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

A chemist is investigating reactions of 3-bromopropan-1-ol, shown below.

In the first experiment, 3-bromopropan-1-ol is heated with an excess of aqueous sodium hydroxide. The C–Br bond is broken and a new organic product, propane-1,3-diol, is formed.

Outline the mechanism for this reaction.

Include curly arrows, relevant lone pairs and dipoles.

The chemist wants to confirm that the bromide ion has been produced.

Describe a chemical test that could be carried out on the resulting solution to confirm the presence of bromide ions.

Include the reagents used and the expected observation.

The reaction in part (a) is repeated using 3-chloropropan-1-ol.

The rate of reaction is observed to be significantly slower.

Explain this difference in rate.

Explain why propane-1,3-diol has a significantly higher boiling point (214 °C) than 3-bromopropan-1-ol (162 °C).

In a second experiment, 3-bromopropan-1-ol is heated with hot, ethanolic potassium hydroxide. This causes an elimination reaction to occur.

Draw the displayed formula of the organic product of this elimination reaction.

Geraniol is a primary alcohol found in rose oil. Its structure is shown below.

Geraniol is heated under reflux with an excess of acidified potassium dichromate(VI). A new product, Z, is formed.

Name the functional group in product Z that is formed from the alcohol group in geraniol.

State the expected colour change during this reaction.

The chemist wants to confirm that the reaction has occurred. The product Z is separated from the reaction mixture.

Describe a chemical test, other than using Tollens' reagent or Fehling's solution, that would give a positive result with product Z but a negative result with the starting material, geraniol.

The chemist performs another test by shaking a sample of geraniol with bromine water.

State the expected observation and name the type of reaction occurring.

In the reaction in part (a), the dichromate(VI) ion, Cr₂O₇^2-, is converted to the Cr³⁺ ion.

Deduce the oxidation state of chromium in the Cr₂O₇^2- ion and write a balanced half-equation for the conversion of Cr₂O₇^2- in acidic solution to Cr³⁺ ions.

Another naturally occurring alcohol is linalool, shown below, which is a structural isomer of geraniol.

Explain why linalool would not react with acidified potassium dichromate(VI).

Strontium is a Group 2 element that reacts with nitrogen gas at high temperatures to form an ionic compound, strontium nitride.

Strontium has a higher first ionisation energy than barium but a lower first ionisation energy than calcium.

Explain why strontium has a higher first ionisation energy than barium.

The nitride ion is formed when a nitrogen atom gains three electrons.

Write the full electron configuration for a nitride ion (N^3-).

A 3.55 g sample of pure strontium nitride is added to 200 cm³ of deionised water. It reacts completely according to the equation below. The resulting solution is transferred to a 250 cm³ volumetric flask and made up to the mark with deionised water.

Sr₃N₂ (s) + 6H₂O (l) → 3Sr(OH)₂ (aq) + 2NH₃ (aq)

A 25.0 cm³ sample of this strontium hydroxide solution is then titrated with 0.150 mol dm^-3 hydrochloric acid.

Calculate the volume of hydrochloric acid, in cm³, required to reach the end-point.

State and explain the shape of the ammonia molecule, NH₃.

An unknown sample of pure magnesium is analysed using a time-of-flight (TOF) mass spectrometer. The sample is ionised by electron impact to form Mg⁺ ions.

Explain why magnesium has a higher melting point than sodium.

The first ionisation energy of magnesium is higher than that of aluminium.

State the full electron configuration of a magnesium atom and use it to explain why this is the case.

In the mass spectrometer, a ²⁵Mg⁺ ion travels along a 1.50 m flight tube. The ion takes 5.58 x 10^-6 s to reach the detector.

Calculate the kinetic energy, in J, of one ²⁵Mg⁺ ion.

(The Avogadro constant, L = 6.022 × 10²³ mol^-1)

The mass spectrum of the sample showed three peaks with the following abundances:

Calculate the relative atomic mass of this sample of magnesium. Give your answer to one decimal place.

A different ion with a mass of 4.32 x 10^-26 kg is accelerated to the same kinetic energy as the ²⁵Mg⁺ ion.

Deduce whether this ion will take a longer or shorter time to travel through the flight tube. Explain your answer.

Carboplatin is a platinum-based anti-cancer drug. It is a coordination complex of platinum(II).

State the oxidation state of platinum in carboplatin, its coordination number, and predict the shape of the complex around the central platinum(II) ion.

The decomposition of carboplatin in an aqueous solution was monitored at a constant temperature. The following data were obtained.

Use a graphical method to show that the reaction is first order with respect to carboplatin. You should include calculations for all the data points you plot.

Use your graph from part (b) to determine the rate constant, k, for this reaction. State the units of k.

At a different temperature, 323 K, the rate constant for the decomposition was found to be 1.52 × 10^-5 s^-1. The activation energy, Eₐ, for the reaction is +115 kJ mol^-1.

Use this information and your answer from part (c) to calculate the temperature, in K, at which the original experiment was performed.

(The Arrhenius equation is ln(k) = - + ln(A) and R = 8.31 J K^-1 mol^-1)

Phosgene, COCl₂, is a toxic gas used in the chemical industry. It is formed by the reversible gas-phase reaction between carbon monoxide and chlorine.

CO (g) + Cl₂ (g) ⇌ COCl₂ (g)

Draw the shape of a phosgene molecule.

Name the shape and justify your answer in terms of the valence shell electron pair repulsion (VSEPR) theory.

The following table gives the standard absolute entropies, S°, for the substances in the reaction.

Calculate the standard entropy change of the system, ΔS_system, for this reaction.

The standard enthalpy change, ΔH^θ, for this reaction is –107.6 kJ mol^-1.
Calculate the Gibbs free-energy change, ΔG^θ, for this reaction at 298 K.

Determine whether the reaction is feasible at this temperature. Show your working.

Write an expression for the equilibrium constant, K_p, for this reaction.

In an experiment, 2.00 mol of CO and 1.50 mol of Cl₂ were mixed in a sealed container. The mixture was heated to 600 K and allowed to reach equilibrium. The total pressure at equilibrium was 250 kPa. The equilibrium mixture contained 1.20 mol of COCl₂.

Calculate the value of K_p for this reaction at 600 K, including its units.

Cinnamyl alcohol is a fragrant compound found in the bark of cinnamon trees. It is a primary alcohol that also contains an alkene and a benzene ring. The structure of the E-isomer is shown below.

Explain why cinnamyl alcohol can exist as E-Z isomers.

The ¹H NMR spectrum of cinnamyl alcohol is recorded.

Predict the splitting pattern observed for the protons in the -CH₂- group.

Explain your answer.

Cinnamyl alcohol is warmed with ethanoyl chloride. An ester, cinnamyl ethanoate, is formed.

Draw the mechanism for this acylation reaction.

Cinnamyl alcohol can be synthesised in the laboratory by the reduction of cinnamaldehyde, C₆H₅CH=CHCHO.

State a suitable reducing agent for this conversion and name the type of mechanism.

Cinnamyl alcohol undergoes two different types of reaction when heated with an excess of hydrogen bromide.

Deduce the structure of the single organic product formed under these conditions.

A student constructs an electrochemical cell under standard conditions by combining two half-cells. The cell diagram is shown below:

Pt (s) | VO₂⁺ (aq), VO²⁺ (aq) || MnO₄^- (aq), Mn²⁺ (aq) | Pt (s)

The relevant standard electrode potentials are:

Half-cell 1: MnO₄^- (aq) + 8H⁺ (aq) + 5e^- ⇌ Mn²⁺ (aq) + 4H₂O (l) E° = +1.51 V

Half-cell 2: VO₂⁺ (aq) + 2H⁺ (aq) + e^- ⇌ VO²⁺ (aq) + H₂O (l) E° = +1.00 V

Both half-cells were prepared using solutions acidified to ensure [H⁺] = 1.00 mol dm^-3.

State the colour of the aqueous VO₂⁺ ion and the aqueous Mn²⁺ ion.

Identify which of the two half-cells contains the negative electrode (anode). Justify your answer.

Write the overall ionic equation for the reaction that occurs when the cell is in operation.

Calculate the standard EMF of the cell.

After the cell has been operating for some time, sodium hydroxide solution is added to Half-cell 1, causing its pH to increase to 4.

Use Le Chatelier’s principle to explain the effect this change would have on the electrode potential of Half-cell 1 and, consequently, on the EMF of the cell.

Hypochlorous acid, HOCl, is a weak acid formed when chlorine dissolves in water. It is the active disinfectant in bleach and swimming pools.

Hypochlorous acid is a weak acid with a K_a of 2.90 × 10^-8 mol dm^-3.

Write an expression for the acid dissociation constant, K_a, of hypochlorous acid.

Calculate the pH of a 0.150 mol dm^-3 solution of hypochlorous acid.

A buffer solution is made by mixing a solution of hypochlorous acid with a solution of sodium hypochlorite (NaOCl).

Explain, with the help of an equation, how this buffer solution resists a large change in pH when a small amount of acid (H⁺) is added.

The reaction between chlorine and water forming hypochlorous acid, HOCl, is a disproportionation reaction:

Cl₂ (g) + H₂O (l) ⇌ HOCl (aq) + HCl (aq)

Justify that this is a disproportionation reaction by stating the relevant oxidation states of chlorine.

Hypochlorous acid decomposes in the presence of sunlight to form hydrochloric acid and oxygen.

2HOCl (aq) → 2HCl (aq) + O₂ (g)

Suggest a suitable method, not involving gas collection, that could be used to monitor the rate of this reaction. Justify your answer.

A solution of sodium hypochlorite (NaOCl) reacts with iodide ions in acidic conditions to reform chlorine.

OCl^- (aq) + 2I^- (aq) + 2H⁺ (aq) → I₂ (aq) + Cl^- (aq) + H₂O (l)

A 25.0 cm³ sample of a commercial bleach solution was diluted to 250 cm³. A 25.0 cm³ portion of this diluted solution was then added to an excess of acidified potassium iodide solution. The resulting iodine was titrated with 0.120 mol dm^-3 sodium thiosulfate solution, requiring 19.50 cm³ for complete reaction.

2S₂O₃^2⁻ (aq) + I₂ (aq) → S₄O₆^2- (aq) + 2I^- (aq)

Calculate the concentration, in mol dm^-3, of sodium hypochlorite in the original, undiluted bleach solution.

A student investigates the reaction between barium hydroxide and ammonium chloride:

Ba(OH)₂⋅8H₂O (s) + 2NH₄Cl (s) → BaCl₂ (aq) + 2NH₃ (g) + 10H₂O (l)

Table 1 gives some standard enthalpies of formation:

Table 1

Use Table 1 to calculate the standard enthalpy change for the reaction.

Table 2 gives the entropy values at 298 K.

Table 2

Use Table 2 to calculate the total entropy change.

 Use your answers in a) and b) to calculate the Gibbs free energy change at 298 K, and state whether the reaction is feasible under standard conditions.

Explain how the solubility of Ba(OH)₂ compares to the solubility of Mg(OH)₂.

Explain why BaCl₂ solution is used to test for sulfate ions and why it is acidified.