4.2 Organic & Inorganic Chemistry Practicals

A student is provided with 2 cm³ samples of the four different compounds shown in Figure 1.

Figure 1

The student is set the challenge of distinguishing between the compounds using the minimum number of tests possible. They suggest that only two tests are needed:

1. Silver nitrate solution
2. Potassium dichromate

Evaluate the student’s suggestion.

Suggest and explain improvements to the student’s suggestion in part (a).

The boiling point of compound Q is higher than compound S.

Use your knowledge of structure and bonding to explain the difference in boiling points.

The student is asked to separate a mixture of all four compounds by fractional distillation.

Explain the order in which the compounds will distil off.

The student sets the equipment up for fractional distillation of the mixture of four compounds as shown in Figure 2.

Figure 2

The boiling point range for all four compounds is from 108 C to 212 C  but the boiling points of three of the compounds are within 47 C.

Explain three improvements that the student could make to the distillation equipment to obtain pure samples of each compound.

The commercial production of ethanol involves the fermentation of glucose.

Discuss the conditions necessary for fermentation to occur alongside the advantages and disadvantages of the fermentation process. You should include equations as appropriate.

A student set up a conical flask with yeast, sugar and water. They placed a balloon over the conical flask to show that fermentation was occurring. After fermentation was complete, the reaction mixture was filtered and then distilled using the equipment shown in Figure 1. 

Figure 1

The boiling point of pure ethanol is 78 C.

Name the piece of equipment labelled X and evaluate if it is necessary for the distillation of the reaction mixture.

The student collected a 25.0 cm³ sample of distillate. When they weighed it, the mass of the sample was 21.7 g.

The density of ethanol is 0.79 g cm³.

Calculate the mass of ethanol in the student’s sample. Assume the volume of the sample is equal to the combined volume of ethanol and water.

The process of fermentation to make ethanol as a biofuel is considered to be carbon neutral. The fermentation process releases carbon dioxide along with the combustion of ethanol. These emissions are absorbed by plants and become locked up in the          glucose that the plant produces during photosynthesis.

Explain why a student might argue that the process of fermentation is not carbon neutral.

Three solid samples, A, B and C, are known to be potassium bromide, potassium chloride and potassium iodide.

A student requires a fume cupboard to perform a chemical test to distinguish between the three samples.

Describe how to complete the student’s chemical test. You do not need to describe how to test for the products of the chemical tests.

Your answer should include safety considerations other than the use of the fume cupboard.

Sample A produces thick, purple fumes, sample B produces white misty fumes and sample C produces brown, steamy fumes.

Using ionic half equations, explain the reaction of potassium bromide with concentrated sulfuric acid in terms of redox.

A student is asked to identify an unknown white solid, A.

Step 1: They first add it to water and it fully dissolves to give a solution of A.

Step 2: They take a clean nichrome wire and dip it into the solution of A. The flame test gives a red colour.

Step 3: A small portion of solution A is added to dilute hydrochloric acid and effervescence is observed.

From these tests and observations, the student concluded that the unknown solid A is strontium carbonate.

Evaluate the student’s conclusion.

Describe the chemical tests, including expected observations, that the student could perform to prove that the unknown, solid A is strontium carbonate, SrCO₃.

A student predicted that an unknown solution of a barium salt was barium chloride solution.

The student acidified silver nitrate solution with sulfuric acid before adding it to the unknown solution. They observed a white precipitate and concluded that their prediction was correct.

Suggest improvements to the student’s method to determine if the unknown solution is barium chloride. Justify your answer.

A group of students are completing test-tube reactions to identify different chemical compounds. The students are given different pairs of compounds and must complete simple tests to distinguish between them.

The first pair of compounds is propan-2-ol and 2-methylpropan-2-ol.

Describe a key chemical test that could be done to distinguish between the compounds and any observations that would be made with each compound.

Another pair of compounds for the students to investigate is propanal and propan-2-one.

Describe a key chemical test that could be done to distinguish between these compounds and give any observations that would be made with each compound.

The students were then asked to complete some simple test-tube reactions on 3 samples of compounds, P, Q and R, to determine their identity.

The samples were all solids, known to be lithium chloride, lithium bromide and lithium carbonate.

Outline the series of steps that the students should take to identify the compounds P, Q and R. Include all necessary observations in your answer.

Finally, the students were asked to complete a simple test-tube reaction to confirm that a fourth sample, S, was lithium iodide.

The test-tube reaction that the students used produced a precipitate. Write the ionic equation, with state symbols, for this reaction.

Some students were given three samples, sodium chloride, sodium bromide and sodium iodide. They were asked to complete a number of steps to identify each of the sodium halides.

Complete the following table.

Give the relevant ionic equations for the reactions taking place.

When precipitates are formed they can be collected and dried. Describe how a student would collect a pure, dry sample of a precipitate that was formed.

Other reactions involving sodium halides include the reactions with concentrated sulfuric acid.

Write an equation to show the reaction which takes place between solid sodium bromide and concentrated sulfuric acid.

Explain why sodium bromide reacts differently to sodium chloride.

Like chlorine, bromine can be added to water to kill microorganisms and ensure that the water is safe to drink.

Explain why chlorine and bromine can be used in this way, despite the fact that they are both toxic.

The alkaline earth metals of Group 2 are similar to the alkali metals of Group 1 but are somewhat less reactive.

A group of students were doing a number of tests involving Group 2 cations and various anions.

Describe how the students could test a solution of strontium chloride to prove the presence of strontium ions. Include any observations that the students would see.

The students then tested two samples - sodium carbonate and sodium sulfate - to find out which was which. They used a pipette to add small amounts of each sample to a boiling tube and then added a few drops of barium chloride solution to each boiling tube to test for the presence of sulfate ions.

Identify a mistake which was made by the students and explain how this would affect their results.

Describe a test and observation that the students could have done on the samples in part (b) to determine if carbonate ions were present.

Strontium carbonate is a white solid, which decomposes when heated in a crucible.

Write an equation, including state symbols, for the thermal decomposition of strontium carbonate and explain why the mass of the white solid in the crucible would change during this reaction.

Different alcohols can undergo oxidation reactions to produce a number of different products. These products not only depend on the type of alcohol used, but also on the reagents and the conditions used during the oxidation. 

A chemist has a small amount of butan-1-ol and butan-2-ol to use in oxidation reactions.

State the reagents and conditions which would be needed to produce butanal, butanoic acid and butan-2-one from the alcohols named above and specify the classification of alcohol needed in each case.

Draw the displayed formula of butanal.

Write equations, using structural formulae, for the possible oxidation reactions of butan-1-ol and butan-2-ol from part (a).

Draw and label the equipment needed to produce butanal from butan-1-ol.

The chemist wanted to do a chemical test to prove that butanal had been formed. Suggest why Tollens’ reagent would be more appropriate for this test, rather than acidified potassium dichromate (VI).