Distillation: Ochem lab-George
Raoult's Law
- Px = P°xXx P°x = Vapor pressure of pure liquid X at a given temperature Xx = Mole fraction of liquid X - Xx = molesx / total moles of solution
Boiling Point
- Temperature at which the vapor pressure of a liquid equals the pressure exerted on it by its surroundings
Vapor Pressure
- The pressure that a liquid exerts on its surroundings - As the temperature of a liquid increases and external pressure stays constant, the liquid's vapor pressure increases
Explain Fractional Distillation
- Used for the removal of two components that are solublehave boiling points less than 60-70 °C apart - The efficiency can be adjusted by altering the apparatus used
high reflux ratio
allows for the highest amount of purity obtained for any distillate in a given apparatus
The condenser will condense the vapor into liquid, and the liquid will pour into a _____
collection vessel
During the experiment, the fractionating column is heated by conduction through contact between the ___ & ___
column and vapor (this means the column will not get hotter than the temperature of the vapor making it)
When the vapor exits the distillation head and comes in contact with the water-cooled glass of the condenser, it _____________________
condenses back to a liquid and pours into the collection vessel
Only ____ vapors will be able pass further up the fractionating column without condensation occurring
cooler (vapors with high concentrations will not reach the top of the fractionating column b/c these vapor's will condense when they come in contact with cooler glass)
two columns of differing length with the same HETP will have ______ of theoretical plates
different amounts
theoretical plate
point where a vaporization-condensation cycle can occur
As the distillation head fills with vapor, the vapor will eventually spill over into the _____ which is connected to the ____
side-arm water-cooled jacketed condenser
Which type of distillation travels a shorter path?
simple distillation
Liquid boils when the vapor pressure of the liquid equals
the pressure that is applied by the atmosphere on the liquid
Dalton's Law
- Ptot = PA + PB Ptot = Total vapor pressure Px = Vapor pressure of compound X at a given temperature
Combine Dalton's and Raoult's Laws
- Ptot = P°AXA + P°BXB
Increasing the path length in fractional distillation causes more points for the vapor to condense on when coming in contact with the apparatus. As surface area increases, ___________ increases
# of vaporization-condensation cycle (theoretical plates)
Explain Simple Distillation
- A max of 2 to 3 condensation/vaporization cycles occur, so any liquid purifications are quite crude - Useful for the removal of non-volatile components that are dissolved in a liquid (Solid organic compound dissolved in solvent) - Useful for the removal of solid impurities from a liquid (Removal of reagents used to chemically remove water in organic solvents) - Useful technique if two compounds have a boiling point greater than 100 °C apart (minimum temp. diff. is 60-70 °C) - Also useful technique if liquid is almost already relatively pure (Typically greater than 90 - 95% pure before distillation)
This lab requires MSDS information for:
- Acetone - Toluene
Maximum boiling azeotrope
- Boiling point of two component system is higher than either individual component - The pot residue approaches the azeotropic concentration as the process advances •Given enough theoretical plates, pure AOR pure B will distill off first • Pot residue will concentrate the components up to the azeotropic concentration • Never distill the flask to dryness; therefore, what is left in the pot at the end of the process is the azeotrope
Minimum boiling azeotrope
- Boiling point of two component system is lower than either individual component - The azeotrope distills first • As ethanol concentration drops in the original solution, the boiling point of the pot residue (what is left in the round bottom flask, which is water) rises - This signifies the pot residue moving away from the azeotropic concentration • Given enough theoretical plates (lets say infinity), the azeotrope will still be the first component to exit the apparatus
Gas chromatography is useful for
- Compound purity -Determine the number of components in a system -Compound identity
GC utilizes a two phase system
- Liquid (stationary phase) - Gas (mobile phase)
what is HETP and its equation?
-the efficiency of a fractionating column -the number of "plates" per length of fractionating column HETP= height of fractionating column (H) / number of theoretical plates (Nt)
760 mmHg = ___________
1 atm at STP
Clamp in 2 locations
1. Distillation head 2. Condenser
Six important parts of Distillation set-up
1. Heat source 2. Round bottom flask 3. Distillation head 4. Thermometer & adapter 5. Condenser 6. Receiving container -Use boiling chips or a stir bar
Two types of Azeotropes
1. Minimum boiling azeotrope 2. Maximum boiling azeotrope
Two types of Distillation
1. Simple Distillation 2. Fractional Distillation
Purpose of distillation experiment (2)
1. To learn how to identify the individual components of binary solution throughout the use of fractional distillation 2. To explore the usefulness of gas chromatography (GC) to qualitatively determine the efficiency of a fractional distillation through evaluation of three samples ( initial mix, first few drops, last few drops)
If we don't know the ratio of the components (also called mole fraction) , then we need to know
1. Total pressure 2.Partial pressures of pure substances at the given temp.
Explain two types of columns in Gas Chromatography
1. capillary column: hollow, smaller diameter, but much longer than a packed column 2. packed column: filled with porous sorbet material
components/steps needed for a gas chromatograph
1. constant source of gas (Hs, He, N2) -all components introduced into a GC must be convertible to (or already) a gas within the instrument 2. the injector (where the sample is introduced to GC using a needle & syringe) -needs to be hot in order to turn the solvent into a gas -connected to a column which contains an immobile sorbent material (nonvolatile liquid) on a solid support backing 3. column is housed in an oven that keep its temperature high (two types of columns) -works by causing an equilibrium for each individual compound determined by the amount of time each compound spends in the gas phase vs liquid 4. connected to the end of the column is a detector -different sector types tailored specific analysis -we will use TCD, which utilizes a heated wire with a voltage applied to it 5. Info from detector charted using physical chart recorder or computer program to be interpreted
lower boiling componenet is _____ the higher boiling component is ____
1. hexane (.70 vapor rate) 2. heptane (.30 vapor rate)
(GC) organic compounds have lower heat capacities than the carrier gas, they will remove _____ heat from the wire which causes the resistance of the wire to ____
1. less 2. increase
Two types of azeotropic systems are ____ & ____ an example of each type are
1. minium boiling azeotropes (or "positive azeotropes)- BP lower than any pure substance Ex: ethanol and water 2. maximum boiling azeotrope (or "negative azeotropes) Ex: HCL dissolved in water
As the hexane is removed and the liquid in the distillation flask becomes more rich in heptane.. what are the 2 consequences of this?
1. the BP of the mixture in the distillation flask increases 2. any new vapor generated from the flask will be richer in heptane
How gas chromatography works
A liquid sample is injected • The gas is then loaded onto the column • Depending on oven temperature, the various components of the sample will spend different amounts of time in the liquid and gas phases • Just like in a drag race, the faster car wins (assuming both started at the same time, of course) • The finish line is where the detector is
When separating in fractional distillation _______ component will be removed first, and the ______ will be removed last
A. Lowest B. Highest (Azeotropes can pose a problem here)
- As external pressure decreases, a compound's _____ will increase and its _____ decreases
A. Vapor Pressure B. Boiling point • Water boils at 71 °C on top of Mt. Everest (29,029 feet above sea level) • Less air pushing down on the liquid causes the vapor pressure of water to increase at a given temperature
As external pressure increases, a compound's _______ will decrease and its _______ increases
A. Vapor Pressure B. Boiling point • The boiling point of water in death valley (285 feet below sea level) is 100.3 °C • More air pushing down on the liquid causes the liquid's vapor pressure to decrease at a given temperature
Height Equivalent of a Theoretical Plate (HETP)
Describes the distance between theoretical plates The smaller the number, the better the fractionating column's efficiency The number of theoretical plates of a distillation column is related to its height and HETP value HETP = Height/Nt Vigreux fractionating columns used in this lab have approximately 6 theoretical plates (Nt) Nt = 6 theoretical plates The height of the Vigreux fractionating columns being used for this experiment are approximately 15 cm long
The separation during fractional distillation can be described by what two laws?
Dalton's Law and Raoul's Law (They can be described pictorially with phase diagrams)
If a solution is an ideal one, then _____ law applies
Raoult's Law
Detector used for this class
Thermal conductivity detector (TCD) - Rhenium and Tungsten are the main components Current across a hot wire changes as the gas composition passing the wire changes • Gas with organic compounds in it will remove less heat from wire, causing the wire's resistance to increase
Water flows ______
against gravity
define azeotropic systems
a solution of two or more liquids that has it's own unique BP that is higher or lower than either individual component
The temperature of the vapor will not exceed its ________ because no additional heat is applied to the vapor outside of the flask
boiling point
Compounds separate by differences in _____ & _____
boiling point and differences in interactions with the stationary phase
Note that Xx is a mole FRACTION, so the sum of all components must _________
equal 1 - XA+XB=1
The more time spent in mobile phase, the ________ the compound moves through the column
faster
the longer a compound spends in a gas phase during GC, the ______ moves through the instrument
faster
the best utilization of fractionating distillation prescribes heating the system slowly because this allows ______
for the largest amount of vapor to be returned to the vessel through condensation and the smallest amount of vapor to escape the system at any given time
A ______ is added to the distillation setup to increase the distance over which the vapor must travel before reaching the distillation head and condenser
fractionating column
The term "plate" comes from distillation columns that have actual plates inside the _______
fractionating column
What is the temperature gradient like in the fractionating column during the experiment?
hotter at the bottom and cooler at the top
As hexane (.70 vapor rate/lower BP) is removed from the liquid in the distillation flask, that liquid becomes ____________
increasingly rich in heptane (.30/higher boiling point)
the more vaporization-condensation cycles a fractionating column is capable of, the more
initial components of a solution can be separated & the more efficient the column is
(GC) The solubility of the compound in the stationary phase is dictated by the
intermolecular forces
Separation in fractional distillation is achieved by __________
lengthening the distance the vapor must travel before it is cooled and collected
Gas Chromatography "how to" & identification
look at powerpoint
Each plate represents a ___________and catches compounds that condense at that temperature
specific temperature -There are holes in the plates to allow vapor to rise until it hits a temperature that cools it enough to cause condensation
Heated liquid cannot exist in the liquid phase past its boiling point at ____________
standard pressure
What separates a simple distillation from a fractional distillation?
the # of vaporization-condensation cycles that occur between the distillation flask and recovery flask
The term "theoretical plate" describes ____
the efficiency of a distillation system (Or any apparatus that separates individual components of a system)
Gas Chromatography Parts
• Carrier gas • Injector - Hot! - Vaporizes sample, turning liquid into gas • Column oven - Oven helps keep the sample volatilized • Column - Interacts with compounds injected to varying degrees and is compound specific - All compounds injected need to be soluble in coating to varying degrees to get separation • Detector • Chart recorder/computer
Different column types
• Packed column - What you will be using! • Capillary column - More accurate - Smaller volumes - More expensive - Easier to destroy
