Liquid Separations Flashcards
raoult’s law
States that the partial pressure of a liquid above a solution is equal to the vapour pressure of the pure solvent at the same temperature
what is vapour pessure?
It is the pressure exerted by vapour molecules on the liquid surface when a liquid is in a container.
what happens when a liquid in a closed vessel reaches dynamic equilibrium?
The rates at which the liquid turns to vapour and vapour condenses back to liquid become equal, resulting in saturated vapour pressure (SVP).
how does SVP vary with temperature?
SVP increases with temperature because more molecules acquire energy to escape into vapour
how does SVP vary with temperature?
SVP increases with temperature because more molecules acquire energy to escape into vapour
What is the difference between miscible d immiscible liquids?
Miscible liquids form a homogeneous solution in any proportion (e.g., H₂O and CH₃OH), while immiscible liquids separate into distinct layers (e.g., H₂O and paraffin oil).
define an ideal solution
An ideal solution is where the forces of attraction between different molecules in the mixture are of the same strength as in the pure liquids, with no volume change on mixing.
how do you calculate total vapor pressure in a mixture?
Add the partial pressures of each liquid in the mixture.
how do you calculate total vapor pressure in a mixture?
Add the partial pressures of each liquid in the mixture.
what is fractional distillation?
It is a process to separate liquid mixtures based on differences in boiling points.
what happens as vapour rises in a fractionating column?
It becomes richer in the more volatile component, eventually producing pure vapour at the top.
what happens as vapour rises in a fractionating column?
It becomes richer in the more volatile component, eventually producing pure vapour at the top.
how does packing improve the efficiency of fractional distillation?
Packing provides a surface area for condensation and ensures equal rates of vapour rise and liquid descent.
what causes negative deviation from Raoult’s law?
Stronger forces of attraction in the mixture than in the pure liquids, leading to lower SVP and higher boiling points.
what happens to volume and heat in mixtures showing negative deviation?
The volume contracts, and heat is released (ΔH is negative).
what happens to volume and heat in mixtures showing negative deviation?
The volume contracts, and heat is released (ΔH is negative).
what is an azeotrope?
A constant boiling point mixture that retains the same composition in liquid and vapour phases during boiling.
provide an example of azeotropic behaviour
H₂O and HNO₃ mixtures form an azeotrope at 121°C with 31.8% H₂O and 68.2% HNO₃.
what is the positive deviation from Raoults Law?
A pair of liquids will mix and form a mixture, which shows a positive deviation from Raoult’s Law when there are weaker forces in the mixture than in the pure liquids.
When these liquids mix, there will be:
i. an increase in volume
ii. an absorption of heat
what are immiscible liquids?
A pair of liquids is said to be immiscible if they separate into distinct layers on mixing, with the less dense liquids above.
properties of immiscible liquids
Whenever a pair of immiscible liquids is in a container, each liquid exerts its total SVP above the mixture.
The mixture has to be agitated by either shaking or stirring for the lower liquid to establish its SVP.
Does boiling point vary with composition for immiscible liquids?
no because each liquid exerts its total SVP above the mixtures so all the mixtures will have the same boiling point
what is the rule of boiling point with immiscible liquids?
The boiling point of a pair of immiscible liquids is always lower than the boiling point of either liquid.
what is stam distillation?
Steam distillation is a method of purification and extraction of compounds which tend to decompose at temperature values less than their boiling points.
when does steam distillation work well?
i. The liquid being purified does not have a very high boiling point since large amounts of steam will be needed, making the process costly.
ii. The impurities must not be volatile at the temperature of the distillation.
iii. The liquid being purified must have a high SVP and a high RMM.
relationship between SVP of liquids and the moles in the liquid
During steam distillation, the number of moles of each liquid in the vapour is proportional to the SVP of the liquid.
factors which affect the value of the partition coefficient
- Temperature- The solubility of a solute in a solvent is affected by temperature.
- Molecular Form- The solute must be in the same molecular form in both liquids.
- Solubility- The solute must not be very soluble in the liquids.
what is solvent extraction?
Solvent extraction is a process by which a liquid is used to partially remove a solute from an original solvent.
properties of an extracting solvent
Immiscible with the original solvent such that the solutions can be easily separated after extraction.
Volatile such that the solute can be easily obtained by evaporation.
best method to do solvent extraction
better to extract the solvent in smaller portions and perform many extractions rather than using all the solvent in one extraction
what is chromatography used to do?
Chromatography is used for separating and analysing mixtures which are difficult to do so by other means.
types of chromatography
- Paper Chromatography
- Thin Layer Chromatography
- Column (liquid) Chromatography
- Gas-Liquid Chromatography
what does chromatography involve?
partitioning of the components of a mixture between a pair of immiscible liquids (gas)
how is partition coefficient obtained?
- A solute X is added to a mixture of two immiscible liquids. Molecules of the solute will dissolve in both liquids.
- Sufficient solute is added until each liquid is saturated with the solute.
- At this point of saturation, there is a state of dynamic equilibrium achieved where molecules of the solute move from one liquid to the other at equal rates.
- At this time, a ratio of the solubility of the solute in the liquids is constant. This constant is called the partition coefficient/ distribution constant.
what are the phases of chromatography?
stationary and mobile phase
what is the stationary phase?
solid/ liquid used to absorb components of the mixture which is being analysed
examples of stationary phase in GLC
wax/ grease
methyl silicone
methyl phenyl silicone
what is the mobile phase?
liquid/ gas which allowed partitioning of components as they moved through the stationary phase
what happens when the mobile phase moves up the paper by capillary action?
The components of the mixture begin to partition between the 2 phases. A substance more soluble in the mobile phase will move higher up the paper.
The known substances also move up the paper
how is Rf calculated?
Rf = (Distance moved by component)/(Distance moved by mobile phase)
what happens in two layer chromatography?
In two way chromatography, the chromatogram obtained by using one mobile phase is turned through an angle of 90o. Another mobile phase is allowed to move up the paper in this new direction.
what are the advantages of TLC over paper chromatography?
- The stationary phase can be changed.
- Since the stationary phase is more compact, the chromatography develops faster.
- Smaller amounts of samples can be analysed by TLC.
what is the process of thin layer chromatography?
- Substances such as silica oxide, aluminium oxide, and cellulose are mixed with a solvent and made into a paste.
- The paste is then used to evenly coat a strip of glass or hard plastic material.
- The strip is allowed to dry within an oven.
- The reference line is drawn and the mixture and known substances are placed on the line and the chromatography is developed in the same way as paper chromatography.
how are the components obtained in column chromatography?
Each component is obtained by running out from the end of the column. The component is then separated from the slurry.
what is replaced in high performance liquid chromatography?
the column is replaced by a series of narrow tubes containing the stationary phase. The mobile phase is pumped through the narrow tubes.
how can HPLC be used?
HPLC is also used to test the purity of pharmaceuticals and to analyse food and beverages for additives and preservatives.
what is analysed in gas liquid chromatography?
a mixture of volatile liquids or gases
what is the solvent front?
the leading edge of the solvent
