Microscale and reduced scale chemistry; Experimental notes Small scale experiments for the “traditional school” from Bob Worley, chemistry adviser at CLEAPSS on [email protected]CLEAPSS, The Gardiner Building, Brunel Science Park, Kingston Lane, Uxbridge, UB8 3PQ Tel: 01895 251496 Fax/Ans: 01895 814372 email: [email protected]Website: www.cleapss.org.uk
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Microscale and reduced scale chemistry; Experimental notes
Small scale experiments for the “traditional school” from Bob Worley, chemistry adviser at CLEAPSS on [email protected]
CLEAPSS, The Gardiner Building, Brunel Science Park, Kingston Lane, Uxbridge, UB8 3PQ
On a large scale, this reaction has caused many explosions and one case did lead to the prosecution of a
teacher by the Health & Safety Executive.
It used to involve passing hydrogen, dried with
concentrated sulfuric(VI) acid, over hot copper(II)
oxide.
If the excess hydrogen was ignited while there was
still a hydrogen/oxygen mix in the glass tube, then
the apparatus exploded.
With the micro-scale approach described here, there is very little dead space for there to be an explosive
atmosphere.
This method is based on work carried out by Bruce Mattson at Creighton University.
Procedure
Wear eye protection.
Do not light any spirit burners while hydrogen is being collected in the syringes.
Notes:
Hydrogen does not diffuse from the syringes when the Luer-lock cap is fitted.
They have been kept several days before using them.
Copper oxide is often “damp”. Heat the oxide in a borosilicate test-tube first and then
allow it to cool before using it in the reduction experiment.
Fill a syringe with hydrogen from a canister or chemical generator.
Secure the Luer-lock cap on the syringe to prevent hydrogen from escaping.
Using a microspatula, place a small amount of copper(II) oxide in a Pasteur pipette.
Set up as shown in the diagram (clamping around the silicone tubing).
Light the spirit burner.
After about 2 minutes, blow out the flame. Hold the
syringe in one hand and push the barrel to force hydrogen over the hot copper(II) oxide.
Let the apparatus cool before disconnecting the pipette.
The exothermic reaction between copper(II) oxide
and hydrogen.
The spirit burner flame has been extinguished –
note the water droplets on the right hand side of the
pipette.
The same approach can also be used for the reduction with hydrogen of:
lead(II) oxide,
iron(III) oxide,
nickel(II) oxide*
cobalt(II) oxide.
The reactions are not noticeably exothermic and the flame needs be kept on. The products of the iron, nickel
and cobalt oxide reductions are magnetic!
Reduction of lead(II) oxide to lead
Reduction of iron(III) oxide to magnetic iron
Reduction of nickel(II) oxide* to magnetic nickel
* There have recently been changes to the hazard classification of nickel compounds.
Discuss with CLEAPSS before attempting this reduction.
Reduction of cobalt(II) oxide to magnetic cobalt
Another variation for performing this reaction is described in CLEAPSS Guide L195, Safer Chemicals, Safer
Reactions.
A diagram of the alternative apparatus arrangement is shown below. The picture on the right shows the resulting
copper mirror.
Vial
Zinc
2 M hydrochloricacid
Sprit burner
Mineral wool
Copper(II) oxide Mineral
wool
Examples of drop chemistry for workshop sessions Drop chemistry (my name) enables “test tube” chemistry to be carried out very quickly on plastic folders with the instructions inside the folder but visible to the user. The surface tension of water keeps al the chemistry inside a hemispherical drop on the water. The reactions can be projected onto a screen with a Veho USB microscope.
Neutral solutions of iron(II) sulfate(VI) should be made as freshly as possible, by adding 1.5 ml of water to 0.1 g of the solid in the vial provided.
Precipitation reactions
Place two drops of 0.1M iron(II) sulfate(VI), iron(III) nitrate(V) and copper sulfate(VI) solutions in to the relevant circles.
To the left circle, add 2 of 0.4M sodium hydroxide or 4 to 5 drops of 0.2M sodium hydroxide solution to each of the drops. To the right circle add 2M ammonia solution.
0.1M iron(II) sulfate(VI)
0.1M iron(III) nitrate(V) 0.1M copper sulfate(VI)
To one drop of 0.1M iron(II) sulfate(VI), add one drop of potassium hexacyanoferrate( III) solution
To one drop of 0.1M iron(III) nitrate(V), add one drop of potassium hexacyanoferrate( III) solution
To one drop 0.1M copper sulfate(VI), add one drop of potassium hexacyanoferrate( III) solution
Redox reactions 1: displacement
Place 3 drops of iron(II) sulfate(VI) solution in the circle below
Add 2 pieces of
magnesium
turnings. Move a
bar magnet slowly
towards the drop..
The magnesium turnings are coated with iron and become magnetic.
Hydrated iron(II) irons are acidic in solution and the reaction with magnesium can be seen as bubbles of hydrogen are produced. This starts the formation of iron(II) hydroxide as a competitive reaction. So it is quite complicated.
Cobalt chloride produces magnetic cobalt. Nickel produces magnetic nickel but it works much better in ammonia solution.
Lead nitrate produces beautiful crystals. Crystal structure in zinc can also be seen.
Redox reactions 2:
Place two drops of iron(II)
sulfate(VI) solution in the brown
circle
Add 1 drop of 1 M hydrochloric acid
and 5 drops of 20 hydrogen
peroxide solution. Stir the solution
with your pipette.
Take 2 drops of the
solution from circle on the
left and add 3 drops of
sodium hydroxide solution
or ammonia.
Disposal
To clear up, wipe the plastic sheet with absorbent paper.
Complexes:
To one drop of 0.1M iron(II) sulfate(VI) and add one drop of potassium thiocyanate solution
To one drop of 0.1M iron(III) nitrate(V) and add one drop of potassium thiocyanate solution
To one drop 0.1M copper sulfate(VI) add 2M thiocyanate solution
Reaction intermediates:
Add one drop of
iron(III) nitrate to the
purple circle, 1 drop of
water, and 2 drops of
0.1M sodium
thiosulfate solution.
The purple intermediate
intermediate slowly
decolourizes.
The equation below is claimed to demonstrate. How could you prove the presence of uiron(II) ions? Reaction
Catalysis:
Copper ions catalyse the react above.
Serial dilutions
0.01M copper
sulfate(VI) can used
instead of water to
show the effect of a
catalyst
Does 0.001M copper(II) solutions still work? If it does, can you go even more dilute with the catalyst.
Other transition metals should work as well.
Disposal
To clear up, wipe the plastic sheet with absorbent paper.
Demonstrations?
Ammonium salt crystals
Place one drop of 1M hydrochloric acid in the circle followed by 2 drops of 2M ammonia solution. Place on a microscope slide and warm on a hot plate until crystals first appear.
Precipitates and diffusion
Show the copper hydroxide diffusion precipitate.
To make 0.01M copper
sulfate(VI) solution, add one
drop of 0.1M copper sulfate(VI)
in the blue circle plus 9 drops of
water.
pH and indicators - using the plastic Comboplate®
Procedure
Wear eye protection - solution B is IRRITANT.
Use the pipette to fill the wells E1 – E6 and F1 – F5 as follows:
E1 20 drops of A
E2 18 drops of A + 2 drops of B
E3 16 drops of A + 4 drops of B
E4 14 drops of A + 6 drops of B
E5 12 drops of A + 8 drops of B
E6 10 drops of A + 10 drops of B
F1 8 drops of A + 12 drops of B
F2 6 drops of A + 14 drops of B
F3 4 drops of A + 16 drops of B
F4 2 drops of A + 18 drops of B
F5 20 drops of B
F6 empty
Add water to each of the wells so the level is about 3mm from the top.
Rinse a pH meter* in clean water. Remove as much water as possible then dip it into the liquid in well E1.
Note the reading (to 1 decimal place).
Dip the pH meter into water and take a reading of well E2. Continue in this way up to F5.
Write the readings on the diagram above.
Fill wells A1, B1, C1 and D1 each with 3 drops from E1.
Fill wells A2, B2, C2 and D2 each with 3 drops from E2.
Continue until wells A11, B11, C11 and D11 are full.
If you have used well F6, you can fill A12, B12, C12 and D12.
Add 1 drop of Universal indicator to wells A1 to A11.
Repeat using the different pure indicators e.g., methyl orange to row B1 - B11.
Extracts from flowers and vegetables can also be used (e.g. red cabbage; petunia flowers).
If you have another Comboplate®, more wells can be filled.
Photograph the Comboplate® from above for a lasting record. Label the photograph.
Solutions Solution A: 3.1 g of boric acid + 2.65 g of citric acid made up to 250 cm
3 of solution.
Solution B: 9.0 g of disodium hydrogen phosphate-12-water + 1 g of sodium hydroxide made up to 250 cm
3 of solution.
Universal indicator solution. Other chemical indicators (e.g. methyl orange). Plant extracts.
Some results
A
B
C
D
E
F
1 2 3 4 5 6 7 8 9 10 11 12
1 2 3 4 65
*A calibrated Checker pH meter (about £30) can be used to find the pH values.
The following pictures were sent to CLEAPSS by a technician. The pupils did this in a science club.
The percentage water in hydrated copper(II) sulfate(VI) using an unbreakable substitute crucible!
The mass of the substitute crucible is measured (M1).
Copper(II) sulfate(VI)-5-water is added and the mass is measured again (M2).
Mass of copper(II) sulfate(VI)-5-water is M2-M1.
The substitute crucible is held by a clamp above the spirit burner as shown on the
right
Once the blue colour has been lost and a colourless solid is in the bottle top, blow
out the flame and allow it to cool
The mass of anhydrous copper(II) sulfate(VI) and substitute crucible is measured
M3.
The mass of water lost is M2-M3
Calculation
The percentage of water is
How close is this to the theoretical value of 36.0%
Notes
The apparatus
A serrated bottle top is heated strongly in a fume cupboard to remove the plastic
insert. After cooling, a hole is drilled and a metal bolt is fitted as shown in the
pictures.
Chemistry
Copper(II) sulfate(VI)-5-water loses 4 of its water molecules at 100°C. The final water molecule is lost at 150°C.
If a Bunsen flame is used, temperatures of over 650°C are reached at which point copper sulfate(VI)
decomposes and a sulfur dioxide (toxic) and sulfur trioxide (corrosive) are released. The solid darkens in colour.
The spirit burner flame is not hot enough to cause this decomposition with copper(II) sulfate(VI).
Other hydrated salts can be used.
Iron(II) sulfate(VI)-7-water looses its water at 70°C and begins to decompose at 400°C. Unfortunately is difficult
obtain the pure green heptahydrate as if looses water to the atmosphere and you can see white specs in the
solid.
Magnesium sulfate(VI) looses all its water at 200°C and does not decompose until 1124°C so this can be used.
However, on heating it liquefies and spits as it re-solidifies, so the bottle top needs to be a little higher and
gradually lowered as the heating progresses.
The Aqueous Chemistry of Ammonia
Wear eye protection
Place the 9 cm-diameter plastic Petrie dish on
a white background.
Dampen the 0-14 pH paper before placing it
on the dish.
Use the drop-technique to place the drops of
the required chemicals onto the Petrie dish.
These are 0.1M solutions of a metal salts
which are to hand.
Also 1 drop of 0.1M hydrochloric or 0.05M
sulfuric(VI) acid containing universal indicator.
Place the generator into the centre of the dish.
Take a photograph of the dish!
Add 0.5 cm3 of 2M ammonia to the generator
(optional: a few granules of anhydrous calcium
chloride can be added; this causes an
exothermic reaction with water so more
ammonia is liberated.)
Place the cover on the dish.
Take a photograph of the dish about 2
minutes!
Leave for another 3 minutes and take a photo
again.
Leave for as long as possible and take a photo
again
Notes
If all the ammonia is released (which it isn’t), then 24 cm3 17 mg of gas would be released. Although the gas can
be detected (odour level is 3.5 mg m3) by our sense of smell, ammonia levels will be below the Short Term
Exposure Level (STEL) level of 25 mg m3 for a large room (of 300m
3) averaged over 15 minutes.
Observations
Indicator paper indicates a pH of 11.
Sulfuric(VI) acid is neutralised and finally goes alkaline.
Some metal salts form precipitates of hydroxides.
Some metal salts form hydroxides and then the precipitates dissolve in excess ammonia to courses solution
as complex ions form.
0.5 ml of 2M ammonia (optional calcium chloride)
Add 2 drops of 0.1M metal salts and other reagents which are to hand
Universal indicator paper
A Microscale Hoffman-type voltameter
Procedure
Wear eye protection.
Set up the apparatus as shown above. Support the Petri dish on a plastic container or platform in Use Blu-
Tack® (or similar) to secure the Petri dish in the platform.
Place ~ 0.8 –1 M sodium sulfate(VI) solution in the Petri dish. (Also add bromothymol blue indicator if you
wish.)
Attach a 10 0r 20 cm3 syringe to one of the 3-way taps.
Adjust the tap and draw up sodium sulfate(VI) solution to fill the syringe and rotate the tap so that the
solution remains in the syringe. You may need to add a further small volume of the sodium sulfate(VI)
solution to the Petri dish
Repeat with the other vertical syringe.
Place Luer-lock caps on the taps connected to the 5 cm3 syringes.
Connect the copper wires to the power pack/battery and note which syringe covers (i) the positive electrode
(anode) and (ii) the negative electrode (cathode).
Switch on the power pack/battery and observe the relative volume ratio of the two gases produced.
Extension 1
Once electrolysis is completed (i.e. the syringes are each full of gas), attach a 20 cm3 syringe to the ‘anode
syringe’ and, by manipulating the 3-way tap correctly, transfer the collected oxygen to the 20 cm3 syringe.
Repeat the above process at the ‘cathode syringe’, transferring the collected hydrogen into the same 20
cm3 syringe. Place a Luer-lock cap on the syringe.
Wear eye and ear protection! Light a Bunsen burner about 1 metre away.
Place a large plastic Petri dish on a tripod, and fill it with bubble mixture.
Warn the students (all standing at least 3 m away) to place their hands over their ears.
Light a splint, bubble a small volume of the hydrogen/oxygen gas mixture into the soap solution and then
light the bubbles with the splint.
Extension 2
Place an ammeter in series to read the current and time how it takes to collect 10 cm3 of hydrogen gas. Various
calculations can be made from this depending on the topic being studied, e.g., volume of a mole of gas.
Two 5 cm3 syringes
3-way taps
1M sodium sulfate(VI)
Platinum or lead anode
Copper wire leads
50 mm Petri dish in
a plastic container
Carbon fibre cathode
Disposal
All the liquids can be washed down the sink into the foul water drain.
Notes
A full-size Hoffman voltameter with Pt electrodes can cost around £100 to £150.
The small-scale equipment described here is less than £20. The cathode can be
carbon-fibre around which copper wire is wrapped tightly around and glue from a
glue gun is applied (see picture right). The platinum anode is soldered onto the
copper but it is not a secure fitting so the join is encapsulated with glue from a glue
gun. Lead foil can be squeezed very tightly with pliers and used as an anode. In
this case just wrap the copper wire around the lead and use glue. Check the Petri
dish is water tight and add more glue to seal it.
In place of 3-way taps syringe can be used. A 10 ml syringe is used to suck the
electrolyte, the silicone tubing is pinched while the syringe is replaced with a closed 5
ml syringe as in the picture on the right
The experimental procedure can be projected onto a large screen.
Hoffman voltameters are usually filled with dilute sulfuric(VI) acid but the sodium
sulfate(VI) solution used here is a low hazard material and safer to use as fingers may
get contaminated.
This practical activity shows that it is water that is being electrolysed at the electrodes
with the surrounding solutions turning acidic or alkaline (with the use of an indicator
such as bromothymol blue).
Sodium ions and sulfate(VI) ions are solvated by water molecules
(remain in solution).
The Hoffman in action (see picture right) You will see that the hydrogen
in the right-hand cathode has twice the volume of the oxygen in the
left-hand anode.
Micro-electrolysis of copper(II) chloride solution
Procedure
Wear eye protection.
To avoid inhaling chlorine gas (which could result in triggering breathing difficulties in those who are
susceptible), do not remove the cover of the Petri dish and at the same time lean closely over the top. The
chlorine can be quickly diffused away with a waft of the hand.
The chlorine levels are, on average, well below the Workplace Exposure Levels (WELs).
Place the following in the Petri dish (see diagram above):
1 drop of potassium bromide solution (~ 0.5 – 2 M);
1 drop of potassium iodide solution (~ 0.1 – 0.5 M);
a piece of damp blue litmus paper
drops of 0.5 M copper(II) chloride solution until the ‘merged’ drop just touches both electrodes.
Place the lid on the Petri dish and then connect the electrodes to a DC source (~ 6 to 8 volts).
Switch on and observe what happens: (i) at the electrodes, (ii) to the test solutions (iii) to the moist litmus paper.
Remove the lid of the Petri dish, but take great care not to inhale the gas. Waft the gas away with your hands.
Look carefully at the electrode regions using a digital microscope.
Warning: make sure the battery is disconnected at the end of your session.
Disposal
All the liquids can be washed down the sink into the foul water drain.
A few drops of 0.5 M copper(II) chloride
solution
Two drops of 0.5 to 2M potassium
bromide solution
Two drops of 0.1 to 0.5 M potassium iodide solution
Moist blue litmus paper Carbon-fibre
electrodes
Results
Photographs can be taken of the equipment.
During the electrolysis:
copper, Cu(s), is produced at the cathode (see picture above).
chlorine, Cl2(g) formed at the anode reacts with the salt solutions to form bromine, Br2, and iodine, I2 (in
solution).
moist blue litmus paper turns red due to formation of hydrochloric acid, HCl(aq), and chloric(I) acid, HClO(aq).
The latter then oxidises the litmus dye to give colourless products.
The results are even more effective if the procedure is viewed via a visualizer.
Notes
The electrodes are 1mm carbon fibre rods available from suppliers of kite materials.
In this procedure, 4 drops (i.e. 0.2 cm3) of 0.5 M copper(II) chloride solution are used. The maximum amount
of chlorine that could be produced is ~ 7.1 mg (i.e. ~ 2.4 cm3 at room temperature).
If 15 sets of equipment were all working at the same time, the Workplace Exposure Limit (WEL) of 1.5 mg m-3
(averaged over the whole room) would not be reached. However, it would be exceeded in localised areas,
i.e., just above the Petri dish when the lid is removed. Hence, great care must be taken to avoid inhaling the
chlorine gas.
Possible extensions
Find out what happens with other salt solutions.
Potassium
bromide
Place 1 drop of 2M potassium bromide in a Petri dish and add 9 drops of water.
Now place the mixture between the electrodes. Based on the experiment with
copper(II) chloride solution, design some additional investigations.
Iron(II)
sulfate(VI)
Place iron(II) sulfate(VI) solution between the electrodes.
If iron is produced at an electrode, it ought to be magnetic. Is it?
Zinc sulfate(VI) Place 0.1M zinc sulfate(VI) solution between the electrodes. Follow the electrolysis
using a digital microscope. Further dilution will slow down the rate of electrolysis but
will this make the appearance of any metal crystals easier to see? Investigate.
Lead nitrate(V) Place 0.1 M lead nitrate(V) solution between the electrodes and follow the electrolysis
using a digital microscope.
Silver nitrate(V) Place 0.05M silver nitrate(V)solution between the electrodes and follow the
electrolysis using a digital microscope.
Hint: the electrodes can be moved closer together or further apart to speed up or slow down the rate of electrolysis.
Electrode potentials
Procedure
Wear eye protection
Place a plastic Petri dish on a flat surface. Place a strip of filter paper in the dish.
Add 1 drop of 0.1M copper(II) sulfate(VI) to one end of the strip. Place a small piece of copper foil on top of the copper(II) sulfate(VI) solution.
Add 1 drop of 0.1M zinc sulfate(VI) to the other end of the strip. Place a small piece of zinc foil (or a zinc granule) on top of the zinc sulfate(VI) solution.
Add 1 drop of 0.1M potassium nitrate solution to the centre of the filter paper. Allow the liquid to spread out so that it touches the other two solution areas. Add another drop if required.
Set the multimeter to a convenient scale, e.g., 2000 mV.
Place one probe on each of the metals and take a reading.
Extension
More metals
Place another strip at right angles across the one
shown in the diagram (see photo). Repeat the
experiment using other metals with their salts at the
ends of the filter paper strips.
Concentration
Concentration cells can be set up with 1M copper(II)
sulfate at one end of the strip and more dilute
copper(II) solutions at the other end.
Complexing
0.1M copper(II) sulfate in water is placed at one end
and 0.1M copper sulfate made up in 2M ammonia
solution is placed at the other.
Micro-titration: why do it? Titration is a very important procedure in chemistry but some teachers are reluctant to pursue it.
Reasons often cited are as follows.
The equipment is expensive and can be quite easily broken by pupils.
Pupils do not have the dexterity and/or patience to carry it out.
Some newly-trained teachers of science and chemistry are not as comfortable with the procedure as “older” chemists are.
The arithmetic is perceived to be difficult.
The concepts of stoichiometry and ‘the mole’ are difficult.
This micro-titration activity can provide a useful, low-cost introduction to titration technique and the related
calculation work. The technique could be adapted to a variety of quantitative investigations.
Notes
Plastic Pasteur pipettes are known as “pastettes”. They are used extensively in