Flashcards in Phase Equilibrium and Solutions Deck (75)
What is a phase?
Part of a system that is homogenous in chemical and physical state throughout and is separated form other phases by a definite boundary
What do the number of phases in liquids depend on?
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
What are partially miscible liquids?
Mixtures that form one phase under some conditions and more than one under other conditions are partially miscible
What number of phases are there in a gas?
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
How do some molecules of a liquid enter the gas phase before the boiling point?
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
What is the vapour pressure and saturated vapour pressure?
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
Solids vapour pressure?
Solids also exert a vapour pressure although the stronger interactions between molecules mean that it is usually lower than for liquids
Partial vapour pressure?
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
When does a liquid boil?
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
What is a phase diagram?
Plot of the pressure and temperatures where phase transitions take place
What do the lines show on the diagram?
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
What is the triple point?
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
Most stable phase for a compound?
The most stale tase at a particular temperature and constant pressure is the phase with the lowest Gibbs energy at that temperature
Value of Sm and Gm?
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
When do phase transitions occur?
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
What does the Claperyon equation give?
The gradient of the lines on a phase diagram in terms of the molar entropy change at the transition
What is the Clausius Calperyon equation?
Relates pressure and temperature for phase transition of vaporisation or sublimation so change in enthalpy is enthalpy of vaporisation or enthalpy of sublimation
What is the enthalpy change of vaporisation related to?
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
What happens during vaporisation?
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
What is Trautons law?
Entropy of vaporisation for a liquid = 85
Even liquid with very different boiling point and chemical natures have smiler values of entropy of vaporisation
What do deviation from Trautons law result from?
Unusally strong interactions between the molecules in the liquid or vapour
What does the phase of substance depend on?
The phase in which a substance exists at a particular temperature and pressure is largely governed by the interactions between its molecules
Intermolecular forces in gases?
Interactions between molecules are small except at high pressures
Intermolecular forces in solids and liquids?
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
What happens when there are attractive interactions between molecules?
The potential energy becomes more negative as the distance between the molecules decreases
What happens when there are repulsive interactions between molecules?
The potential becomes more positive as the molecules move closer together
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
Non covalent interactions?
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
Dipole dipole interactions?
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