Thermodynamics- Pure Substances

Question 1

What is a pure substance?

Answer 1

A substance that has a fixed chemical composition throughout. Even though air is a mixture of gases, it is considered to be a pure substance.

Question 2

Is a mixture of liquid and gaseous air a pure substance?

Answer 2

No because the composition of liquid air is different to gaseous air due to the different components condensing at different temperatures at a specified pressure.

Question 3

What is a compressed (subcooled) liquid?

Answer 3

A substance in the liquid phase that is not about to vaporise.

Question 4

What is a saturated liquid?

Answer 4

A substance in the liquid phase that is about to vaporise.

Question 5

What is a: saturated vapour, saturated liquid-vapour mixture and superheated vapour?

Answer 5

Saturated: a vapour that is about to condense.
Saturated liquid-vapour mixture: the state at which the liquid and vapour phases coexist in equilibrium.
Superheated: a vapour that is not about to condense.

Question 6

What is latent heat?

Answer 6

The amount of energy absorbed or released during a phase change process.

Question 7

Describe the process diagram (graph) for the heating process of water at a constant pressure

Answer 7

Temperature against specific volume. Straight diagonal line up to 100°C for compressed liquid. Horizontal line at 100°C for saturated mixture (greater proportion of vapour as go along). Straight diagonal line up for superheated vapour.

Question 8

What is saturation temperature and pressure?

Answer 8

The temperature/pressure at which a pure substance changes phase at a given pressure/temperature.

Question 9

Graph of saturation pressure against saturation temperature

Answer 9

Starts at origin. Curves up like y=x^2 curve.

Question 10

What happens to the T-v diagram for heating a liquid at a constant pressure at different pressures?

Answer 10

As you increase the pressure, the horizontal section gets higher, starts slightly further right but is shorter and ends further left. Above a critical pressure, the horizontal section is removed and instead the line starts to decrease in gradient and then increases again beyond the critical point (no stationary point).

Question 11

What is the critical point?

Answer 11

The point at which the saturated liquid and saturated vapour states are identical. This point is at the critical temperature, pressure and specific volume for the substance.

Question 12

Saturated liquid and vapour lines on a T-v graph

Answer 12

Saturated liquid line: almost straight diagonal up until near the critical point where the gradient decreases to 0 (at the critical point).
Saturated vapour line: curves down from critical point like the right side of a bell curve.
Left of curve is compressed liquid region. Under curve is saturated liquid-vapour region. Right of curve is superheated vapour region.

Question 13

Graph of P against v for a pure substance in relation to the T-v graph for liquids and vapour

Answer 13

The same shape but flipped vertically. The saturated liquid and vapour lines are the same shape as for the T-v graph.

Question 14

How to extend P-v diagrams (curve) for pure substances to include solid phase (see pic on phone)

Answer 14

For a substance that contracts on freezing:
Triple line (horizontal) from where normal curve starts to other side of curve. Vertical line from where triple line meets normal curve. Another vertical line further left from where triple line starts. This has small curve down to left. For a substance that expands on freezing:
Similar but 2 vertical lines are no diagonal (-ve m) and cut into normal curve. They sort of swap with each other and triple line starts further right (where small curve is).

Question 15

Define sublimation

Answer 15

Passing from the solid phase directly to the vapour phase

Question 16

Describe the pressure-temperature diagram (phase diagram) for a pure substance

Answer 16

Curved line up from origin to triple point for sublimation (medium gradient). Straight dotted line up left from triple point for melting for substance that expands on freezing. Curved line up from triple point continuing from sublimation curve for melting for substance that contracts on freezing. Curved line up from triple point to critical point for vaporisation (shallower gradient).

Question 17

When would a pure solid sublime?

Answer 17

When the temperature increases but the pressure never exceeds that of its triple point.

Question 18

On the P-T diagram, why does the vaporisation line end at the critical point?

Answer 18

Because no distinction can be made between the liquid and vapour phases above the critical point.

Question 19

Formula for enthalpy

Answer 19

Internal energy plus pressure times volume. Can have specific enthalpy for per unit mass.

Question 20

Enthalpy of vaporisation

Answer 20

Also latent heat of vaporisation. The amount of energy needed to vaporise a unit mass of saturated liquid at a given temperature or pressure.

Question 21

What is the quality (x) of a saturated liquid-vapour mixture?

Answer 21

The ratio of the mass of vapour to the total mass of the mixture. Related to the horizontal distances on the P-v or T-v diagrams.

Question 22

In a saturated liquid-vapour mixture what is the formula for average specific volume?

Answer 22

vavg=vf+xvfg
This can also be applied for internal energy and enthalpy.
vfg=vg-vf

Question 23

In a saturated liquid-vapour mixture, which state has the greater enthalpy, specific volume, internal energy?

Answer 23

The vapour

Question 24

What is an equation of state?

Answer 24

Any equation that related the pressure, temperature and specific volume of a substance.

Question 25

What is the ideal gas equation of state?

Answer 25

Pv=RT
Where R is the gas constant for that specific gas
Also PV(bar)=RuT
Where Ru is universal gas constant and is R over molar mass 
V(bar) is volume per unit mole

Question 26

How to represent a property per unit mole

Answer 26

Add a horizontal line on top of the symbol (bar).

Question 27

When is the ideal gas equation of state most accurate?

Answer 27

For gases at low densities (greater specific volumes). Density decreases at lower pressures and higher temperatures.

Question 28

What is the compressibility factor of a gas?

Answer 28

Symbol Z. A factor that accounts for the deviation of real gases from ideal gas behaviour at a given temperature and pressure.
It is equal to (Pv)/(RT)
Also actual specific volume over ideal specific volume

Question 29

Define reduced pressure and reduced temperature of gases

Answer 29

Reduced pressure/temp is actual pressure/temp over critical pressure/temp.

Question 30

What is useful about reduced pressure and temperature values?

Answer 30

The Z factor for all gases is approximately the same at the same reduced pressure and temperature. This is called the principle of corresponding states.