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The two test-tubes on the left in the diagram contain pure samples of A and B, different gaseous hydrocarbons, one of which is unsaturated - Leaving Cert Chemistry - Question 2 - 2016
Question 2
The two test-tubes on the left in the diagram contain pure samples of A and B, different gaseous hydrocarbons, one of which is unsaturated. The pure liquids C, D an... show full transcript
Worked Solution & Example Answer:The two test-tubes on the left in the diagram contain pure samples of A and B, different gaseous hydrocarbons, one of which is unsaturated - Leaving Cert Chemistry - Question 2 - 2016
Step 1
Describe how to chemically test sample A or B for unsaturation.
Answer
To test for unsaturation in hydrocarbon samples A or B, add bromine water or an acidified potassium permanganate solution to the sample. If the hydrocarbon is unsaturated, the bromine water will turn from brown to colourless, indicating that a reaction has occurred. In the case of potassium permanganate, it will change from purple to colourless.
If there is no change in color, the sample is saturated.
Step 2
At room temperature (20 °C) it was observed that D evaporated more quickly than either C or E. Based on this observation decide the identity of liquid D.
Answer
Liquid D is identified as ethanol (C2H5OH) due to its lower boiling point compared to the other liquids, which explains the quicker evaporation observed.
Step 3
Describe a chemical test to confirm that C can be very easily oxidised.
Answer
To confirm that liquid C (ethanol) can be easily oxidised, carry out an oxidation reaction by adding K2Cr2O7 (potassium dichromate) to a sample of C in an acidic medium. If ethanol is oxidised to ethanoic acid, the solution will change from orange (Cr2O7^2-) to green (Cr^3+), confirming the oxidation.
Step 4
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Step 6
Describe with the aid of a labelled diagram a technique that could be used to measure the melting point range of benzoic acid.
Answer
To measure the melting point of benzoic acid, a capillary tube method can be used. Fill a thin-walled capillary tube with a small amount of benzoic acid crystals. Insert the tube into a heating apparatus with a thermometer alongside. Gradually heat the area around the tube while monitoring the temperature at which the crystals begin to melt, as well as when the substance is completely liquid. Record the temperature range during melting.
A diagram should label the heating mantle, capillary tube, benzoic acid, and thermometer.
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Step 9
Name a laboratory technique that could be used to purify impure benzoic acid.
Answer
A suitable technique to purify impure benzoic acid is recrystallization. The impure substance is dissolved in a suitable solvent at elevated temperatures and then allowed to cool, which facilitates the formation of pure benzoic acid crystals.