Here are three animations comparing the reactivity of lithium, sodium and potassium in water. Use the images to support your observations in classroom.
Movie clips can be viewed here:
What is the order of reactivity of the three metals?
You can test if an organic compound is an alkene by mixing it with a little bromine water and shaking or stirring the mixture. Alkenes decolourise bromine water.
Alkenes turn bromine water from orange to colourless.
Cyclohexane was added to one tube and cyclohexene to the other. It is easy to tell which of the two tubes received the alkene – only one of the two tubes turned colourless.
Alkenes undergo addition reactions with bromine. For example, ethene reacts with bromine to form 1,2-dibromoethane and this may appear in IGCSE Chemistry exam questions.
Alkenes like ethene and cyclohexene contain a double bond and it is the double bond that reacts with the bromine. At Advanced Level we learn about the electrophilic addition reaction mechanism for the reaction of bromine with alkenes.
Last time there were some questions on the distillation of mouthwash experiment. Here are the answers to those questions:
1a Boiling occurs in the round bottomed flask. Ethanol molecules left the surface of the liquid mixture and became ethanol gas (or vapour).
1b The ethanol vapour passed down the Leibig condenser and turned back into a liquid on cooling. The gas molecules condensed into a liquid.
2 The distillate was ethanol.
3 The distillate was not pure but contained other volatile molecules from the mouthwash. You could tell this by smelling the distillate.
4 One might separate the volatile molecules from the ethanol in the distillate by carrying out fractional distillation.
A mouthwash that consists of an alcohol/water mixture can be distilled quite easily. One should be careful to heat gently when doing so as the mixture has a tendency to foam and may ‘boil over’ if heated too rapidly. A clear colourless liquid can be obtained as the major distillation product, as outlined below.
Here is a link to the movie on You Tube
1. Describe what is happening in each of the key stages of the distillation process?
2. Name the distillate
3. Is the distillate pure? Or are there other minor volatile components in the mouthwash that are distilled over with the main product? How can you tell?
4. How might one separate all the volatile components in mouthwash more efficiently by modifying the distillation apparatus?
Crush a few blueberry skins in water and a dilute solution of the skin colouring material is produced. The coloured pigments in the blueberry skin are called anthocyanins and they make a good acid/base indicator.
The blueberry skins can also be smeared over filter paper producing a reddish/purple canvas on which to carry out ‘electrolysis art’.
First the filter paper is moistened with a few drops of brine (saturated sodium chloride solution) and placed on a thin sheet of aluminium foil.
Then using a 4.5v dc laboratory power pack the aluminium foil is connected to the positive terminal with an electrical lead and a crocodile clip.
A paper clip was held in a second crocodile clip and connected to the negative terminal of the power pack.
The supply was turned on and when the paper clip (cathode) was passed over the filter paper electrolysis of the brine took place producing bubbles of colourless gas and turning the blueberry juice blue, green and yellow.
We also experimented by adding drops of white vinegar to the filter paper and these areas turned pinky-red.
Can you explain the colours produced by the blueberry juice?
What other dark coloured fruits and vegetables have pigments in their skins that might be used to in the same way?
If you carry out an experiment such as the one described above beware of touching the filter paper with your fingers since sodium hydroxide is produced around the cathode during the electrolysis process.
Add various salts to sodium silicate solution and watch ’em grow.
Spooky Halloween Chemistry?
Copper (II) oxide is reduced to copper by heating with carbon.
Casein is the main protein in cows milk. It precipitates rapidly when vinegar is added to warm milk. The precipitated casein and other milk solids can be filtered, shaped and dried into a hard plastic like material frequently referred to as milk plastic.
Both full cream milk and low fat milk contain casein and give a good yield of product. The casein obtained from full cream milk gives a more pliable, less crumbly material, presumably due to the higher fat content. However, the solid also exudes more oily liquid during the drying stage.
We used the following recipe:
1. Heat 300ml milk to 65 C.
2. Add 20ml of white vinegar and stir.
3. Allow the precipitated casein to cool a little before filtering through a sieve.
4. Press the filtered solid between several layers of paper towel.
5. Mould the solid into the desired shape and dry in a low temperature oven or on a warm shelf near to a radiator.
Making milk plastic
Add vinegar and stir to precipitate out the casein and other milk solids.
A closer look:
Cottage cheese anyone?
Filter it through a sieve
Press out the excess fluid before drying
What shape will you make?
You can add food dye to colour the milk during the heating stage
The drying stage takes several days or even weeks, until a very hard plastic-like material is produced.
Hey presto! Milk plastic.
It’s too hard to eat, but it may still go mouldy under the right conditions.
Finally, on the chemical reaction in the baking powder submarine last time
The baking powder used in the toy submarine last time contained two active ingredients, disodium diphosphate and sodium bicarbonate. These two chemicals react to produce carbon dioxide which was released as bubbles that caused the submarine to rise.
Na2H2P2O7 + 2NaHCO3 → Na4P2O7 + 2H2O + 2CO2
This reaction tends to release carbon dioxide at a slower rate than the sodium bicarbonate reacting with an acid like vinegar.
Baking powder submarines are great fun.
The baking powder provides the chemical reactants needed to produce the gas which raises the sub.
At the surface the sub takes on water and sinks to the bottom of the water trough. A chemical reaction takes place producing gas inside the sub which causes it to rise back to the surface. The gas is emitted from the sub and the cycle repeats, diving and rising.
A video clip from which the above animation was constructed can be viewed here.
1. What is the chemical reaction that takes place when the baking powder dissolves in water?
2. What is the gas produced that raises the sub?
Answers next time.
Thin layer chromatography of three analgesics and caffeine under U.V. light was carried out in order to show the separation taking place. It is not a recommended technique in the laboratory.
Due to the nature of the uv hazard polycarbonate safety spectacles (which absorb short wavelength U.V. light) and rubber gloves were worn throughout.
Five samples were run on a single TLC plate.
The samples were (left to right on the plate):
IBU = Ibuprofen
CAF = caffeine
u? = a commercial ‘pain relief’ medicine, used as an unknown
PAR = paracetamol
ASP = aspirin
The samples were dissolved in ethanol for spotting onto the plate. The TLC plate was run in an open beaker under short wavelength u.v. light using ethyl ethanoate as the eluting solvent.
The movement of the dark purple spots (samples) during the running of the plate can be observed in the animation. The original movie can be viewed here.
It is easy to see which are the two active ingredients in the commercial pain relief medicine by comparison of the spots with the standard reference materials running on either side.
This kind of thin layer chromatography experiment is often carried out at school.
At GCSE level Chemistry making salts is divided into three categories:
1. by reacting an acid with an excess of an insoluble base
2. by titration
3. by making a precipitate
Making the salt copper (II) sulfate from copper (II) oxide falls into the first category, since copper (II) oxide is a basic oxide which is insoluble in water, but it reacts with sulfuric acid in a typical acid/base reaction.
acid + base -> salt + water
Making ammonium sulfate from aqueous ammonia requires a titration technique, since both the acid (sulfuric acid in this case) and the base (ammonia) are soluble in water. We call bases that are soluble in water alkalis, so we can re-write the general equation here as:
acid + alkali -> salt + water
Making copper (II) sulfate
Do not boil!
Stop adding copper (II) oxide when no more reacts and the black solid collects at the bottom of the beaker. At this stage all of the acid has reacted.
The excess copper (II) oxide is removed by filtration.
Transfer the filtrate to an evaporating basin.
Water is removed by evaporation over a Bunsen burner. Stop heating when crystals start to form at the edge of the solution.
Place the evaporating basin on a windowsill or in a drying cabinet to allow crystals to form.
Making ammonium sulfate
In an approximation to carrying out a titration, small volumes of ammonia can be added to sulfuric acid in a beaker using a dropping pipette. After each addition of ammonia a sample of the reacting mixture is spotted onto a small piece of Universal Indicator paper using a glass rod. Stop adding ammonia when the Universal Indicator turns green or green/blue.
In the picture below the beaker in the centre shows the reaction mixture, the small squares of Universal Indicator paper on the tiles have been spotted after adding aliquots of ammonia (from right to left).
Stop adding ammonia when the indicator turns green. The last four squares of paper on the bottom left (of the tile of the left) were not used.
The third method of making a salt, by precipitation, is not illustrated above.
Which sulfate is formed as a white precipitate and used as a test for the presence of the sulfate ion in GCSE Chemistry?