Phase Equilibrium and Solutions

Question 1

What is a phase?

Answer 1

Part of a system that is homogenous in chemical and physical state throughout and is separated form other phases by a definite boundary

Question 2

What do the number of phases in liquids depend on?

Answer 2

Nature of the components, for a single component there can only be one liquid phase, when more than one component is involved they can form one phase id they mix or two phases if they don’t, whether two liquid mix may change depending on the conditions, mixtures that form a single phase are said to be miscible and those that form more than one are said to be immiscible

Question 3

What are partially miscible liquids?

Answer 3

Mixtures that form one phase under some conditions and more than one under other conditions are partially miscible

Question 4

What number of phases are there in a gas?

Answer 4

In gases and vapours there is plenty of space for the molecules to mix so that there is only ever one phase irrespective of how many components are involved

Question 5

How do some molecules of a liquid enter the gas phase before the boiling point?

Answer 5

Some molecules near the surface of the liquid will have sufficient energy to overcome the attractive interactions with neighbouring molecules and escape into the vapour phase. At higher temperatures liquids have a higher vapour pressure since more molecules have significant energy to escape the liquids

Question 6

What is the vapour pressure and saturated vapour pressure?

Answer 6

If some liquid is placed in a container the vaporised material will exert a pressure - the vapour pressure. if the container is now closed and the temperature is constant the vapour and liquid come to equilibrium and the equilibrium vapour pressure is then called the saturated vapour pressure po

Question 7

Solids vapour pressure?

Answer 7

Solids also exert a vapour pressure although the stronger interactions between molecules mean that it is usually lower than for liquids

Question 8

Partial vapour pressure?

Answer 8

If there are other gases present above the liquid then the pressure due to the vapour in its partial vapour pressure the total pressure in the container will be the vapour pressure of the liquid plus the partial pressure of all other gases present

Question 9

When does a liquid boil?

Answer 9

When the vapour pressure of a liquid becomes equal to the external pressure the liquid boils the pressure of the vapour is large enough to overcome the external pressure on the liquid so bubbles of vapour can form anywhere within the liquid as well as at the surface

Question 10

What is a phase diagram?

Answer 10

Plot of the pressure and temperatures where phase transitions take place

Question 11

What do the lines show on the diagram?

Answer 11

T-A shows the melting temperature changes with pressure at the melting temperature solid and liquid exist in equilibrium
T-C shows how the boiling temperature changes with pressure
B-T solid vapour curve and the liquid vapour curve and the liquid vapour cure T-C also shows how the equilibrium pressure for solid and liquid respectively depends on pressure

Question 12

What is the triple point?

Answer 12

Where the solid, liquid and vapour exist in equilibrium, only set of conditions where the three phases can be in equilibrium, the triple point also represents the lowest temperature at which the substance can exist in the liquid state

Question 13

Most stable phase for a compound?

Answer 13

The most stale tase at a particular temperature and constant pressure is the phase with the lowest Gibbs energy at that temperature

Question 14

Value of Sm and Gm?

Answer 14

The value of Sm is always positive so that Gm decreases aa the temperature rises, the gradient of a plot of Gm against T is negative and equal to -Sm, the molar entropy of the solid Sm is smaller than that of a liquid which is in turn smaller than that of the vapour, therefore the line for the solid in the graph has the smallest slope and the line for the vapour has the steepest slope. At low temperatures the solid phase has the smallest Gm so this is the most stable phase between the melting and boiling temperature Gm is smallest for the liquid phase, the gas phase is more stable above the boiling temperature since it has the smallest Gm at these temperatures

Question 15

When do phase transitions occur?

Answer 15

Melting or freezing at Tm and boiling or condensation at Tb occur where the line intersect, at these temperatures and pressures Gm values for the two phases are equal

Question 16

What does the Claperyon equation give?

Answer 16

The gradient of the lines on a phase diagram in terms of the molar entropy change at the transition

Question 17

What is the Clausius Calperyon equation?

Answer 17

Relates pressure and temperature for phase transition of vaporisation or sublimation so change in enthalpy is enthalpy of vaporisation or enthalpy of sublimation

Question 18

What is the enthalpy change of vaporisation related to?

Answer 18

The energy needed to overcome the intermolecular interactions for 1 mol of liquid, if is therefore a good measure of the strength of these interactions

Question 19

What happens during vaporisation?

Answer 19

A liquid turns into a vapour, provided that there are no very strong interactions between the molecules such as hydrogen bonding most liquids have similar arrangements of molecules, the vapour phase is highly random and disordered no matter what substance is involved, therefore the same degree of change in molecular order and hence in entropy is to be expected as a result of vaporisation whatever the substance provided there are no strong interactions between the molecules

Question 20

What is Trautons law?

Answer 20

Entropy of vaporisation for a liquid = 85
JK-1mol-1
Even liquid with very different boiling point and chemical natures have smiler values of entropy of vaporisation

Question 21

What do deviation from Trautons law result from?

Answer 21

Unusally strong interactions between the molecules in the liquid or vapour

Question 22

What does the phase of substance depend on?

Answer 22

The phase in which a substance exists at a particular temperature and pressure is largely governed by the interactions between its molecules

Question 23

Intermolecular forces in gases?

Answer 23

Interactions between molecules are small except at high pressures

Question 24

Intermolecular forces in solids and liquids?

Answer 24

Held together b attractive interactions between the molecules there are also repulsive interactions due to the overlapping of electron clouds and the place between attractive and repulsive interactions keeps the molecules at equilibrium separation

Question 25

What happens when there are attractive interactions between molecules?

Answer 25

The potential energy becomes more negative as the distance between the molecules decreases

Question 26

What happens when there are repulsive interactions between molecules?

Answer 26

The potential becomes more positive as the molecules move closer together

Question 27

Ionic interactions?

Answer 27

The strongest interactions occur in ionic systems, the energy of the interaction differs according to whether the two ions are in a vacuum, in air in water or in a hydrocarbon solvent. The permittivity accounts for this difference, if the ions have opposite charges the terms in the equations have opposite signs and the potential energy is negative as the ions come closer together, the energy becomes less negative as the distance between the ions increases but the 1/r dependence shows that ions interact even at quite long distances compared with the size of the ions

Question 28

Non covalent interactions?

Answer 28

Interactions that occur between molecules the have no overall charge are known as non covalent interactions to distinguish them from covalent bonds with molecules they are also sometimes known as van der waals interactions

Question 29

Dipole dipole interactions?

Answer 29

Two molecules that have dipoles can interact, the larger the dipole moment the stronger the interaction, the potential energy of interaction varies depending on the relative orientation of the molecules

Question 30

Dipole dipole interactions 1/r^6 dependence?

Answer 30

The 1/r^6 dependence means that the potential energy approaches zero very quickly at the molecules get further away so that the intentions are only significant when the molecules are very close together.

Question 31

Dipole dipole interactions 1/T dependence?

Answer 31

The 1/T dependence means that the magnitude of the interaction energy is smaller at higher temperatures this is because at higher temperatures the molecules have more thermal energy which leads to a more random orientation of the dipole

Question 32

Dipole induced dipole interactions?

Answer 32

When a molecules with a dipole approaches a molecules without one it induces a movement of the electrons causing a temporary dipole in the non polar molecules so there is an attractive interaction

Question 33

Strength of dipole induced dipole interactions?

Answer 33

Depends on how easily the electron cloud of the non polar molecule can be moved around, this is measured by the polarisability of the molecule, the higher the polarisability the more easily the electrons can be distorted by a nearby dipole or charge

Question 34

Induced dipole induced dipole interactions?

Answer 34

Non polar molecules have attractive interactions due to the formation of instantaneous induced dipoles, these are known as dispersion or london interactions a theoretical model for dispersion forces showed that the strength of the interaction depends on how easily the electrons in the molecules can be distorted and hence on their polarisability as well as their ionisation energy

Question 35

Induced dipole induced dipole interactions compared to dipole dipole interactions?

Answer 35

While the magnitude of dispersion energies are much smaller than those of chemical bonds they are comparable with and usually slightly larger than dipole dipole interactions, they are usually the major contribution to intermolecular interactions

Question 36

Overall attractive interaction?

Answer 36

Sum of those given by previous interactions, all of the contributions to the attractive interactions depend on separation as 1/r^6 this means that they are short range and their magnitudes rapidly decreases as the separation increases, they become very small once the separation is more than a few molecular diameters, at constant temperature all then terms except r^6 are constant for a particular compound

Question 37

Hydrogen bonding?

Answer 37

Attractive intermolecular interaction, interaction between a hydrogen atom and an electronegative tom, they are intermediate between ionic and non covalent interactions

Question 38

Repulsive interactions?

Answer 38

The electron clouds of two adjacent molecules repel each other, the repulsive force is small until the molecules approach close to each other, at very small separation the repulsive interaction energy rise rapidly as the molecules move closer together

Question 39

value fo potential energy for repulsive interactions?

Answer 39

The value of the potential energy is positive indicating that it is a repulsive interaction, it varies as 1/r^12 which means that it is significant one when the molecules are very close together

Question 40

Total interaction?

Answer 40

For two molecules the overall interaction energy is the sum of the attractive and repulsive contributions given by the Lennard Jones potential

Question 41

Interaction at large separations?

Answer 41

At large separations there is no interaction between the molecule as they approach they begin to attract each other and the potential energy falls when the molecules come bear close the repulsive interactions becomes dominant and the potential energy corresponds to the equivalence separation

Question 42

Minimum potential energy?

Answer 42

Corresponds to the equilibrium separation between the molecules r0, at this minimum the total potential energy is equal to -E where E is the energy needed to overcome the attractive interactions and completely separate the molecules, the larger the value of E the stronger the interactions between the molecules, the distance r0 is the equilibrium separation where the net force between the molecules is zero

Question 43

Vapour phase?

Answer 43

There can only be one vapour phase no mater how many gases are present since gases mix in all proportions

Question 44

Solid phases?

Answer 44

Mixtures of two solids can like single components display any number of solid phases, the major difference is that where pure substance only have one liquid oases mixtures can have more than one

Question 45

The phase rule?

Answer 45

simple rule that helps interpret phase diagrams

Question 46

Degree of freedom?

Answer 46

The number in intensive variable such as temperature and pressure that can be varied independently without changing the number of phases in equilibrium

Question 47

When is mixing spontaneous?

Answer 47

mixing is spontaneous if it results in a lowering of the Gibbs energy of the system, the change in Gibbs energy is the difference between the molar Gibbs energy of the solution and that of the separate components

Question 48

What is he composition of a mixture represented by?

Answer 48

The mole fraction of each component Xa and Xb

Question 49

Value of delta mixS?

Answer 49

Usually positive since the mixture will be more disordered and will have a higher entropy than the separate components, the entropy change favours mixing whether two liquids actually mix depends on the enthalpy change of mixing

Question 50

What does the enthalpy change of mixing arise from?

Answer 50

Differences in the intermolecular interactions between molecules in each of the pure liquids and between the components in solution

Question 51

If delta mix H is positive?

Answer 51

Means an unfavourable contribution to delta mix G, if delta mix H is large and positive then its magnitude may be greater than that of

• T delta mixS, in this case delta mix G will be positive so the separate components will be thermodynamically more stable than the mixture, they do not mix spontaneously, however if the value of delta mix H is small and positive mixing may occur of the magnitude of delta mix h is less than that of
• T delta mixS

Question 52

Why does delta mix G change with temperature?

Answer 52

Since the T delta mixS term becomes more significant at higher temperatures, two liquids that do not mix at low temperatures may mix on heating

Question 53

If -T delta mixS is negative and enthalpy change of mixing is negative what are the interactions, Gibbs energy change of mixing and the overall effect?

Answer 53

A-B stronger than A-A and B-B, enthalpy change of mixing is negative, Gibbs energy change of mixing is negative and of large magnitude and the overall effect is that the components mix

Question 54

If -T delta mixS is negative and enthalpy change of mixing is positive and large what are the interactions, Gibbs energy change of mixing and the overall effect?

Answer 54

A-B much weaker than A-A and B-B, enthalpy change of mixing is positive and large, Gibbs energy change of mixing is positive and large, components do not mix but may do so at high temperatures

Question 55

If -T delta mixS is negative and enthalpy change of mixing is positive and small what are the interactions, Gibbs energy change of mixing and the overall effect?

Answer 55

A-B much weaker than A-A and B-B, enthalpy change of mixing is positive and small, Gibbs energy change of mixing is negative and small, components may mix but depends on temperature and composition

Question 56

Phase behaviour of solutions?

Answer 56

Can be more complicated than this since the enthalpy change depend son the relative amounts of components involved it can be small for some composition but high for other, hence some compounds mix in some compositions but not other they are said to be partially miscible also the enthalpy change can be temperature dependent

Question 57

Dissolving spontaneous?

Answer 57

For dissolving to be spontaneous the overall Gibbs energy change must be negative and this can be split into the contribution form the enthalpy change even but he difference in interaction between molecules in the solution and those in the solid and pure liquid and the entropy change related to the change in order of the molecules

Question 58

Value of delta mix S?

Answer 58

Most solutions are more disordered than the separate solid and liquid so that the value of delta mix S is positive and favours dissolving

Question 59

Dissolving a non polar solid in a non polar solvent?

Answer 59

Interactions between each of the components will be relatively weaker will be similar in solution and in the separate components so the magnitude of enthalpy change of mixing will be small this means that the entropy change will be the most important contribution to the Gibbs energy change so the value of Gibbs energy change of mixing will be negative and the substance will dissolve. A similar situation occurs with a polar or ionic substance dissolving in a polar solvent the difference in interactions and hence the magnitudes of enthalpy change of mixing will be small so that dissolving will be favoured

Question 60

Dissolving a polar or ionic solid in a non polar solvent or a non polar solid in a polar solvent?

Answer 60

The solute solvent interactions will be relatively weak in the solution and will not overcome the stronger interactions in the solid, the value of enthalpy of mixing will be positive and may lead to overall positive Gibbs energy change so that the solid does not dissolve

Question 61

Vapour pressure of a liquid?

Answer 61

Related to the intermolecular forces in the liquid, if the molecules strongly attract each other few molecules escape the liquid to enter the vapour phase so the liquid exerts a low vapour pressure

Question 62

Raoult’s law?

Answer 62

For many solutions the vapour pressure of each component p wad proportional to its mole fraction X, for a solution with two component A and B
pA = Xap0A pB = Xbp0B
Ptotal = Pa + Pb = XaP0A + XbP0B
where p0 is the vapour pressure of the pure component at the temperature of the solution

Question 63

Obey Raoult’s law?

Answer 63

In practise vary few solutions obey Raoult’s law precisely and it can only be regarded as an approximation, however it works quite well for mixtures of similar compounds such as hexane and hepatic or benzene and methylene, mixtures that obey Raoult’s law are known as ideal solutions

Question 64

Requirements for a solution to behave ideally?

Answer 64

The solution should consist of molecules that:
Are similar in chemical nature so that they have similar intermolecular interactions
Are similar in size and shape

Question 65

Why are the molecules being of similar chemical nature more important than size and shape?

Answer 65

Difference in size and shape of the compounds are only significant of the difference is large such as in a solution of a polymer in a solvent, the major requirement for an ideal solution is that the intermolecular interactions in the solution are similar to those in pure compounds

Question 66

Intermolecular interactions in an ideal solution?

Answer 66

In an ideal solution the intermolecular forces in the solution are the same as in the pure components so the enthalpy change of mixing is zero, and the value of Gibbs energy change of mixing is negative

Question 67

Ideal solutions?

Answer 67

Few solutions behave ideally, it is looking at deviations from ideal solutions behaviour where the model is useful because these deviations provide information about the interactions that occur between molecules in the solution

Question 68

How can the behaviour of real solutions be described?

Answer 68

By modifying Raoults law and introducing an additional parameter, the activity coefficient
Pa(real) = yAXaP0A

Question 69

Activity coefficient for an ideal gas?

Answer 69

1
If Y > 1 the vapour pressure of the component over the solution is higher than it would be if the solution behaved ideally. If y < 1 the vapour pressure is lower than for an ideal solution

Question 70

Positive deviations from Raoult’s law?

Answer 70

In most solutions the interactions in solution are weaker than those in pure liquids in this case molecules are held less tightly into the solution and can more readily escape into the vapour, as a result the vapour pressure is greater than would be the case if the solution conformed to ideal behaviour, such systems are said to display positive deviations from Raoult’s law since p > p(ideal) so that y > 1 the higher the value of y the greater the difference in intermolecular interaction between the solution and pure liquid

Question 71

Negative deviations from Raoult’s law?

Answer 71

If the components show stronger intermolaulcar interactions in solution that in pure liquids so that few molecules escape to the vapour, as a result the vapour pressure is smaller than would be the case if the solution confirmed to ideal behaviour so p < p (ideal) and y < 1 such systems are said to display negative deviations from Raoult’s law

Question 72

When does Raoult’s law work best?

Answer 72

When the component is present in excess, ie the solvent in a dilute solution. As Xa gets closer to 1 each A molecule is more likely to be surrounded by other A molecules and experience an environment similar to that in the pure liquid

Question 73

How is Raoult’s law a limiting law?

Answer 73

It works better as the concentration of one component approaches 100% it desired the behaviour of the solvent in a volute solution quite well but does not usually describe the behaviour of the solute well. The solute molecules B are in a minority so they are in an environment quite unlike that of pure B, this means Raoult’s law does not describe the vapour pressure of B at all well, this is particularly the case for gases dissolved in liquids

Question 74

What rule does the vapour pressure of the solute B follow?

Answer 74

Henry’s law
pB = XbKb
solutions that behave according to Henrys law are called ideal dilute solutions, henrys law is also a limiting law and solutions obey it better as the solution becomes more dilute

Question 75

Consequence of Henry’s law?

Answer 75

For a given mass of solvent the amount of gas dissolved is proportional to the partial pressure of the gas over the solution, the solubility of the gas rises as the pressure increases, when the pressure is released the solubility falls and the gas rapidly comes out of solution