AQA A Level Chemistry Paper 3 November 2021 question paper 7405/3

This question is about ethanedioic acid (HOOCCOOH) and the ethanedioate ion (⁻OOCCOO⁻).

Ethanedioic acid reacts with propane-1,3-diol (HOCH₂CH₂CH₂OH) to form a polyester.

Draw the repeating unit of this polyester.

Explain why polyesters are biodegradable but polyalkenes are not biodegradable.

Sodium ethanedioate is used to find the concentration of solutions of potassium manganate(VII) by titration. The equation for this reaction is

A standard solution is made by dissolving 162 mg of Na₂C₂O₄ (M_r = 134.0) in water and making up to 250 cm³ in a volumetric flask.

25.0 cm³ of this solution and an excess of sulfuric acid are added to a conical flask. The mixture is warmed and titrated with potassium manganate(VII) solution. The titration is repeated until concordant results are obtained.

Calculate the concentration, in mol dm^–3, of the potassium manganate(VII) solution.

Figure 1 shows the 25.0 cm³ pipette used to measure the sodium ethanedioate solution.

On Figure 1, draw the meniscus of the solution when the pipette is ready to transfer 25.0 cm³ of the sodium ethanedioate solution.

Potassium manganate(VII) is oxidising and harmful. Sodium ethanedioate is toxic.

Suggest safety precautions, other than eye protection, that should be taken when:

• filling the burette with potassium manganate(VII) solution

• dissolving the solid sodium ethanedioate in water.

Filling the burette

Dissolving the solid

State the colour change seen at the end point of each titration.

Figure 2 shows the burette containing potassium manganate(VII) solution.

Give two practical steps needed before recording the initial burette reading.

When Na₂C₂O₄(aq) is added to a solution containing [Fe(H₂O)₆]³⁺ ions, a reaction occurs in which all six water ligands are replaced by ethanedioate ions.

Explain why the replacement of the water ligands by ethanedioate ions is favourable. In your answer refer to:

• the enthalpy and entropy changes for the reaction

• how the enthalpy and entropy changes influence the free-energy change for the reaction.

The protein fibroin can be broken down into amino acids using an enzyme.

A student uses thin-layer chromatography (TLC) to identify these amino acids.

The student identifies two of the amino acids as alanine and serine.

Use Figure 3 to calculate the R_f value of the unknown amino acid. Show your working.

Use your R_f value and Table 1 to identify the unknown amino acid.

The amino acids cannot be seen as they move during the experiment.

State how the amino acids can be made visible at the end of the experiment.

State why each amino acid has a different R_f value.

This question is about ketones.

Solution X reacts with liquid ketones to form a crystalline solid.

This reaction can be used to identify a ketone if the crystalline solid is separated, purified by recrystallisation, and the melting point determined.

Describe how the crystalline solid is separated and purified.

Propanone (CH₃COCH₃) reacts with the weak acid HCN to form a hydroxynitrile.

This hydroxynitrile is usually made by reaction of propanone with KCN followed by dilute acid, instead of with HCN

State the hazard associated with the use of KCN

Suggest a reason, other than safety, why KCN is used instead of HCN.

Why KCN is used

Outline the mechanism for the reaction of propanone with KCN followed by dilute acid.

This question is about Group 7 chemistry.

Give an equation for the reaction of solid sodium bromide with concentrated sulfuric acid to form bromine.

State one observation made during this reaction.

Equation

A solution that is thought to contain chloride ions and iodide ions is tested.

1. Dilute nitric acid is added to the solution.

2. Aqueous silver nitrate is added to the solution.

3. A pale yellow precipitate forms.

4. Excess dilute aqueous ammonia is added to the mixture.

5. Some of the precipitate dissolves and a darker yellow precipitate remains.

Give a reason for the use of each reagent.

Explain the observations.

Give ionic equations for any reactions.

A mixture of methanoic acid and sodium methanoate in aqueous solution acts as an acidic buffer solution.

The equation shows the dissociation of methanoic acid.

Calculate the mass, in g, of sodium methanoate (HCOONa) that must be added to 25.0 cm³ of 0.100 mol dm³ methanoic acid to produce a buffer solution with pH = 4.05 at 298 K

For methanoic acid, pK_a = 3.75 at 298 K

Assume that the volume of the solution remains constant.

A student plans an experiment to investigate the yield of propanoic acid when a sample of propan-1-ol is oxidised.

Figure 4 shows the apparatus that the student plans to use for the experiment.

The student’s teacher says that the apparatus is not safe.

Give two reasons why the apparatus shown in Figure 4 is not safe.

Give one additional reagent that is needed to form any propanoic acid.

State two more mistakes in the way the apparatus is set up in Figure 4.

State the purpose of the small glass beads in the flask in Figure 4.

After correcting the mistakes, the student heats a reaction mixture containing 6.50 g of propan-1-ol with an excess of the oxidising agent. The propanoic acid separated from the reaction mixture has a mass of 3.25 g

State the name of the technique used to separate the propanoic acid from the reaction mixture.

Calculate the percentage yield of propanoic acid.

Which does not involve the absorption of ultraviolet radiation or visible light?

Which statement about chloride ions is not correct?

What is the mole fraction of 1.0 g of a compound of relative molecular mass 100.0 dissolved in 30.0 g of a solvent of relative molecular mass 50.0?

Which has the electron configuration of a noble gas?

Which statement does not support the suggestion that an unknown organic

Which statement about inorganic ionic compounds is always correct?

Which species has a lone pair of electrons on the central atom?

In which substance do covalent bonds break when it melts?

In which molecule are all the atoms in the same plane?

Which molecule has a permanent dipole?

Which statement about (CH₃)₂CHCH₂COOH is correct?

A mixture of 2 dm³ of hydrogen and 1 dm³ of oxygen is at room temperature.

Which statement is correct?

Which statement about the distribution curve of molecular energies in an ideal gas at a given temperature is correct?

Which statement about the addition of a catalyst to an equilibrium mixture is correct?

Which equation does not show the reduction of a transition metal?

Which substance contains delocalised electrons?

Which compound has E – Z isomers?

Which polymer has hydrogen bonding between the polymer chains?

Which compound needs the greatest amount of oxygen for the complete combustion of 1 mol of the compound?

Which compound is produced when 1-phenylethanol reacts with acidified potassium dichromate(VI)?

Which is the correct general formula for non-cyclic compounds in the homologous series?

Which compound forms a white precipitate when added to aqueous silver nitrate?

Nitration of 1.70 g of methyl benzoate (M_r = 136.0) produces methyl 3-nitrobenzoate

What mass, in g, of methyl 3-nitrobenzoate is produced?

A two-step preparation of propylamine is shown.

bromoethane → X → propylamine

What is X ?

Which compound reacts with warm dilute aqueous sodium hydroxide?

Methylamine reacts with bromoethane by nucleophilic substitution to produce a mixture of products.

Which is not a possible product of this reaction?

Which is the repeating unit of a polyamide?

Which type of polymer is not hydrolysed by heating with concentrated aqueous sodium hydroxide?

Which is the structure of a zwitterion of an amino acid?

Which row shows a pair of bases that can link two strands of DNA with three hydrogen bonds?

Use the Data Booklet to help you answer this question.