CM 1021 and 1031
Laboratory Manual: Aug-Nov 2020
Division of Chemistry and Biological Chemistry
School of Physical and Mathematical Sciences
Nanyang Technological University
CM1021-CM1031 combined lab manual 2020
CONTENTS
Introduction
2
Grading
2
Online Resources, QR codes
3
Schedule of Experiments
4
Safety
5
Glassware Guide
9
Laboratory Accuracy
11
Experiments:
1. Potentiometric Titration (1021)
12
2. Recrystallisation (1031)
15
3. Calorimetry – the Oxidation of Magnesium (1021)
17
8. Acid-Base Extraction (1031)
20
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CM1021-CM1031 combined lab manual 2020
Introduction
Practical work is at the heart of chemistry. In industry and research,
most chemists are working at making molecules or measuring their properties
and concentrations. The CBC laboratories classes are an essential part of
your training as a professional chemist. Lab experiments can also
demonstrate chemical principles more effectively than any lecture.
Due to the COVID-19 pandemic, workplace distancing is required in
the labs. A result of this is that fewer students can be present in the lab at any
one time and, therefore, fewer experiments can be done “hands on”. You will
do only four hand-on experiments, two for CM1021 and two for CM1031.
Other experiments will be done virtually. Details of virtual experiments will be
given separately on NTU Learn.
The four experiments in this manual are intended to start your practical
training and illustrate important principles from the CM1021 and CM1031
lectures. We suggest that you keep this manual after the semester is over.
We can promise that you will be using the techniques that it teaches many
times in the coming years.
We welcome your feedback. Send us your suggestions on how we can
make the course better.
Grading & Lab Reports
Your lab work will contribute to your grade in each of CM1021 and
CM1031. The contents page shows which experiments belong to which
course.
For each experiment, there will be a prelab quiz. These are available
through the NTU Learn system and must be completed by the specified
deadline. The quiz contributes 10% of the lab grade. Safe and appropriate lab
behaviour, assessed by the supervising staff and teaching assistants,
contributes an additional 20%. Your report, using the pro-formas supplied to
you, contributes the remaining 70%. Reports that do not use our pro-formas
will not be accepted. All lab reports must be submitted online (see NTULearn
for instructions). This means that all written parts must be completed by typing
into the pro-forma provided in the LAMS sequence. Chemical structures
should be created using Chemdraw (see the e-mail from CBC on how to get
this programme for free) and pasted in. Other drawings should be created
using an appropriate programme (even Chemdraw) and pasted in.
Alternatively, drawings can be done by hand and converted into jpg images.
There is a compulsory online safety quiz which must be taken before starting
laboratory work.
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CM1021-CM1031 combined lab manual 2020
Online Resources
When you go into NTU Learn for CM1021 Lab or CM1031 Lab, you will
first need to view the lab video. Watch this carefully and read the lab manual.
When the video is over, you will be able to take the online quiz. Only after
that, will you be able to download the pro-forma.
It is your responsibility to get yourself in front of a working
computer in good time. You are strongly advised to not procrastinate – do
not leave it until the last minute. Please note that neither CBC staff nor CITS
are able to address technical problems outside of normal working hours.
Copies of the lab manual and the pro-formas can be obtained through
the NTU Learn system. You can also find photographs of the experimental
set-ups for each experiment. The video may also be viewed during the lab on
the computers provided. This is NOT a substitute for not viewing the video
before coming to lab.
The videos can also be accessed through QR codes, which have been
inserted into this lab manual. If you have a smart phone with the right app,
you can scan the codes to watch the video, even in the lab. Please note that if
you print out this manual at less than one A4 page per page, then the QR
codes may not work. QR codes will work just as well in black & white.
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CM1021-CM1031 combined lab manual 2020
Schedule of Experiments
The detailed schedule of experiments will be posted on NTU Learn.
When you arrive at the lab, you must check in, then check out when
the experiment is over and you leave. You will need to have your matric card
with you for check in and check out. You will also need to bring your matric
card to enter the lab!
The lab staff will check the condition of your experiment equipment as
you leave.
Please arrive promptly for your labs and please have everything
cleaned up and be ready to leave by the official closing time (12.30 or 4.30).
Students who are late leaving the lab will lose points.
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CM1021-CM1031 combined lab manual 2020
Safety Practices in the Chemistry Laboratory
Safety in the chemistry laboratory depends on a cautious attitude and an
awareness of potential hazards. Each person in a laboratory is responsible for
the safety of everyone present. Risk assessment before coming to the lab is
an essential part of accident prevention. The risk assessment table on each
pro-forma must be completed before coming to the lab.
Risk Assessment
Identify one hazard associated with this experiment and give one reasonable
precautionary measure.
Hazard
Precaution
1 concentrated sulphuric handle in the fume cupboard; wear gloves;
acid: corrosive
treat spills with Na2CO3 or a similar reagent
An accident in a chemistry laboratory can cause serious injuries or even
death. However, potential hazards can usually be anticipated, thus preventing
most accidents. The number of laboratory accidents can be reduced if all
safety precautions and directions given for each experiment are strictly
followed by every student. Special note should be taken of specific
instructions concerning potential hazards. In general, remember these three
rules:



Read the experiment before coming to the laboratory
Use common sense when working with laboratory materials and
apparatus
Know how to get help in case of accident
Fire is a particular hazard in chemistry laboratories. All students taking CBC
laboratory courses must have completed the online safety course.
General Rules For Laboratory Safety
Here are several rules found to be essential in promoting safety in the
chemistry laboratory:
1. SAFETY GLASSES MUST BE WORN AT ALL TIMES!
There is no excuse for injured eyes in a laboratory accident, because
adequate eye protection is always available and should always be worn.
However, even when goggles are worn, contact lenses must not be worn
in the laboratory. It is always possible that corrosive material might flow
under the edge of a contact lens and cause permanent damage to the eye.
Also, contact lenses will hinder immediate and complete flushing of eyes in
case a chemical splashes into them. Ordinary glasses do not provide
adequate protection. If you wear glasses, you must wear safety goggles
over them.
2. Do only the experiment assigned by your laboratory instructor
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CM1021-CM1031 combined lab manual 2020
Never do an unauthorized experiment in place of the one assigned by your
instructor. Do not alter the designated procedure in any way without
obtaining permission from your instructor
3. Know the exact location and operation of all safety equipment
Identify the location of the eyewash fountain, safety shower, fire alarm, fire
blanket, fire extinguisher and emergency exit nearest to your laboratory
bench. Learn the locations of this safety equipment and how and when to
use it. Your actions during an emergency might prevent a classmate from
suffering a serious or fatal injury.
4. Eating (including chewing gum) and drinking is strictly prohibited in the
laboratory.
5. Shoes should be worn that provide full coverage of the feet. Open-toed
shoes, sandals, slippers and high heeled shoes must not be worn in the
laboratory. There should be no gap between the pants and the shoe.
6. Appropriate personal clothing should be worn. A laboratory coat must be
worn unless staff give permission otherwise. However, do not wear your
lab coat outside of the teaching lab. Shorts, Bermudas and skirts may not
be worn in the laboratory. Long pants must be worn. Students who come
to the lab incorrectly dressed will not be allowed in.
7. Long hair should be tied back and ensure that long or large necklaces are
safely tucked away.
8. Use of mobile phones, personal audio equipment and other electronic
items is not permitted in the laboratory. Anything that interferes with your
ability to hear what is going on in the laboratory is a potential hazard.
9. Handle all chemicals with care. When dispensing chemicals great care
should be taken not to contaminate the balances or the benches. All spills
should be cleaned up immediately by the person concerned. Procedures
involving the liberation of volatile, toxic or flammable materials should be
performed in a fume cupboard. Dispose of solvents properly. Return any
chemicals you have used to the shelves IMMEDIATELY for other students
to use.
10. Keep your work area clean at all times and free from chemicals and
apparatus which are not in use. Clean up any spills, including water, on
the floor.
11. Immediately report all physical and chemical injuries to your laboratory
instructor, no matter how minor the injury might seem.
12. Wash your hands thoroughly with soap or detergent before leaving the
laboratory so as to remove all traces of reagents from your skin. Be careful
to avoid transferring reagents to your mouth, eyes, face or other parts of
your body.
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CM1021-CM1031 combined lab manual 2020
13. Any laboratory is a dangerous place – running, jumping or horseplay is
forbidden.
14. Do not wear your lab coat and gloves outside of the laboratory, including in
the toilets. Take your gloves off before touching anything that you would
not want to contaminate. These include door handles, your eyes, face or
hair, your hand phone or smart phone and your iPad.
Unacceptable:
shorts of any kind are not permitted, neither are sandals or flip-flops
No, these are not acceptable either
Finally!
But do tie back your hair if it is long
illustrations © Aekjira Kuyyakanont 2008
Note that these cartoons were drawn before Covid and do not show face masks
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CM1021-CM1031 combined lab manual 2020
Additional Safety Rules due to Covid-19
1. Students must wear a mask at all times.
2. All students must maintain workplace distancing of 1.5 m. When you are
working in the lab you must remain 1.5 m away from other students and lab
staff except when closer proximity is absolutely unavoidable.
3. When queuing (for instance to check into the lab), maintain a safe distance
of 1m from other students.
4. Students must comply with all check in requirements both to the lab and to
the SPMS building.
5. Everyone is encouraged to download the TraceTogether mobile
app (https://www.tracetogether.gov.sg) and do their part to stop the spread of
COVID-19 through community-driven contact tracing.
6. Students who feel unwell with symptoms of covid-19 should seek
immediate medical attention. Symptoms can include difficulty breathing, a dry
cough, a fever, loss of the sense of taste or smell, a sore throat, body ache.
We expect that all students will show common sense and keep to these
safety practices. However, we will deduct marks for unsafe behavior and
we will exclude persistent offenders from the lab.
There is a safety quiz in a LAMS sequence the NTU Learn site for the lab
course. You must pass the quizzes in order to download essential
documents. The safety quiz scores will also affect your final grade. If
you do not pass the safety quiz, you may not work in the lab.
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CM1021-CM1031 combined lab manual 2020
GLASSWARE AND EQUIPMENT GUIDE
Some of the glassware that you will use this semester may be unfamiliar to
you. The glassware guide on this page and the next is to help you identify the
equipment.
beaker
conical flask
or
Erlenmeyer
flask
filter flask
or
Buchner
flask
crystallising
dish
petri dish
round
bottomed
flask
volumetric
flask
separatory
funnel
measuring
cylinder
test tube
filter funnel
Hirsch funnel
thermometer
spatula
desiccator
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CM1021-CM1031 combined lab manual 2020
thermometer
adapter
distillation
adapter or
still head
condenser
fractionating or
distillation
column
buret
Pasteur pipet
or dropper
volumetric
pipet
graduated pipet
10
receiver bend or
adapter
CM1021-CM1031 combined lab manual 2020
Laboratory Accuracy Guidelines
Physical quantities should only be quoted with a degree of accuracy that is
sensible. Many instruments with digital read-outs will give many more
significant figures than can be justified. Students should follow these
guidelines when quoting such quantities.
Quantity
Yield
molecular mass
weights/ masses
number of moles or mmoles
melting points (ranges)
boiling points
reaction temperatures
pressure
1H NMR chemical shift ()
1H NMR coupling constants (J)
IR frequency
Rf
[]D
magnetic moment
Accuracy
whole %age
2 decimal places
3 sig. fig. – but not less than a whole
mg
3 sig. fig.
nearest whole degree
nearest whole degree
nearest whole degree
whole mmHg
2 decimal places
1 decimal place
nearest whole wavenumber (cm-1)
2 decimal places
1 decimal place
2 decimal places
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CM1021-CM1031 combined lab manual 2020
POTENTIOMETRIC TITRATION
1
Titration using an indicator allows us to
see only one event during the titration (the “end
point”). More information can be obtained by
following the pH. This is most conveniently
done using a pH meter. In this experiment, you
will be titrating one of several acid solutions and
plotting the pH against the amount of base
added. From the curve, you can determine the
pKa of the acid. pKa values are used as a
measure of the strength of the acid. For the
equilibrium
We use pKa instead of Ka because it gives us numbers that are easier
to handle. It is important to note that it is a log scale, so a difference of one
unit involves a factor of ten. The pKa scale runs backwards. Less acidic
compounds have higher values. The most acidic compounds have low or
negative values.
HNO3 –1.0
NH4+ 9.3
H2O 14.0
At the start of titration, we just have an acid in water and the pH will
depend on the strength of the acid, and its concentration. Between the start
and the equivalence point, a buffer exists as the acid has only been partly
neutralised. At equivalence, the acid will have been converted to its conjugate
base and the pH will depend on its properties. After equivalence, the pH is
dominated by the amount of strong base (NaOH) that has been added.
You will be assigned one of four organic acids: formic, acetic,
propanoic and chloroacetic. The purpose of the experiment is to determine
the pKa of the acid using potentiometric titration.
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CM1021-CM1031 combined lab manual 2020
Potentiometric Titration – Calibration of the pH Meter
Calibrate your pH meter according to the instructions supplied with the
instrument.
Do not leave the electrode/probe dry for too long when not in use. Either
leave it in sample solution between measurements or in 3M KCl storage
solution if not using for a prolonged period of time. Clean electrode
between measurements with deionized water.
Do not turn off the pH meter.
Collecting the Data
After calibrating your pH
meter, set up your experiment
as illustrated in the picture.
Secure the probe and ensure
that the magnetic stir bar
does not damage the probe.
Start with 50 ml of the acid
solution that you are provided
with. Do a rough titration with
0.1 M NaOH (aq), recording
the pH against the volume of
NaOH (aq) used.
Complete the titration with about another ten readings after the climb starts to
flatten out. Repeat the titration with 0.1 M NaOH (aq), recording more data
points around where you expect the “end-point” to be (the region of steepest
climb in pH in your rough titration). You should expect to take pH readings
every 0.05 – 0.1 mL of NaOH added.
Post-laboratory data analysis
Plot a graph of pH versus volume of sodium hydroxide solution (mL)
added using the data from your accurate titration. You may do this manually
or using graphing software. After you obtain the pH curve, visually pin-point
the part of the curve with the steepest slope (you may want to expand the pH
curve in your spreadsheet programme and zoom in on the relevant portion).
This corresponds to the end-point in a traditional titration using an indicator.
Record both the value of the pH at the end-point and the volume of NaOH
(aq) required to reach the end-point.
Knowing the volume of NaOH (aq) required to reach the end-point of
the titration, you can estimate the half-equivalence point and hence the pK a of
the acid. As the name suggests, the half-equivalence point is where you can
determine half the volume of NaOH (aq) required to reach the end-point. At
the half-equivalence point, the concentrations of the acid and its conjugate
base are equal, and the associated pH of this estimated half-equivalence
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CM1021-CM1031 combined lab manual 2020
point gives you the pKa of your acid, according to the Henderson-Hasselbach
equation.
[H + ][A - ]
Ka =
[HA]
- lgK a = -lg[H + ] - lg[A - ] + lg[HA]
pH = pK a + lg
[A - ]
[HA]
At the half equivalence point,
[A-] = [HA]
 log[A-]/[HA] = log 1 = 0
 pH = pKa
(where HA is a weak acid, A- is the corresponding conjugate base, and pH
and pKa are defined as –lg[H+] and –lg[Ka] respectively)
Using your curve, estimate your half-equivalence point and record the
corresponding pH of that point as the pKa of your acid.
(An alternative, more mathematical way to get to the answer is to fit your pH
curve to a mathematical function and plot the second derivative curve of this
function. The point where the second derivative curve cuts the horizontal axis
gives you the volume of NaOH (aq) needed to reach the end-point and the pH
at the end-point.)
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CM1021-CM1031 combined lab manual 2020
2
RECRYSTALLISATION
This experiment is based on
real research situations. You are
provided with an unknown sample to
purify. It is contaminated with a black
impurity. First you will have to find an
appropriate solvent or solvents by
experimenting on a small scale. You
will then use the solvent that you
have selected to purify the sample.
Recrystallisation depends on the differences in solubility at different
temperatures. The idea is to find a solvent in which the desired substance is
quite soluble when hot (usually the solvent is boiling), but very poorly soluble
when cold (usually either room temperature or ice bath). Impurities that do not
dissolve in the hot solvent can be removed by filtration of the hot solution.
Impurities that are soluble at all temperatures remain in solution when it is
cooled. The pure substance is then obtained by a second filtration. For good
recovery, it is essential to use the minimum amount of solvent.
A more advanced technique involves the use of two solvents, but for
this experiment one is enough.
You will measure the melting point of your purified sample. Actually,
the term “melting point” is a misnomer. You must always report a melting
range, from the temperature at which melting starts to the temperature at
which it finishes. Melting points should be reported to the nearest whole
degree. The melting point of a pure compound should be over a narrow range
(1-3°C). An impure compound will have a broader range. In addition,
impurities tend to lower the melting point of a substance – think about the
Arctic Ocean. How does the water up there stay liquid at below 0°C?
Procedure
Prepare four small test tubes. Place a small
amount of your unknown in each. To each tube
add a little of one of the following solvents:
ethanol, ethyl acetate, hexane and water.
How much solvent?
How much sample?
About the same as
you saw in the video
By observing what happens before and during heating, choose a
suitable solvent for crystallisation, recording the data in the table provided.
Use a hot water bath for heating. A suitable solvent will be one that your
sample (apart from the black impurities) will dissolve in when hot, but will
crystallise out on cooling.
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CM1021-CM1031 combined lab manual 2020
Now recrystallise the remainder of the sample using the solvent that
you have selected. Follow the procedure that you saw in the video.
Prepare for the hot filtration first. Place a filter funnel and a conical flask
in an oven now! Prepare for the cold filtration too: cool some of the solvent
that you will use in an ice bath. Do this by standing a conical flask containing
some of the solvent in a dish or beaker of ice.
Place the substance to be purified in a conical flask. Cover the crystals
with just enough solvent and heat the flask until gentle boiling with occasional
swirling. If the crystals do not dissolve completely, add additional solvent until
they do, bringing the solution to the boil (gently!) after each addition. While
you are doing this, you will also have to warm some of the solvent in another
conical flask. This is for washing the filter after the hot filtration.
Set up the funnel and flask with a filter paper. Rinse the filter paper with
a little hot solvent, then pour the hot solution into the funnel. After the filtrate
has come through, allow the flask to cool slowly. When it has reached room
temperature, you can cool it further using an ice bath. The key to a good
crystallisation is slow cooling. Then filter again (this time use Buchner
filtration), washing the crystals with a little cool solvent. Dry them in a
desiccator under reduced pressure. If you used water as solvent, place some
anhydrous calcium chloride in the bottom of the desiccator. This will help to
absorb the water from your sample.
Measure the melting point of your purified sample after it is dry (ask a
TA to demonstrate the use of the melting point apparatus) and compare your
result with that given below for the compound assigned to you. In the proforma, comment on any difference.
compound
A
B
C
D
E
name
phthalic acid
nicotinamide
4-hydroxybenzoic acid
acetanilide
anthracene
melting point/°C
210-211
128-131
213-217
113-115
210-215
Students must submit their recrystallised compounds for grading.
They must not be either thrown away or taken out of the lab.
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CM1021-CM1031 combined lab manual 2020
3
CALORIMETRY – THE OXIDATION OF MAGNESIUM
The objective of the experiment is to
determine the amount of heat released when
magnesium burns (the enthalpy change of the
reaction). The reaction is
Mg + 1/2 O2  MgO
As this reaction involves the formation of the
compound from its elements, we are
measuring the heat of formation (Hf). The
violence of the reaction makes it difficult to
determine directly, but can be easily done by
considering a series of reactions that achieve
the same overall transformation. If we know
the heat change involved with the two
reactions shown with solid lines, we can
calculate the heat change involved in the third
reaction, shown with a dotted line, as long as
we carefully allow for the energetics of any byproducts too.
How do we measure the amount of heat produced? This is done by
observing the temperature change during the reactions. The heat produced
(or absorbed, if the reaction is endothermic) warms (or cools) the reaction
mixture (which is mostly the solvent, water, in our case) and the surroundings.
We will do the reaction in an insulated container, called a calorimeter, to
minimise the amount of heat that flows out of the system. Therefore, the heat
generated in the reaction, q, will warm the water and the calorimeter.
eqn. 1
q = qw + qcal
The amount of heat absorbed by the water, qw, is related to the
temperature change, T, through the heat capacity. The specific heat capacity
of water, cw, is known, and we have to include the mass of water,m w, too.
That leaves us with the
heat capacity of the calorimeter,
Ccal. We will first measure this
using a reaction with a known
enthalpy change – the reaction
between acid and hydroxide.
Why “c” and “C”?
Ccal is the heat capacity of the entire
object, in this case the calorimeter.
The units are JK-1.
cw is the specific heat capacity of a
substance – here it is water – per
unit mass. The units are JK-1g-1.
So the heat released by a given reaction in our calorimeter is:
q = cwTmw + TCcal
17
eqn. 2
CM1021-CM1031 combined lab manual 2020
This will be equal to the standard enthalpy change for this reaction,
multiplied by the number of moles of water formed, N1 (there is a minus sign
in the equation because, by convention, the enthalpy change for an
exothermic reaction is negative).
-H1N1 = q = cwT1mw + T1Ccal
eqn. 3
1. Measure the heat capacity of the calorimeter
Record the weight of the empty calorimeter with the lid.
Measure out 25 mL of 2 M HCl. Pour this into the calorimeter and measure
the temperature. This will be T0 and it will be the same as room temperature.
Measure out 25 mL of 2 M NaOH, and pour this into the calorimeter. Measure
the temperature at 30 s intervals until it reaches a steady value over a minute.
The increase in temperature is T1. You can record the temperatures to the
nearest 0.5 °C.
Reweigh the calorimeter (plus contents). The increase in weight is equal to
mw1.
Rearrange equation 3 so that you can calculate Ccal. The enthalpy change for
the neutralisation reaction is:
and
H+ + HO-  H2O
H1 = -57.1 kJmol-1
cw = 4.18 JK-1g-1
2. Mg + 2 HCl  MgCl2 + H2
Pour 50 mL of 2M HCl into the calorimeter. Record the total weight.
Record the temperature until it reaches a stable value as before. Weigh out
approximately 0.25 g of magnesium, recording the exact weight.
Add all of the magnesium to the calorimeter. Swirl gently, recording the
temperature every 30 seconds until a stable T is reached (and look inside to
check that all of the magnesium has dissolved). Calculate the temperature
change, T2.
3. MgO + 2 HCl  MgCl2 + H2O
Repeat procedure 2, but using between about 1 g of MgO. Calculate the
temperature change , T3.
4. Calculations
As we have already seen, the amount of heat evolved in either of these
reactions, q2 or q3, is given by the following equation:
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CM1021-CM1031 combined lab manual 2020
qn = cwmwTn + CcalTn
eqn. 2
To get the enthalpy change, we have to allow for the amount of substance
involved. From the quantities of reagent (magnesium or magnesium oxide),
you can calculate the number of moles, Nn. The enthalpy change for the
reaction, Hn is given by:
-Hn = qn/Nn = {cwmwTn + CcalTn } /Nn
eqn. 4
We now have H for two reactions and we can start to look at adding the
equations. You will see that we need a third reaction describing the formation
of water. The enthapy change,HH2O, for this reaction is -241.8 kJmol-1.
subtract
add
Mg + 2 HCl  MgCl2 + H2
H2
MgO + 2 HCl  MgCl2 + H2O
H3
Mg – MgO  H2 – H2O
H2 – H3
H2 + 1/2 O2  H2O
Mg – MgO + H2 + 1/2 O2  H2 – H2O + H2O

cancel
and
rearrange
H2– H3 + HH2O
Mg + 1/2 O2  MgO
 HfMgO = H2– H3 + HH2O
19
HfMgO
CM1021-CM1031 combined lab manual 2020
8
ACID-BASE EXTRACTION
This is the longest experiment in the
lab course. You must be properly
prepared. Make sure that you have
watched the video carefully. Use the
3 hours wisely.
A useful rule of thumb for judging solubility is “like dissolves like”.
Organic compounds tend to be soluble in organic solvents. Polar materials,
such as salts tend to be more soluble in polar solvents, such as water.
Some organic compounds are exceptions and are
more soluble in water than in organic solvents. This is
because they have a large number of polar functional
groups, such as hydroxyl groups, relative to the
amount of hydrocarbon. Glucose is an example of
this kind of molecule.
In general, an organic compound and an inorganic salt can be easily
separated using these solubility differences. If a mixture of 1,4dimethoxybenzene and lithium chloride is dissolved in a mixture of diethyl
ether and water, both will dissolve. The mixture will separate into two clear
layers. One will be the “organic layer” and it will contain the 1,4dimethoxybenzene. The other will be the aqueous layer, and it will contain the
lithium chloride. If the organic layer is separated from the aqueous, dried to
remove traces of water and then evaporated, pure 1,4-dimethoxybenzene will
be obtained.
Acidic and basic organic compounds come into a separate category.
While they are usually soluble in organic solvents, but not in water, they can
be made soluble in the aqueous phase by a change of pH. Carboxylic acids
dissolve in aqueous base because they are converted to the corresponding
carboxylate salts. Acidification, converting them back to their neutral forms,
makes them insoluble in water again. The bases that can be used for this
include sodium hydroxide, sodium bicarbonate and sodium carbonate.
Phenols are less acidic than carboxylic acids. Usually, they will
dissolve in aqueous sodium hydroxide, but not in aqueous carbonate or
bicarbonate.
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CM1021-CM1031 combined lab manual 2020
Amino compounds are organic bases. In their neutral form they
dissolve in organic solvents, but will dissolve in aqueous acid by formation of
cations. They are returned to their neutral form by addition of base.
These properties can be useful for achieving purification of mixtures.
This technique, called “extraction” is probably the most common purification
method in organic chemistry and you will use it again in the coming years.
In this experiment, you will be given a mixture of 1,4dimethoxybenzene and benzoic acid. 1,4-Dimethoxybenzene has no acidic or
basic groups, so it will always remain in the organic layer. Benzoic acid, on
the other hand, will dissolve in aqueous base. If we then separate the two
layers, we can evaporate the organic layer to obtain pure 1,4dimethoxybenzene and acidify the aqueous layer to obtain pure benzoic acid.
This kind of separation is most easily understood using a flow chart. The
glassware used is called a separatory funnel.
How can we tell which phase is
aqueous and which is organic?
It is a question of density. The density
of water is about 1 g/mL. The density
of diethyl ether, which we will use in
this experiment, is 0.71 g/mL. With
this combination, ether will be the
upper layer and the aqueous layer
will be lower. If we use chloroform as
the organic solvent (d = 1.49 g/mL) it
will be the other way round.
There are some other tricks that you need to know about. Usually, we
do not extract once, but twice or even more times. This is to ensure that all
traces of the phase are removed. Look out for this trick in the experimental
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CM1021-CM1031 combined lab manual 2020
procedure. Also, organic extracts must be dried after being separated from
the aqueous layer. This is because organic solvents will contain small
amounts of dissolved water. This is usually done by addition of a “drying
agent” such as anhydrous magnesium sulfate or anhydrous sodium sulfate.
These absorb the water from the organic solution and can then be removed
by filtration. In some cases, it helps if the organic layer is washed with brine
(saturated sodium chloride solution) to extract most of the water content.
Students often put a lot of effort into extractions by shaking the
separatory funnel vigorously to mix the two layers. This can be a bad idea.
Such vigorous mixing, especially of alkaline solutions, can result in the
formation of an emulsion – an intimate and inseparable mixture of the two
layers. Emulsions can be broken – but not easily! Milk is an everyday example
of an emulsion. Swirling the separatory funnel is a better technique.
Procedure
You are provided with about 1g of a
mixture of 1,4-dimethoxybenzene and
benzoic acid. These must be
separated, purified and characterised.
Preparation: label one beaker as “waste” and another as “base extract”. Label
a conical flask as “organic”.
Dissolve the sample in about 10 mL of diethyl
ether and transfer the solution to a separating
funnel. Add about 5 mL of bench sodium
hydroxide solution (about 2M). Swirl, then allow
the layers to separate. Run the aqueous layer
into the “base extract” beaker. Repeat the
process using another 5 mL of sodium
hydroxide, adding the second aqueous layer to
the same beaker.
A
good
rule
to
remember for extraction:
don’t
discard
any
solution until you have
isolated the compounds
that you need. This is
just in case you discard
the wrong layer
Wash the diethyl ether solution with water (5 mL). Drain the aqueous layer
into the waste beaker. Repeat the washing with saturated NaCl (brine) (5 mL).
Also drain the brine into the waste beaker. Run the diethyl ether into a conical
flask (rinsing the funnel with a little more diethyl ether). Add sufficient MgSO 4
or Na2SO4 to dry the solution. Leave it to stand for about 15 minutes, then
filter off the drying agent and evaporate the solvent. Use a rotary evaporator
to evaporate the solvent. A TA will demonstrate how to use it.
Cautiously acidify the contents of the “base extract” beaker using 6M HCl (you
may use blue litmus paper to check that it is sufficiently acidic). Carefully heat
the beaker so that the solid redissolves, then allow it to cool slowly. Filter off
the crystals and dry them under reduced pressure in a desiccator containing
some anhydrous calcium chloride.
Record the melting points of the two compounds, and compare to the
literature values.
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CM1021-CM1031 combined lab manual 2020
Students must submit their compounds for grading.
They must not be either thrown away or taken out of the lab.
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