Lectures 3 & 4

Question 1

what is Gibbs definition of a phase

Answer 1

Gibbs defined phase as “State of matter that is uniform throughout in chemical composition and physical state”

Question 2

what is the gibbs phase rule

Answer 2

𝐹=𝐶−𝑃+2

This equation simply tells us the maximum number of intensive independent variables that can be specified (F) to define a system of C components in P phases. Its best illustration is on a phase diagram:

Question 3

what is the critical point

Answer 3

The highest possible temperature and the highest possible pressure for which distinct liquid and gas phases can be observed.

Question 4

what happens the gas and liquid phases at the critical point

Answer 4

Saturated gaseous and liquid phases become identical.

Question 5

what happens to the gas above the critical temperature

Answer 5

Above the critical temperature T c it is not possible to liquefy the gas by isothermal compression alone.

Question 6

what happens to the liquid above the critical liquid

Answer 6

Above the critical pressure P c it is not possible to vaporize the liquid by increasing temperature alone.

Question 7

is there a critical point between solids and liquids

Answer 7

here is no critical point between solid and liquid states. This is because it is impossible to pass from a state of disorder (liquid) to order (solid) without having a sudden change. There will always be a noticeable transition, an interface.

Question 8

why do the vaporization and sublimation curve have positive slopes for all substances

Answer 8

For all substances, the boiling curve and the sublimation curve have positive slopes. (If pressure increases, more energy is required to escape the liquid/solid, boiling point will increase)

Question 9

what slope does the melting line generally have

Answer 9

For most pure substances, the melting curve has a positive slope.

Question 10

what slope does the melting line have for water

Answer 10

For water, the melting curve has a negative slope. This is mostly due to the fact that ice is less dense than liquid water (this relates to the Gibbs free energy)

Question 11

why does the melting line go to infinity

Answer 11

because it is a line that shows equilibrium between solids and liquids which doesnt have a critical point so it goes to infinity

Question 12

using gibbs phase rule for C = 1 and P = 1: Single phase, F = 2

Answer 12

At most 2 intensive variables need to be specified in a pure component system to fully define its thermodynamic properties and state. Need to pre-set T and p .

Question 13

use gibbs phase rule for C = 1 and P = 2: Phase boundary, F = 1

Answer 13

The two-phase boundaries are fully specified with one variable (Clausius-Clapeyron).

Question 14

use gibbs phase rule for C = 1 and P = 3: Triple point, F = 0

Answer 14

At most three phases can be found at coexistence in a pure component system. Triple point is already pre-determined. But there can be more than one triple point.

Question 15

why can’t we specifiy P,T and Vm at the same time

Answer 15

although the system will have that specific volume, it will in fact be split into two separate phases, each with compositions at the bubble and dew points (for VLE).

Question 16

does only one triple point exist

Answer 16

no several triple points exist which can be seenon the P-Vm phase diagram

Question 17

what problem does the P-V diagram face

Answer 17

although you can have a volume in the phase envelope, it does not represent the volume of either phase.

Question 18

what is the chemical potential

Answer 18

in a pure component is simply the molar Gibbs free energy

Question 19

how do we determine if two phases are in equilibrium

Answer 19

If we then consider two phases, depending on their respective chemical potentials, it will determine whether or not they are in phase equilibrium:

Question 20

what are the phase equilibrium equations between the two phases

Answer 20

no net transfer of heat, no net transfer of work, no net transfer of mass

Question 21

what do we know about chemical potential at a specified pressure

Answer 21

As we can see, due to the gradients, for a range of temperatures, the solid phase has the lowest chemical potential until it intersects with the line for the liquid phase. Beyond this, the liquid has the lowest chemical potential. This is equally true for when the vapour phase becomes the lowest chemical potential.