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Background Infomation on Chemistry Coursework

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Background Information, Reactions and Safety Precautions

Background Information

In the reactions below, it involves alkenes, alkanes, alcohols, acids, and water. In the beginning of the experiments they are unknown and it is needed that a test is used to identify them.

Alkenes are unsaturated compounds with a C=C double bond. The high electron density of the double bond makes alkenes more reactive than alkanes. The best way to test for alkenes is to use bromine which is used in one of the reactions below.

Alkanes are generally unreactive. Alkanes contain only C-H and C-C bonds, which are relatively stron ang difficult to break. The similar electronegativies of carbon and hydrogen give molecules which are non-polar and do not mix with water. This is why water is not used to test for it.

The functional group in alcohols is the hydroxyl group, C-OH. They can be classified as primary, secondary or tertiary depending on how many alkyl groups are bonded to the C-OH. In the reactions below that contain an alcohol, they are classified both as primary.

With alcohols, they are oxidized to aldehydes and then to carboxylic acids. The aldehydes are carbonyl compounds and they have functional group C=O. They have a hydrogen and one alkyl group attached to the carbonyl carbon atom.

Equipment Required

These will be the items required to perform the reaction tests below.

  1. The six colourless liquids each in their own test tube.
  2. Test tube rack
  3. Small piece of foil
  4. 2cm3 of 2% solution of bromine water
  5. Test tube with 2cm3 of dilute sulphuric acid
  6. 1cm3 of dilute potassium dichromate (VI)
  7. Pipettes (3)
  8. 2cm3 of ethanoic silver nitrate
  9. 2 Stoppers
  10. An available fume cupboard
  11. 2cm3 of phosphorus pentachloride
  12. A “squeezed out” dropper
  13. A Bunsen burner and its holder
  14. Small amount of anhydrous sodium hydrogen carbonate
  15. Blue chlorine cobalt paper
  16. Boiling tubes (2)

Safety Precautions

In the tests to follow, wearing a lab coat, goggles (safety spectacles), and gloves are required. These experiments contain acid and other materials that can seriously harm health.

Reaction 1: Test for Cyclohexene

This is an alkene and as all alkenes, it may have a few uses, but they are reactive because of their carbon-carbon double bonds. When you mix bromine water with cyclohexene, it turns reddish brown bromine water colourless under dark conditions. Ethanol can be added to help aid the mixing. The equation for reaction of cyclohexene and bromine water is:

                C6H10(l) + Br2 (l)  →  C6H10Br2 (l)

This is an addition reaction. The carbon-carbon double bond in the alkene ‘opens up’ and a bromine atom is ‘added’ on to both carbon atoms

  • Procedure:
  • Put 2cm3 of cyclohexene into a test tube and place on a test tube rack.
  • Wrap foil around the test tube.
  • Add 2cm3 of 2% solution of bromine water into the test tube.
  • Wait two minutes and then take the foil off.
  • The liquid should decolourise.
  • Safety: This liquid is highly flammable and the vapours mixtures are explosive. It should not be exposed to any open flame, sparks or any kind of smoking. It should also be kept in a closed system and any reactions carried out in a fume box. If exposed can cause redness to the eyes and skin, drowsiness, nausea and dry skin. Protective clothing, gloves and safety spectacles should be worn.

Reaction 2: Test for Butan-1-ol

Acidified potassium dichromate oxidise a primary alcohol like butan-1-ol. The reaction involves breaking the O-H bond of the alcohol. To test for Butan-1-ol using Acidified Potassium Dichromate(VI) is best as you will see it change colour from orange to green to show it’s a primary alcohol because an aldehyde has formed.

The equation for the reaction of Butan-1-ol and Acidified Potassium Dichromate (VI) is:

                C4H9OH (l)  +  K2Cr2O7  →  C4H8O  +  H2O

  • Procedure:
  • In a boiling tube, mix 2cm3 of dilute sulphuric acid with 1cm3 of dilute potassium dichromate (VI) solution
  • Add 5 drops of butan-1-ol and warm the mixture gently. Only allow to just boil.
  • Cautiously smell the product.
  • Safety: This liquid is flammable and should be kept away from any open sparks or flames. If above 29oC, explosive vapour mixtures may be formed. If used at this temperature use in a close system and make sure there is enough ventilation. If exposed can cause sore throat, headache, dizziness, redness to the eyes, abdominal pain and vomiting. Avoid all contact by using protective clothing, gloves and face shields. Do not eat or drink during the procedures.

Reaction 3: Test for Bromoethane

When testing for bromoethane it is best to use silver nitrate to identify the halide and you know it must be bromoethane because it’s your only chemical containing a halide. It should give a cream precipitate as AgBr if formed.

The equation for the reaction of bromoethane and silver nitrate is:

C2H5Br (aq)  +  AgNO3 (aq)  →  AgBr (s)  +  NO3 (g)  +  C2H5 (aq)

  • Procedure:
  • Add 2cm3 of bromoethane to a test tube.
  • Add 2cm3 of ethanoic silver nitrate to the test tube.
  • Put a stopper on the test tube and give it a slight shake.
  • The test tube should show a cream precipitate.
  • Safety: This liquid is extremely flammable and should be kept away from open flames, sparks and any kind of smoking. It can also cause drowsiness, or redness of the eyes. To prevent this protective clothing, gloves and goggles should be worn. No eating or drinking should be done during the work.

Methlypropan-2-ol

In order to test this liquid it would be necessary to use Phosphorus Pentachloride PCl5 this will release hydrogen chloride gas which forms white misty acidic fumes in the air.

The equation for this reaction is:

        C4H9OH  +  PCl5  →  C4H9Cl  +  HCl  +  POCl3

  • Procedure:
  • Add 2cm3 of methlypropan-2-ol to a test tube.
  • In a fume cupboard, add a small amount of phosphorus pentachloride.
  • Collect some of the fumes evolved in a “squeezed out” dropper and identify them by bubbling through some silver nitrate solution.
  • OR identify the fumes with the stopper from an ammonia bottle.
  • Safety: This liquid is colourless with a characteristic odour. The vapour mixes well with air and explosive mixtures are easily formed. The substance can be absorbed into the body by inhalation of its vapour and by ingestion. A harmful contamination of the air can be reached rather quickly on evaporation of this substance at 20oC. The vapour of this substance is irritating to the eyes and may cause effects on the central nervous system. Exposure at high levels could cause lowering of consciousness.

Reaction : Test for Ethanoic Acid

To test for Ethanoic Acid, you should add it to Anhydrous Sodium Hydrogen Carbonate. At the end of this reaction you should get a white crystal like solid, water and carbon dioxide.

CH3COOH  +  NaHCO3  →  CH3CONa  +  CO2  +  H2O

  • Procedure:
  • Add 2cm3 of ethanoic acid to a test tube.
  • Add a small amount of the anhydrous sodium hydrogen carbonate to the test tube.
  • Safety: This liquid is flammable and should be kept away from any open sparks or flames. If above 39oC, explosive vapour mixtures may be formed. If used at this temperature use in a close system and make sure there is enough ventilation. If exposed can cause sore throat, headache, dizziness, blisters, redness to the eyes, abdominal pain and vomiting. Avoid all contact by using protective clothing, gloves and face shields. Do not eat or drink during the procedures.

Water

This is a very simple test that only involves a piece of blue chlorine cobalt paper pink.

  • Procedure:
  • With the liquid in the test tube, get a piece of blue chlorine cobalt paper and place in the test tube.
  • The paper should turn pink, indicating that the liquid is water.

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