Synoptic Exam Questions (AQA A Level Chemistry): Exam Questions

A student prepares the ester propyl ethanoate by reacting ethanoic acid with propan-1-ol, using concentrated sulfuric acid as a catalyst. The mixture is heated under reflux.

CH₃COOH + CH₃CH₂CH₂OH ⇌ CH₃COOCH₂CH₂CH₃ + H₂O

The student starts with 5.75 g of propan-1-ol and an excess of ethanoic acid. After purification by distillation and separation, the student obtains 6.55 g of propyl ethanoate.

State the role of the concentrated sulfuric acid in this reaction.

The reaction is reversible. Suggest a reason why the apparatus is set up for distillation after the initial reflux period to separate the ester from the reaction mixture.

Calculate the percentage yield of propyl ethanoate.
(M_r of propan-1-ol = 60.0; M_r of propyl ethanoate = 102.0)

The student uses infrared spectroscopy to check the purity of the final product.

State one key absorption that would be present in the spectrum of the starting material, propan-1-ol, but absent from the spectrum of the pure product, propyl ethanoate.

You should describe the bond responsible and its approximate wavenumber.

The student analyses a sample of the pure propyl ethanoate using high-resolution ¹H NMR spectroscopy.

Predict the number of peaks, the relative peak areas (integration ratio), and the splitting pattern for each peak in the ¹H NMR spectrum of propyl ethanoate.

In a separate experiment, the kinetics of the reaction were studied by varying the concentration of the sulfuric acid catalyst, [H⁺]. The initial rate was measured in each case.

Further experiments showed the reaction to be first order with respect to propan-1-ol and zero order with respect to ethanoic acid.

Determine the rate equation for this reaction. Use the data from Experiment 1 to calculate a value for the rate constant, k, and state its units.

Ethanol can be produced industrially by two main methods: the fermentation of glucose and the direct hydration of ethene.

The hydration of ethene uses a catalyst of solid silicon dioxide coated in phosphoric(V) acid.

State the conditions of temperature and pressure used in this process.

The fermentation of glucose (C₆H₁₂O₆) produces ethanol and carbon dioxide.

Write a balanced equation for this reaction.

 A student carries out the fermentation of 15.0 g of glucose. After purification, 6.25 g of ethanol is collected.

Calculate the percentage yield of ethanol.

A sample of ethanol is gently heated with acidified potassium dichromate(VI) in a flask with distillation apparatus attached.

Describe a simple chemical test to confirm the identity of the organic product, stating the reagents used and the positive result.

Explain why the first ionisation energy of calcium is greater than that of strontium.

Draw a 'dot-and-cross' diagram to show the bonding in calcium iodide.
Show outer shell electrons only.

Solid calcium iodide is reacted with concentrated sulfuric acid. This is a redox reaction where the iodide ion acts as a reducing agent.

8I⁻ (s) + H₂SO₄ (l) + 8H⁺ (aq) → 4I₂ (s) + H₂S (g) + 4H₂O (l)

Explain why the iodide ion is a more powerful reducing agent than the chloride ion.

State the element that is oxidised in the reaction shown in part (c) and the change in its oxidation state.

Identify the element that is oxidised in the reaction shown in part (c) and state the change in its oxidation state.

CaCO₃ (s) + 2HI (aq) → CaI₂ (aq) + H₂O (l) + CO₂ (g)

The resulting solution is transferred to a 250.0 cm³ volumetric flask and made up to the mark with deionised water.

Calculate the concentration, in mol dm^-3, of iodide ions (I^-) in the final solution.

(M_r of CaCO₃ = 100.1)

A student is given a sample of an unknown straight-chain carboxylic acid, X.

The student dissolves 1.85 g of X in deionised water and makes the solution up to 250.0 cm³ in a volumetric flask. A 25.0 cm³ sample of this solution is titrated with 0.100 mol dm^-3 sodium hydroxide solution. The mean titre required for neutralisation is 25.0 cm³.

State the colour change that would be observed at the end-point if the student used phenolphthalein.

Assuming X is a monoprotic acid, use the titration data to calculate the relative molecular mass (M_r) of X.

The empirical formula of X is C₃H₆O₂.

Use your answer from part (b) to determine the molecular formula of X.

Draw the displayed formula of X and state its systematic name.

Name the primary alcohol that can be oxidised under reflux to form X.

Identify two key absorptions that would be expected in the infrared spectrum of X.

For each, state the bond responsible and the approximate wavenumber.

Cyclohexene can be prepared in the laboratory by the acid-catalysed dehydration of cyclohexanol. The reaction is reversible.

Draw the mechanism for the acid-catalysed dehydration of cyclohexanol to form cyclohexene.

In an experiment, 0.200 mol of cyclohexanol was heated with an acid catalyst in a sealed container of volume 1.50 dm³. At equilibrium, 0.160 mol of cyclohexene had been formed.

Calculate the value of the equilibrium constant, K_c, for this reaction.

Cyclohexene can undergo addition polymerisation.

Draw the repeating unit of poly(cyclohexene).

Explain why poly(cyclohexene) is a solid at room temperature, whereas the monomer, cyclohexene, is a liquid.

A student does an experiment to determine the formula of a soluble ionic chloride by a precipitation reaction using silver nitrate solution. The reaction is carried out as a titration, and aqueous potassium chromate(VI) is used as an indicator.

Method:

• A sample of mass 1.20 g of a Group 2 chloride is dissolved in 50 cm³ of distilled water. 

• The solution is transferred to a volumetric flask and made up to 250 cm³ with distilled water.

• Transfer 25.0 cm³ of the solution to a conical flask.

• Add 0.0500 moldm^-3 silver nitrate solution to the conical flask from a burette until a permanent pink precipitate is seen.

The student uses exactly 30.0 cm³ of silver nitrate solution.

The equations for the reactions in this experiment are:

Ag⁺ (aq) + Cl^- (aq) AgCl (s)

Excess silver ions form a red precipitate in the presence of chromate(VI) ions:

2Ag⁺ (aq) + CrO₄^2- (aq) Ag₂CrO₄ (s)

Use all the information to calculate the mass of the Group 2 element present in the 250 cm³ of the solution.

Give your answer to 3 significant figures.

Determine the identity of the Group 2 element.

Tick (✓) one box.

Silver chloride dissolves in aqueous ammonia solution to form a colourless solution, which can be used to identify halide ions.

Name and give the formula of the complex ion present in the colourless solution.

Draw the structure of the complex ion, giving the relevant bond angles.

Explain what is meant by a complex ion.

State the type of bonding between silver and ammonia in the colourless solution in c).

This question is about the essential oils citral (C₁₀H₁₆O), carvone (C₁₀H₁₄O), and limonene (C₁₀H₁₆). Their structures are shown in Figure 1.

Figure 1

Mark on the molecules the presence of a chiral centre using an asterisk (*).

State the structural feature that gives rise to optical isomerism.

Optical isomers can be distinguished using a polarimeter.

Figure 2 shows an incomplete diagram of a polarimeter assembly.

Figure 2.

Complete the diagram with the labels analyser, detector, sample cell, and polariser.

Show the direction of the light path.

The infrared spectra are those of citral, carvone and limonene, but not necessarily in that order.

A

B

C

Which spectrum belongs to limonene?

Use the Data Booklet to support your answer.

Justify your answer.

A student adds a few drops of citral, carvone, and limonene to separate test tubes containing Tollens' reagent. The test tubes are warmed.

Give equations for any reactions that occur to the organic species.

Describe what is observed.

Complete combustion of 1.00g of one of the three oils, in excess oxygen, yielded 3.23 g of carbon dioxide and 1.06 g of water

Determine which oil was combusted. Show your workings.

Write an equation for the reaction.

The Fenton reaction involves the use of iron(II) ions to catalyse the decomposition of hydrogen peroxide. This reaction is used in environmental chemistry to break down organic pollutants. The overall equation for the decomposition is:

2H₂O₂ (aq) → 2H₂O (l) + O₂ (g)

Explain, in terms of their electronic structure, why iron ions can act as catalysts in this reaction.

The rate of reaction was investigated at a constant temperature by measuring the initial rate for different concentrations of H₂O₂ and Fe²⁺.

Use the data to deduce the rate equation for the Fenton reaction.

Use the data from Experiment 1 to calculate the rate constant, k, and state its units.

The standard enthalpy change for the decomposition of hydrogen peroxide is –98 kJ mol^-1.

Draw an enthalpy profile diagram for both the catalysed and uncatalysed decomposition of hydrogen peroxide.

Label the activation energies for both pathways (E_a and E_{a, cat}) and the overall enthalpy change (ΔH).

Phenyl ethanoate is an ester that can be synthesised by the reaction of phenol with ethanoyl chloride.

Explain why phenol is more reactive than benzene towards electrophilic substitution.

Phenol is a weak acid. Write an equation for the reaction of phenol with aqueous sodium hydroxide.

The standard enthalpy change, ΔH^θ, for the synthesis of phenyl ethanoate from phenol and ethanoyl chloride is –56.1 kJ mol⁻¹. The standard entropy change of the system, ΔS^θ_system, is +34.5 J K⁻¹ mol⁻¹.

Calculate the Gibbs free-energy change, ΔG^θ, for this reaction at 298 K.

Explain, with reference to your answer in part why this reaction is feasible at 298 K.

Explain, in terms of ΔH^θ and ΔS^θ, why this reaction is feasible at all temperatures.

Alanine is an amino acid. In aqueous solution, it exists predominantly as a zwitterion.

Draw the structure of the species formed when alanine reacts with:

• an excess of aqueous HCl.

• an excess of aqueous NaOH.

The zwitterion of alanine can act as a weak acid:

⁺H₃NCH(CH₃)COO^- (aq) ⇌ H⁺ (aq) + H₂NCH(CH₃)COO^- (aq)

The K_a for this equilibrium is 2.51 × 10^-10 mol dm^-3.

A buffer solution is prepared by mixing a solution containing the alanine zwitterion with a solution containing its conjugate base, the alaninate ion. The buffer has a pH of 9.90.

Calculate the ratio of [conjugate base] : [weak acid] in this buffer solution.

The alaninate ion, H₂NCH(CH₃)COO^-, can act as a bidentate ligand, forming an octahedral complex ion with cobalt(III). The formula of the complex ion is [Co(H₂NCH(CH₃)COO)₃].

State the coordination number of cobalt in this complex and explain why the complex is neutral.

The complex ion [Co(H₂NCH(CH₃)COO)₃] can exist as a pair of optical isomers.

Draw 3-D diagrams to show the two optical isomers of [Co(H₂NCH(CH₃)COO)₃].

You may represent the bidentate alaninate ligand as N—O.

Phosphorus pentachloride, PCl₅, is a solid that decomposes upon heating in a reversible gaseous equilibrium.

PCl₅ g) ⇌ PCl₃ (g) + Cl₂ (g) ΔH^θ = +92.5 kJ mol⁻¹

Phosphorus(V) chloride reacts vigorously with water to form phosphoric acid and one other product.

Write a balanced equation for the reaction of phosphorus(V) chloride with an excess of water.

State the shape of the PCl₅ molecule and the PCl₃ molecule.

Explain the difference in their bond angles.

In an experiment to determine K_p, 41.7 g of solid PCl₅ was placed in a sealed 5.00 dm³ container and heated to 500 K. The total pressure at equilibrium was 250 kPa.

(M_r of PCl₅ = 208.5)

Calculate the value of the equilibrium constant, K_p, for this reaction at 500 K. Include units in your answer.

Use Le Chatelier's principle to state and explain the effect of increasing the temperature on the equilibrium yield of PCl₃.

Phosphorus trichloride, PCl₃, is used in organic synthesis.

Write an equation for the reaction of PCl₃ with propanoic acid to form an acyl chloride and phosphorous acid, H₃PO₃.

Acyl chlorides are useful intermediates. The acyl chloride formed in part (e) is reacted with ethanol.

Name the organic product and the mechanism for this reaction.