Experiment 4 Part 1: Distillation
The efficiency of separation achieved in a fractional distillation depends on a number of factors:
1. SURFACE AREA OF THE PACKING MATERIAL IN THE COLUMN: the heat transfer between the liquid and Vapor phases in the column is more efficient when there is a large surface area 2. LENGTH OF THE DISTILLING COLUMN : the longer the column the greater the number of condensation vaporization Cycles there will be as material moves up the column 3. DIFFERENCE IN THE BOILING POINTS OF THE COMPONENTS BEING SEPARATED: the greater the difference the more easily the components may be separated 4. THE RATE OF HEATING: if heating is to rapid the quantity of vapor produced may exceed the capacity of the column to effect equilibration, and separation efficiency will suffer .On the other hand, if too little heat is supplied the rate of distillation will be too slow to be practical
What is azeotrope?
2 or more compounds with different boiling points that forms a soln that boils at a constant BP with constant composition.
Example of low-boiling azeotrope?
95%ethanol and 5%water, the ethanol-water azeotrope has BP of 78.15 C which is below BP pf pure 100% ethanol which has BP of 78.3 so max degree of purity using distillation is 95% ethanol
What is a solution?
A homogeneous mixture of two or more components
If you have a equimolar mixture of volatile compound x and nonvolatile compound y explain the pressure
If BP of pure x at atm pressure is 100 C the vapor pressure exerted would be 760 torr but in the hypothetical solution the vapor pressure would be lower than 760 torr bc half of the sites at the surface of the soln are occupied by nonvolatile compound y meaning only half of x can reach the surface and escape so the pressure of the solution (at 100 C) would be 380 torr
What does the the constant BP (at atm pressure) of a pure compound tell you?
In distillation the pot temp would remain constant from boiling to completion. Also the distillation head temp will remain constant.
Explain what happens in the distillation pot and distillation head
In the pot the comp of the liquid becomes richer in nonvolatile com y so the mole fraction will decrease. So BP will increase as distillation progresses and pot temp will need to be increased to maintain boiling. In distillation head the temp remains constant and corresponds to BP of pure x.
What effect did the introduction of nonvolatile solute have on vapor pressure?
It decreases vapor pressure. The extent by which vapor pressure is reduced will be directly related to the proportion of NONVOLATILE material added.
What is the force of attraction?
It is what allows to keep molecules in condensed phase.
How does a simple distillation work?
Liquid to be distilled is placed in distillation pot and heated to boiling. Vapor rises into head and passes through side arm. Thermometer placed in distillation head to record temp of vapor before it escapes to condenser. Cold water passing through jacket of condenser cools vapor so that it condenses. The condensate flows down to condenser, where it is delivered through bent adapter to receiver vessel.
Lab manual reference: figure 3 page 45
Make sure you know how to read this graph for example, if you have 30% Y and 70% X the initial temperature is 85 C. At 85 C the initial comp of X and Y is 10% Y and 90% X. The paragraph following that graph also shows how to read the graph.
Van der Waals (London dispersion)
Molecules that have no permanent dipole moment exhibit these intermolecular interactions that are very weak.
CASE 1
Nonvolatile solute in volatile solvent
Mole fraction formula
Nx=nx/(nx+ny+nz) this solution contains nx moles of x, ny moles of y, and nz moles of z.
Liquid-vapor interface
Phase equilibrium between pure liquid and pure vapor phases.
Hydrogen Bonding
Polar molecules that have dipole-dipole interactions between sites of higher electron density of one molecule and low electron density in neighboring molecules. Structures include functional groups in which hydrogen is attached to a EN atom (F, O, N)
If compound X is more volatile than compound Y, which is greater Px° or Py°?
Px°
What is fractional distillation?
Separates 2 or more volatile components with different boiling points it is like multiple simple distillations
CASE 2:
Soln of 2 volatile compounds
What do you call the initial liquid if it is NOT a pure compound?
Solution
What is always true about vapor pressure in relation to volatility?
The composition of the vapor is always richer in the more volatile compound than the liquid with which it is at equilibrium. What this means is a more volatile compound will lead to a greater contribution to vapor pressure.
How are the three states of matter distinguishable?
The degree of order in the arrangement of molecules or ions present.
Difference between simple and fractional distillation
The distilling column is "fractioning column" which is used in fractional, simple uses condenser
Behavior of boiling point/temp
The initial BP of soln is between the 2 boiling points of comp X and Y and gradually rises to reach the BP of the higher boiling compound.
Why is the original mixture so important?
The nature of the original mixture determines the degree of purification. Simple Distillation: If the compound is much more volatile than the contaminants present, separation is easy. Fractional Distillation: Mixtures of more than 1 volatile compound.
What is distillation?
The separation and purification of volatile liquids.
What needs to happen for pressure to = 760 torr?
The temp has to be much higher than 100 C, the extent of BP elevation resulting from into of nonvolatile solute depends on concentration of compounds in soln. Thus, initial BP would be higher than BP of pure x.
What is the thermometer in the distillation head measuring?
The temp of a vapor-liquid equilibrium, some of pure vapor condenses to liquid on surface or thermometer bulb at this point.
What is vapor pressure?
The vapor pressure of a liquid is a measure of the number of molecules that are escaping into the gas phase. # of molecules which reach surface of liquid moving with sufficient kinetic energy, and travelling in the right direction to overcome intermolecular forces and external pressure.
In distillation of a pure substance the pot and head temp are constant and equal corresponding to BP of distillate. True or False
True
The temp in distillation head will always be lower than pot temperature. True or False
True
What is a heterogeneous mixture?
Two immiscible liquid phases, it involves "steam distillation" which involves water as one of the two phases. (stem is useful for organic comps)
What contributes to total vapor pressure in case 2?
both volatile compounds and the total vapor pressure can be expressed by Dalton's law
Lab manual reference: page 47
middle of top paragraph talks about figure again
What is the significance of pot and head temps of distillating a solution (NOT a pure compound)?
pot and head temps are never = and vapor and liquid phases are not of = composition
What is mole fraction?
proportion of all of the molecules in the solution that are molecules in the solution that are molecules of the given compounds.
What do azeotropes do?
some boil at temps lower than BP of any of the individual components while some have BP higher than those of any of the pure components
What is another way of defining boiling point?
temp at which pure liquid and pure vapor are in equilibrium.
Information from figure 2: Kinetic Energy Distribution of Molecules (page 41 of lab manual)
"E" on the graph represents kinetic energy a molecule must possess in order to escape from the liquid phase into the vapor phase. At low temperatures (Plot A), very few molecules have sufficient energy to make the transition. If temp is increased the curve is shifted to the right and a very large portion of molecules have kinetic energies in excess of that required to overcome the forces holding them in the condensed phase.
How can you lower boiling point? What do you use? What cases would this apply to?
- Lower BP by lowering pressure over system - Use aspirator or vacuum pump to reduce pressure in closed system - This applies for purification of compounds whose BP at atm pressure are high enough that thermally-induced decomposition occurs during distillation
What is gas chromatography?
- analyze mixtures of volatile components - obtain info about number of components and relative proportions using a sample as a mg or less -data is presented in a graphic plot known as "gas chromatogram" or "GC Trace"
How does the fractional distillation work?
-The vapor in pot rises up to column and it is partially condensed as it cools in the column -Condensate runs back down column and encounters hot vapors rising up the column -Heat transfer between condensate is partially vaporized while vapor is partially condensed -Each time partial condensation-revaporization occurs the vapor becomes further enriched in the more volatile component and returned into pot -After a few more partial condensation-revaporization cycles, the vapor reaching the head at the top of the distilling column would be nearly pure x
Distilling columns
-they function as a inefficient condenser, this column is constructed with a jacket that makes is less susceptible to drafts -it is very similar to a condernse but it is NOT cooled with water -the material inside the column is some sort of inert material like glass beads, rings, steel wool, etc. which provides large surface area
How does distillation work?
An impure liquid is vaporized by heating, and the vapor is separated from the less volatile material, collected, and cooled. Successful distillation means the "distillate" (liquid that is collected as the vapor condenses) will be in a higher state of purity than the original sample before distillation.
When does vapor pressure stop increasing?
As heat is applied to distillation pot the temp of the liquid increases as well as the vapor pressure, the vapor pressure increases until the boiling point is reached. At this temp the vapor (of substance being distilled) displaces air molecules from region above surface of liquid.
Why is simple distillation not a effective method of separating 2 volatile compounds?
Because it is very hard to separate them and the exact compositions may not be known you would have to do multiple distillations to collect the more volatile distillate.
Why does the head temp not immediately rise when BP begins?
Because it takes time for vapor to be produced in pot to reach thermometer. (in early stages you will see a ring of liquid condensing on inside walls before glass heats to equilibrium)
Why does the head temp remain constant?
Because only the volatile compound reaches the thermometer in the head so temp in the head is in equilibrium between pure vapor of x and pure liquid x as partial condensation takes place on thermometer bulb.
Formulas
Daltons law: Ptotal=Px+Py Contributions: Px=Px°Nx and Py=Py°Ny (x is more volatile in this case) Starting mixture: Nx=Ny=1/2 Raoult's law: Px=Px°/2 and Py=Py°/2
In a liquid non state of non-ionic compound, what do the forces of attraction depend on?
Depend on the nature of the substance.
What is vacuum distillation?
Distillation in a system under reduced pressure.
What happens to the vapor pressure exerted by a liquid if the temp increases?
Vapor pressure increases
Vapor behavior
Vapor produced in the pot is richer in the more volatile component but vapor composition also changes until vapor and liquid in the pot are nearly pure y when distillation is almost complete
What is boiling point?
When the vapor pressure of a liquid is = tp external pressure over the liquid (atm pressure) Usually 760 torr unless otherwise mentioned
What does it mean when something is more volatile?
has a lower bowling point
Raoult's Law
the vapor pressure of a solution is directly proportional to the mole fraction of solvent present. This law is used to describe the relationship between vapor pressure and comp of a species. Px=Px°Nx (x is subscript) Px=partial pressure of x at any given temp Px°= vapor pressure of pure x at that temp Nx=mole fraction of x in solution
