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In a practical examination, chemistry students were required to perform a number of tasks in a laboratory - Leaving Cert Chemistry - Question 2 - 2013
Question 2
In a practical examination, chemistry students were required to perform a number of tasks in a laboratory. They had access to all the necessary reagents and glasswar... show full transcript
Worked Solution & Example Answer:In a practical examination, chemistry students were required to perform a number of tasks in a laboratory - Leaving Cert Chemistry - Question 2 - 2013
Step 1
How could a student have carried out a simple chemical test to confirm that a colourless liquid sample was ethanoic acid and not ethanol?
Answer
To distinguish between ethanoic acid and ethanol, the student could add a few drops of an appropriate test reagent, such as bromine water. If the sample is ethanoic acid, the bromine water will remain brown (due to the presence of the carboxylic acid functional group). However, if the sample were ethanol, a reaction would occur producing colorless and fruity smells, indicating a different outcome.
Step 2
Describe fully how the gas could have been shown to be unsaturated.
Answer
To demonstrate that ethene is unsaturated, the student could bubble the gas through a solution of bromine water in a colorless test tube. Initially, the bromine water is colored red-orange. If ethene is present, the solution will lose its color, turning colorless, indicating the addition reaction typical of unsaturated hydrocarbons.
Step 3
Describe with the aid of a labelled diagram how a student could have used chromatography to separate a mixture of indicators.
Answer
The student should set up a chromatographic apparatus, using either paper chromatography or column chromatography. In paper chromatography, a spot of the mixture to be analyzed is placed on a strip of chromatography paper. This strip is then placed vertically in a suitable solvent, separating the mixture components as they travel up the paper at different rates based on their solubility. For column chromatography, the mixture is added to the top of a column filled with solid adsorbent; the solvent moves downward through the column, separating the components based on their differential retention. Each component can then be identified based on the distance traveled.
Step 4
Which was the purer sample? Justify your answer.
Answer
Sample B is the purer sample since it has a narrower melting point range (120 - 121 °C), compared to sample A (117 - 120 °C). A pure substance will generally have a sharpe melting point, and the narrower the range, the purer the sample is likely to be.
Step 5
What solvent should they have used for the recrystallization? Explain why the solvent used is suitable.
Answer
The students should have used water as the solvent for recrystallizing the benzoic acid. This choice is suitable because benzoic acid is soluble in hot water and less soluble at lower temperatures. This allows for the selective crystallization of pure benzoic acid upon cooling, as impurities will remain dissolved.
Step 6
Identify the flaw in the assembly and state how it should have been rectified.
Answer
One common flaw in a steam distillation assembly is improper positioning of the condenser, where it may be horizontal instead of inclined, preventing effective condensation of vapor. To rectify this, the condenser should be set up in an inclined position to facilitate better cooling and ensure that vapor condenses back into liquid effectively.