Microscale and reduced scale chemistry; Experimental notes

Small scale experiments for the “traditional school” from Bob Worley, chemistry adviser at CLEAPSS on bob.worley@cleapss.org.uk

CLEAPSS, The Gardiner Building, Brunel Science Park, Kingston Lane, Uxbridge, UB8 3PQ

Tel: 01895 251496 Fax/Ans: 01895 814372 email: science@cleapss.org.uk Website: www.cleapss.org.uk

Index to experiments

There are many more in my list. If you would like to access more experiments then please contact me on

bob.worley@cleapss.org.uk. I will ten give you access to the google docs file that I have.

1. Please let me know of any improvements or twists you make.

2. No charge for this as I do not believe in payment for articles in education and the pursuit of knowledge. A

photograph of an activity being carried out in the far flung reaches of the world is reward enough. I am

semi-retired from my advisory work at CLEAPSS and it is good to have a hobby which satisfies myself

and sometimes helps other teachers of chemistry.

3. I can do workshops but doing them out of the UK is difficult. I am also getting older but still fit to travel

and it is an interesting way of meeting people and seeing the world.

Bob Worley (bob.worley@cleapss.org.uk)

Activity Reference

Safe exothermic reduction of copper(II) and iron(III) oxide oxide with hydrogen

Demo http://www.youtube.com/watch?v=b9UF6wycia8

Drop chemistry on plastic sheets

workshop http://www.youtube.com/watch?v=sk3ZolhPyWM

pH and indicators Demo

Crown bottle top crucibles; formula of copper(II) sulfate(VI) crystals

Workshop http://www.youtube.com/watch?v=3b1V38YV0wo

Chemistry of ammonia in a Petri dish

workshop

Electrolysis of copper chloride and the chemistry of chlorine

workshop http://www.youtube.com/watch?v=sk3ZolhPyWM

Hoffman voltameter with added demonstrations on hydrogen/oxygen mixtures

Demo http://www.youtube.com/watch?v=3yj1ZazuYRg

Electrode potentials in minutes

Demo

Titration: acidity of Canadian vinegar

Workshop http://www.youtube.com/watch?v=YzipDbdzgTc

Microscale alkane cracking with no suck back dangers

Demo http://www.youtube.com/watch?v=qQh2YXyFD7I

Reduction of metal oxides with hydrogen

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

microbiology laboratories.

One supplier, “Alpha Labs” sells 500 non-sterile, extended fine-tip pastettes (which deliver

50 drops per cm3), for £17.20, i.e., 3.4p each (LW4233).

Micro-titrations can be carried out by counting drops or by weighing the titration vessel. This

document describes the latter technique using a pastette as the ‘burette’.

Micro-titration by weighing: Most school science departments possess balances weighing to 2 decimal places

(and some have balances that read to 3 d.p.). Making the assumption that all the solution densities are the same

and then measuring the mass of the vials and solutions appears to work well – experimental results obtained are

very similar to those when standard titration equipment is used.

Using a pastette as a ‘burette’: To control the drop-wise addition of a solution, the bulb of the pastette is fixed

between the claws of the clamp (Figure a). Turning the adjusting screw on the clamp (Figure b) allows the

delivery of one drop at a time, until the endpoint is reached (Figure c).

(Figure a) (Figure b) (Figure c)

Acidity of vinegar by micro-titration

Procedure

Wear eye protection.

Part A: Preparing the sample

To enable the endpoint to be more visible, the samples of vinegar can be diluted by 4 in one of two ways:

(i) pipette 25 cm3 of vinegar into a 100 cm

3 volumetric flask and make up to 100 cm

3 of water, OR

(ii) place 25 cm3 of vinegar into a 100 cm

3 measuring cylinder and make up to 100 cm

3 with water.

Part B: Carrying out the micro-titration

Place 1 drop of phenolphthalein indicator solution in a glass vial. Weigh the vial (M1).

Now add about 1 cm3 of the diluted vinegar solution and reweigh the vial (M2).

M1 = M2 =

You now need one of the special thin tipped pipettes (‘pastette’).

Squeeze the pastette bulb tightly and draw the sodium hydroxide solution (0.2 M) into it.

Clamp the pastette as shown in Figure a (and the picture below right).

Gently turn the clamp screw to add drops of sodium hydroxide solution to the vial. Agitate

the vial between additions. You will see the pink/mauve colour appear more

dramatically after a while and then disappear on agitation.

You must stop the ‘titration’ when the addition of one drop of alkali results in the

pink/mauve colour appearing in the vial but NOT disappearing on agitation.

Weigh the vial and contents (M3).

M3 =

Part C: Calculation

Mass of ethanoic (acetic) acid in 100 cm3 vinegar is (M3-M2) x 4.8/ (M2-M1).

This is the % (w/V) value given on the bottle!

Reasoning behind the calculation

Assume that the densities of all the solutions are the same and are equal to that of water (i.e. 1 g cm–3

).

From experimental results: Volume of vinegar solution used is M2 – M1 cm3.

Volume of the 0.2 M sodium hydroxide solution is M3 – M2 cm3.

No moles of sodium hydroxide used is (M3 – M2) x 0.2/1000

Sodium hydroxide reacts with ethanoic acid in a 1:1 ratio so the number of moles of ethanoic acid reacted (in the

~1 cm3 sample of diluted vinegar) must also be equal to (M3 – M2) x 0.2/1000.

Hence (M2 – M1) x acid concentration/1000 = (M3 – M2) x 0.2/1000

Rearrange to give concentration of ethanoic acid (in mol dm–3

) = (M3 – M2) x 0.2/ (M2 – M1)

Mr of ethanoic acid is 60 g/mol so the mass of acid in 1 dm3 (1 litre) of diluted vinegar

= (M3 – M2) x 0.2 x 60/ (M2 – M1)

So – the mass of acid in 1 dm3 (1 litre) of shop-bought vinegar

= (M3 – M2) x 0.2 x 60 x 4/ (M2 – M1) which is the

same as (M3 – M2) x 48/ (M2 – M1)

Mass of ethanoic acid in 100 cm3 of vinegar = (M3 – M2) x 4.8/ (M2 – M1)

This is the % (w/V) value given on the bottle!

(KS3/4 pupils could work out the % value by just using the last equation.)

Microscale cracking

Procedure Wear eye protection.

Seal the glass Pasteur pipette by heating the end (tip) in a Bunsen burner flame. Allow to cool.

Use a long-tip glass Pasteur pipette (or similar) to add some liquid paraffin down the sealed pipette (~ 0.5

cm3).

Insert some mineral wool into the pipette so that the liquid paraffin is all absorbed by the wool.

Use an adapted pipette* as a micro-scale spatula to place some aluminium oxide powder into the sealed

Pasteur pipette.

Set up the sealed pipette as shown in the diagram above. Support/hold the sealed Pasteur pipette (at its

wide end, around the silicone tubing) with a clamp.

Place a small test tube containing bromine water in position (see diagram) so that the bubbling gas can

pass through it.

Acidified 0.002 M potassium manganate(VII) solution can be used in place of bromine water. The colour

change is more noticeable.

Place the spirit burner so that the flame is at the junction of the mineral wool and aluminium oxide.

Light the spirit burner.

Once it is bubbling through quickly, the gas could be lit as it emerges from the pipette.

Notes

This procedure avoids explosions caused by ‘suck-back’.

Making the hydrogen oxygen microscale rocket

The idea for this rocket came from Dr John Baum (Senior Outreach Technician) at Reading University Chemistry

Department. CLEAPSS would like to thank him for his help and advice in the making of the document.

Prepare a 10, 20 or 60 ml syringe with hydrogen and oxygen in a 2 to 1 ratio.

Fill the syringes with the individual gases either by

using a gas cylinder or canister

using gas generators

using Hofmann apparatus

the Bruce Mattson procedure http://mattson.creighton.edu/Microscale_Gas_Chemistry.html

The syringes should be capped with a special cap or with silicone tubing and a clip. Capped syringes of gas

have been kept for several days.

Connect one syringe to another with a short length of tubing (silicone tubing is preferred) and with help from

another person push oxygen gas into the syringe of hydrogen gas ensuring that there is a volume ratio of 2 to1.

Again cap or secure the syringe.

Creating the “solid base” to the launch pad.

Make a mixture of custard (cornflower is an alternative) and water in a small container adding

water slowly until there is a tick liquid. This liquid is thixotropic. Place this liquid into the cap on the

firing mechanism.

Filling the rocket with fuel

Cut a 3 ml plastic bulb pipette at the 3 ml mark. Working as quickly as possible (and you may

require some help),

remove the cap on the syringe containing the mixture,

attach silicone tubing the length of the bulb of the pipette,

hold the pipette bulb, vertically, and inject between 5 and 10 ml of gas mixture,

place the bulb over the firing mechanism and into the custard and

recap the syringe.

Pull the syringe attached to the firing mechanism a little to suck custard into the neck of the

rocket

Attaching the piezo igniter firing mechanism

Now attach the igniter to the 2 copper wires at the base of the syringe and fire the rocket

Marking the firing mechanism

A commercial gas lighter is used. In this

model the covering is carefully removed

and Copper wire is soldered on to the central

terminal. The covering is removed so that

copper wire is wrapped around the other

terminal. Wrap black tape around it.

Making the hydrogen oxygen microscale launch pad

1. Cut off the end of a hypodermic

standard length needle

2. Coil copper wire around the needle

and solder it to the needle. This does not

need to be perfect but a connection is

vital

3. Cover the soldered wire with glue from

a glue gun, covering wire to the base.

Leave it to set for 5 minutes,

4. Glue a second copper wire

alongside the needle making

sure there is no connection

between the two wires and the

base of the needle. There

should be a gap with exposed

wires at the tip.

5. Trim excess glue and check the pipette bulb (cut off at the 3 ml mark

will fit over the arrangement. Make a hole in a plastic bottle cap, fit the

needle through and glue in place. The two copper wires are then fitted

with crocodile clips which then fit onto the firing mechanism.