solutions Flashcards
isssues affected by phase equilibria
whether an interface exists between 2 liquids
distribution of a drug between different media
how will a substance transfer (phase)
a substance will transfer spontaneously from a phase in which it has a higher chemical potential to a phase on which it has a lower chemical potential
what is a phase
the homogenous, physically distinct parts of a system are known as phases. these are separated from one another by interphases e.g. ice, water, water vapour
co-existence at phases
gases at equilibrium only one phase
liquid - miscible one phase immiscible 2 phases
solids- each solid is a separate phase unless a solid solution is formed as solutions only have one phase
the number of components in a system
the minimum number of independent species necessary to define the composition of all phases present in the system
e.g. ice water water vapour 3 phases one component
when does C = no of species
only when there is no chemical reaction
degrees of freedom
(F)number of factors which can be varied independently without noticeably altering the number of phases
what does phase rule tell us
the number of degrees of freedom for any particular number of components and phases in any system
what is Gibbs phase rule
P+F=C+2 at equilibrium
p is no of phases
c is no of component
a colligative property depends on the:
properties of the solvent
concentration (mole fraction) of the dissolved substance
number of particles of dissolved substance in the solution
4 main colligative properties
vapour pressure lowering
freezing point depression
boiling point elevation
osmotic pressure
(all thermodynamically related to each-other)
osmotic pressure has enormous importance as it contributes to flow of nutrients through biological cell walls and through plants
osmosis
tendency of solvent molecules to pass through a membrane from a more dilute toma more concentrated solution
what is osmotic pressure
if a solution is seperated from its solvent by a membrane then the excess pressure which has to be applied to prevent the flow of solvent is known as the osmotic pressure (uppercase pi)
osmotic pressure = RTx molarity
what is hydrostatic pressure
pressure exerted by a column of aqueous solution or water is called hydrostatic pressure
what is reverse osmosis
reverse osmosis is when a pressure (greater than osmotic pressure) is applied to the solution and forces the solvent through the membrane
types of membranes
semi permeable- permeable to solvent molecules only e.g. they will allow the passage of water but mot other molecules
biological (selectively permeable) these allow the passage of water and other molecules of low molecular weight but will restrain molecules of higher molecular weight
if red blood cells are immersed in solution of greater osmotic pressure than cells content:
water will pass out of cell to reduce chemical potential across cell membrane (cell shrinks)
if red blood cells immersed jn solution of lower osmoticpressure than cells content:
water will enter the cell (cell swells) and eventually lysis (cell breakdown) may occur
isotonic solutions
solutions between which there is no net flow of solvent when the solutions are prepared by a biological membrane are called isotonic - notably the RBC cell membrane
what must the effective osmotic pressure of a solution for injection be
approx same as blood serum
termed tonicity
not always same as osmolality
importance of tonicity
injections- effect of surrounding medium on RBC
tonicity in regards to osmotic pressure with blood serum
lower>hypotonic>cell swells, haemolysis may result
higher>hypertonic> cells shrink, crenation
same>isotonic>no net flow of liquid
relationship between osmotic pressure and rate of infusion
the higher the osmotic pressure the slower the rate of infusion required
relationships between colligative properties
as colligative properties are inter related it is impossible to determine one from the other
osmotic pressure is not easy to measure therefore makes use of other colligative properties to calculate e.g. freezing point depression
molality=moles/kg does not change with temp
molarity= moles/L changes with temp
freezing point depression
this is the lowering of the freezing point of the solvent by adding a non volatile solute
e,g, add salt to roads to drop the freezing point of water in winter
delta Tf= Kf x m
kf is freezing point constant of the solvent
is concentration of solute
converting Tf to osmotic pressure
at a given temp, osmotic pressure is proportional to change in Tf
also for dilute ideal solutions change in Tf is proportional to concentration
what does solubility depend on
pressure, temperature,presence of salts and chemical reactions
what is henrys law
effect of pressure “the vapour pressure exerted by a gas dissolved in a solvent at constant temperature is proportional to the mole fraction of the gas” pb=KhXb Kh henrys law constant Xa mole fraction of gas pa vapour pressure of gas
effect of temperature on solutions
as temp increases, solubility of most gases decrease due to the greater tendency for the gas to expand
what is salting out
salting out- gases are often liberated out of solution by addition of NaCl or sucrose
the solute added extracts the water molecules, reducing the density of the aqueous environment adjacent to the gas molecule
micellar solubilisation
if gas molecules can be incorporated into the interior of micelles then this will increase solubility
3 divisions of liquids in liquids
immiscible, partly miscible, completely miscible
immiscible liquid mixtures
mo solution is formed
P+F=C+2
P=2 and C=2 so degrees of freedom =2
temp and conc. can be varied without necissarily changing number of phases
completely miscible liquid mixture
solution is formed
P(1)+F=C(2)+2
F=3
temp, pressure and conc can be varies without necessarily moving from a single phase system
partially miscible liquid mixture & phase behaviour observed (3)
no of phases can me 1 miscible or 2 immiscible
behaviour; solubility increases with an increase in temperature, solubility decreases with an increase in temperature, high solubility only at low or high temperatures
what is critical temperature
above which both solutions are completely miscible
what is the upper critical solution temperature
highest temp at which phase separation occurs
exists because the greater the thermal motion overcomes any potential energy advantage in molecules of one type being close together
above this temp both are fully miscible
lower critical solution temperature
below which they mix in all proportions and above they form two products
at low temp two components are more miscible because they form a weak complex
at higher temp the complex breaks up and two components are less miscible
what is the max value of F for a single component system
(C=1;P=2) therefore F=2
systems where vapour phase is ignored and only solid and/or liquid phase are considered are termed
condensed systems
eutectic point
represents the lowest temperature at which the liquid solution can exist
what does solid dispersions enhance
the solid dispersions enhance the dissolution of poorly soluble drugs
the drug is mixed with a freely soluble “carrier” and enhanced dissolution is also related to particle size
two main types of solids in solids
an intimate crystalline mixture of one component in the other
a solid solute dissolved in a solid solvent to give a mixed crystal, this is a true solid solution
intimate crystalline mixtures - eutectic point
the eutectic point is the point at which the liquid and solid phases have the dame composition (eutectic composition)
the solid phase is an intimate mixture of fine crystals of the two compounds
when a mixture of 2 components with composition E are cooled, A and B crystallise out simultaneously
mixed crystals
these involve a solid solute being dissolved in a solid solvent, generally these have minor component inserted into the structure of the host crystal
these solid solutions can exhibit higher, lower or unchanged melting behaviour
difference between henrys and raoults law
raoults law applied to the solvent in a solution while henrys law applies to the solute in solution
raoults law
the relative lowering of vapour pressure is equal to the mole fraction of the solute in solution
solutions which obey raoults law are called
ideal solutions
solutions which do not obey raoults law are called
real or non ideal solutions, most are this thpe
