer 23

1. Temperature is 0 K

2. A process which is isothermal and perfectly reversible

Question 24

Thermodynamic Arrow of Time

Answer 24

Irreversible processes separate the past from the future. Therefore since entropy doesn’t decrease (technically it could but yeah) a point where the universe has higher entropy must come after a point with lower entropy in time, as defined by this arrow.

Question 25

2 cosmological Implications of the Second Law

Answer 25

1. The universe had a very low entropy to begin with, meaning it was in a very improbable state.
2. One way in which the universe could end is a heat death (aka big freeze) where the entropy of the universe reaches a maximum value and so no further useful work can be done.