Simple Distillation Flashcards
Boiling Point
Temperature at which the VAPOR PRESSURE of a liquid equals the EXTERNAL PRESSURE on the surface of the liquid
Vapor Pressure
Pressure exerted by the vapor right above a liquid
–> All liquids have some kind of vapor pressure exerted at any given time right above the liquid!
Relationship between liquid volatility and vapor pressure
The higher the volatility of a liquid, the greater the vapor pressure
(think of vapor pressure kind of like the “amount” of vapor above a liquid; a volatile substance evaporates more and so it produces more vapor and therefore has a greater vapor pressur)
Why do volatile substances boil so easily?
Because the vapor pressure is already so high that it is almost equal to atmospheric pressure
–> Therefore these substances typically only need a little bit of heat in order to increase the vapor pressure to be equal to atmospheric pressure
Why does vapor pressure need to equal atmospheric pressure for boiling to occur?
Think of the atmospheric pressure as a force pushing down on the escaping gas molecules and trying to force them back down into liquid
Think of the vapor pressure as a measure of how hard the liquid molecules are trying to escape out as a gas
–> It’s like a battle, and the atmospheric pressure “loses” (can’t beat down the gas molecules anymore) once vapor pressure is equal to or greater than the atmospheric pressure = boiling (in which the liquid can evaporate freely!)
–> However, if vapor pressure is lower than atmospheric pressure, the atmosphere “wins” and effectively pushes down or contains the gas molecules to the liquid
How can we make things boil?
1) Increase the vapor pressure (heating)
2) Decrease the atmospheric pressure (vacuum)
Why does heating a liquid lead to boiling?
Heating increases vapor pressure!
–> Makes it easier to overcome the atmospheric pressure
Why does a vacuum lower the boiling point of a liquid?
Because it decreases the atmospheric pressure (making it easier for the vapor pressure to overcome it!)
Distillation
Process of heating a liquid to its boiling point, allowing evaporation to occur, cooling the hot vapor, and then collecting the condensed liquid in a separate receiver
Distillation Set Up
1) RBF clamped to ring stand
2) Heating mantle connected to Variac with RBF sitting in it
3) Microdistillation apparatus attached to the RBF
4) Thermometer inserted into Microdistillation apparatus with an O-RING
5) Thin-walled tubing connected to the water inlet (top) and outlet (bottom)
6) Receiving container (flask, graduated cylinder, etc.)
What is the part of the microdistillation apparatus that has cold water running through it?
The Jacket
Distillate
The substance being distilled and then collected (gets condensed)
How can distillation be utilized for identifying compounds?
Through measuring boiling point!
When distilling a substance, you can measure its boiling point using the thermometer and compare it to the literature value to see what it may be
Other than identification, what else can distillation be used for?
Purification of a substance or separation of a mixture
–> Mixture MUST be of compounds with LARGE boiling point differences (> 50 degrees C)
What are the factors that impact boiling point?
1) Intermolecular forces (stronger intermolecular forces = HIGHER BP)
2) Molar mass
3) Branching
How does length or size of a molecule impact boiling point?
The greater the size/length of a molecule (higher molecular weight), the greater the boiling point
–> Increasing the size of the molecule increase van der waals forces by creating a larger surface area by which other molecules can interact with a given molecule through dispersion forces
Strength of intermolecular forces
Strongest to weakest:
1) Ion-dipole interactions (ionic compounds)
2) Hydrogen bonding
3) Dipole-dipole interactions
4) Van der waals
What effect does molecular weight have on boiling point?
For a given set of molecules with the SAME functional group/s
–> Increasing molar mass = higher boiling point!
What decreases boiling point?
BRANCHING
The greater the amount of branching, the lower the surface area of the molecule that can have intermolecular forces, and therefore the lower the boiling point!
Which has a higher boiling point: ketones or esters?
KETONES –> Have a double bonded Oxygen (RCOR)
esters = Have a double bonded and single bonded oxygen (RCOOR)
Ketones have the higher boiling point DUE TO GREATER DIPOLE MOMENT
–> In esters, the resonance present delocalizes the concentration of the charge and therefore decreases the dipole moment
–> Ketones do not have this resonance and so they have STRONGER dipole moment = HIGHER BP
What does the distillation curve look like for a PURE species?
Plot of Volume of distillate vs Temp:
Rapid increase followed by prolonged + steady plateau, followed by a drop off at the end
1) Rapid increase portion = FORERUN
2) Plateau = Boiling point of distillate!
3) Drop off = run out of sample to create enoguh vapor to condense
What is T(head)?
A measure of the temperature of the VAPOR right before it condenses
Binary Mixture
A mixture of miscible liquids
Vapor pressure for binary mixtures
Each component (each liquid’s individual vapor pressure) contributes to the overall vapor pressure of the mixture
P(total) = P(A) + P(B) + P(C) + … (Dalton’s Law)
What determines the partial pressure of each liquid in a binary mixture?
Dependent upon the MOLE FRACTION (percentage of a specific compound’s presence in a mixture)
P(A) = P(pure A) x (mole fraction of A)
When you start boiling a mixture, what will the composition of the distillate be?
The composition of the distillate will be the same as the composition of the VAPOR
–> The composition of the VAPOR will NOT be equal to the composition of the liquid mixture (due to different boiling points and thus different amount of each liquid vaporizing out of solution)
Vapor-Liquid Phase Diagram
Describes the distribution of a chemical species between the vapor phase and a liquid phase.
Has two curves:
Bubble point curve
Dew point curve
Bubble Point Curve
LOWER curve on graph
–> Indicates the temperature at which a mixture of liquid BOILS
Think bubbles = boils
Example:
Dew Point Curve
UPPER curve on a graph
–> Indicates the temperature at which a mixture of vapors CONDENSES into liquid
What are the axes of a Vapor-Liquid Phase Diagram?
X-AXIS = Mole fraction of a component of a given mixture
Y-AXIS = Temperature
Finding boiling point of a mixture from the Vapor-Liquid Phase Diagram:
1) On the X-Axis, locate the mole fraction of the substance represented on the axis that matches the composition of your mixture
2) In a straight line, go up from that point until you hit the bottom (bubble point) curve
3) Check the Y-Axis at this point and whatever the value is, that is the BP for that specific mixture composition
For pure species, what is the relationship between the bubble point and dew point curve?
The bubble point and dew point will be EQUAL for a completely PURE species (100% composition)
–> Pure species can be found at INTERSECTIONS of the dew point and bubble point curves
Determining the VAPOR composition for a given liquid mixture composition
(from the Vapor-Liquid Phase Diagram)
1) On the X-Axis, locate the mole fraction of the substance represented on the axis that matches the composition of your mixture
2) In a straight line, go up from that point until you hit the bottom (bubble point) curve
3) Once at this point, go in a straight line to the RIGHT until you hit the dew point (UPPER) curve
4) The X-axis value of this point on the dew point curve is the composition of the vapor at that earlier specific liquid composition
How does the X-Axis differ in meaning for the bubble point and dew point curves?
Bubble Point Curve –> X-axis = composition of the LIQUID that is boiling
Dew Point Curve –> X-Axis = composition of the VAPOR that is CONDENSING
Fractional Distillation
Distillation in “steps” or a series of condensation-evaporation events
The starting mixture will boil and certain components of the mixture will vaporize out (the vapor produced will condense) and this then causes the composition of the mixture to change
This mixture with a new composition then will boil and more components will vaporize out and the entire process will keep repeating until the components are separated
Components of a distillation curve
1) Forerun (RISE)
2) Pure LOW boiling component (being vaporized out of solution) (PLATEAU)
3) “New” mixture heating period (new as in new composition due to the vaporization of the low boiling component (RISE)
4) Pure HIGH boiling component (being vaporized out of solution (PLATEAU)
What happens to the composition of the liquid in the flask as the low boiling component gets distilled?
The composition of the liquid in the flask gets enriched with the HIGH boiling component
Azeotrope
A liquid mixture in which at its boiling point, the composition of the vapor and the liquid are EQUAL (occurs in NON-Ideal solutions)
–> Mixtures that BEHAVE as pure substances (dew point and bubble point curves INTERSECT)
Represent a DEVIATION from Raoult’s Law!!!
–> The azeotropes are the points of strongest deviation from the ideal solution
Positive vs negative azeotrope
Positive Azeotrope= Has a boiling point LOWER than the pure lower boiling component (MINIMUM)
Negative Azeotrope = Has a HIGHER boiling point than any of its individual components (MAXIMUM)
What occurs when you distill a mixture that has a POSITIVE azeotrope?
The azeotrope has a LOWER BP and therefore is the FIRST point at which boiling
of the mixture will occur!
–> Therefore, the LOWER boiling species cannot be isolated
What occurs when you distill a mixture that has a NEGATIVE azeotrope?
The azeotrope has a HIGHER BP and due to compositional changes of the liquid, is the FIRST high boiling point of the mixture to be reached
–> Therefore, the HIGHER boiling species cannot be isolated
What is this?
Positive azeotrope
What is this?
Negative Azeotrope
