Thermodynamics

Question 1

Conservation of energy

Answer 1

the internal energy of the universe is constant. If a system loses energy the surroundings must gain the same amount.

Question 2

What is entropy

Answer 2

How likely something is to exist ‘by chance’.
‘amount of randomness’

Question 3

order of entropy of liquids, gases and solids.

Answer 3

gas > liquid > solid

Question 4

What is equilibrium and its conditions?

Answer 4

equilibrium is the condition under which no spontaneous overall change can take place.
No heat energy transfer, thermally insulated
no mechanical energy transfer
No phase change or chemical changes.

Question 5

what is the ‘universe’

Answer 5

system + surroundings

Question 6

what do we assume about surroundings

Answer 6

Constant pressure P, and temperature T.
Heat q lost to the surroundings raises their entropy by q/T

Question 7

What is an extensive property?

Answer 7

extensive properties are proportional to the amount of substance present

Question 8

What is an intensive property?

Answer 8

Independent of the amount of substance

Question 9

Is molar Gibbs free energy extensive or intensive?

Answer 9

Intensive

Question 10

What is adiabatic heating?

Answer 10

Rapid compression of a fluid raises its temperature. Mechanical energy is converted into heat energy

Question 11

Enthalpy of a ideal gas mixture.

Answer 11

ΔH = 0
components of an ideal gas mixture do not interact

Question 12

Entropy of an ideal gas mixture.

Answer 12

ΔS > 0
entropy favours mixing, ideal gases mix completely randomly.

Question 13

Raoults law

Answer 13

A mixture of liquids can be kept at equilibrium with a gas mixture
The chemical potential of each component is the same in both phases.

Question 14

If a non volatile solute (A) is dissolved in a liquid (B) what happens to the boiling and melting points

Answer 14

boiling temperature of the liquid usually increases and its freezing point decreases.
This is because the chemical potential of B in the vapour and solid remains the same, but the chemical potential of B in the liquid decreases, stabilising the liquid.

Question 15

What does the Gibbs duhem equation tell us?
dG = – SdT + VdP + µAdnA + µBdnB…

Answer 15

Shows us the chemical potentials of different components are related.
It can be used to work out the change in chemical potential of A, if that of B is known.

Question 16

why does non ideal behaviour occur?

Answer 16

Non-ideal behaviour occurs because of attraction and repulsion between molecules

Question 17

What is a convenient way of defining an excess quantity?

Answer 17

Often convenient to define an excess quantity as the difference from an ideal gas at the same temperature and pressure.

Question 18

What is vapour pressure?

Answer 18

a measure of the tendency for a material to change into the gaseous or vapour state.

Question 19

Explain how the phase rule applies to different areas, lines and points on the phase diagram of a one component system.

Answer 19

F + P = C + 2
F = 3 - P
One equilibrium phase has 2 degrees of freedon
Two equilibrium phases give 1 degree of freedom
Three equilibrium phases give no degrees of freedom, this applies to points on the diagram (the triple point)
The critical point only has 2 phases in equilibrium. However an additional condition applies at the critical point: the two phases become indistinguishable, which means that they have the same density. This induces the number of freedom to zero.

Question 20

Explain why the liquid/solid line on a phase diagram is usually steeper than the
liquid/vapour and solid/vapour lines. Explain in molecular terms why this occurs. What does it imply about the
thermodynamics of the phase transition along this region of the line?

Answer 20

Entropy always increases from solid to liquid, and from liquid to gas. Volume is largest in the gas phase, volumes of solid and liquid are similar. Given that dP/dT = dS/dV, gradient is steep whenn the change in volume is small.

Question 21

Explain the difference (in molecular and bulk terms) between boiling and evaporation,
with reference to the change in the boiling points of liquids with altitude.

Answer 21

Evaporation is the exchange of molecules between the surface of the liquid and the gas. Realtively slow. the liquid evaporates when its vapour pressure exceeds its partial pressure in the gas phase.
Boiling refers to the creation of bubbles of gas in the body of the liquid. In order for this to occur, the vapour pressure must exceed the total pressure in the gas phase. faster as all molecules are able to participate.

Question 22

For water, part of the liquid/solid line has a negative
gradient. Explain in molecular terms why this occurs. What does it imply about the
thermodynamics of the phase transition along this region of the line? What would be
observed as a result of isothermal compression of water through this region?

Answer 22

For water, the volume of the liquid is smaller than the volume of the solid where the phase line has a negative gradient. This is because the solid has a rather ‘open’ structure with each water molecule surrounded by a tetrahedron of other molecules. Compressing the gas gives a solid, and compressing the solid further gives a liquid.

Question 23

Process of fractional distillation

Answer 23

when a liquid micture is boiled, the mixture in the vapour phase is usually has a different composition to the liquid.
The vapour is collected and re-condensed in a seperate container (distillation)
This produces a liquid with a higher concentration of the most volatile component than the starting liquid.
Repeating the process concentrates it further.