Edexcel A Level Chemistry Paper 3: General and Practical Principles in Chemistry June 2018 question paper 9CH0/03

This question is about some halogens and their compounds.

The intermolecular attractions between halogen molecules are London forces.

Describe how London forces form between halogen molecules.

The boiling temperatures of chlorine and bromine are shown in the table.

Explain why bromine has a higher boiling temperature than chlorine.

A student carries out experiments to determine the order of reactivity of three halogens: bromine, chlorine and iodine. The student is provided with aqueous solutions of the following five substances:

bromine

iodine

potassium chloride

potassium bromide

potassium iodide.

The student has no access to chlorine gas or chlorine water. The student uses cyclohexane, an organic solvent, to identify the halogen present at the end of each experiment.

The student carries out the smallest number of experiments required to determine the order of reactivity of the halogens.

Describe the experiments and the expected observations.

Include in your answer ionic equations for any reactions that occur.

State symbols are not required.

This question is about lactic acid (2-hydroxypropanoic acid), CH₃CH(OH)COOH. Lactic acid is used to make biodegradable polymers.

Lactic acid can be made in a two-step synthesis starting from ethanal, CH₃CHO.

Devise a reaction scheme for a two-step synthesis.

Include in your answer all reagents and conditions, the type of reaction occurring at each step, and a balanced equation for each reaction. State symbols are not required.

Polymerisation of lactic acid forms poly(lactic acid) as shown in the diagram.

State the type of polymerisation occurring in this reaction.

On the diagram, draw a circle around the repeat unit of the polymer.

This question is about the identification of a Group 2 carbonate.

A chemistry teacher found a bottle containing lumps of a white solid. The original label was missing from the bottle. However, someone had written ‘Group 2 carbonate’ on the bottle. The lumps of the anhydrous white solid were pure and dry.

The chemistry teacher tried to identify the carbonate with the help of three students. The three students worked under identical conditions and shared the same weighing balance.

Student 1 recognised that if an acid is added to a carbonate, carbon dioxide is evolved. The student decided to measure the volume of carbon dioxide evolved when the Group 2 carbonate reacts with excess nitric acid.

The student knew that 1 mol of a Group 2 carbonate produces 1 mol of carbon dioxide.

Student 1 set up the apparatus shown below.

Student 1 weighed out some of the Group 2 carbonate and added it to a 250 cm³ conical flask.

Student 1 then added 100 cm³ of 0.200 mol dm⁻³ nitric acid to the conical flask and replaced the bung.

Student 1 measured the volume of gas collected in the inverted measuring cylinder at room temperature and pressure (r.t.p.) when all the Group 2 carbonate had reacted.

Student 1 obtained the results shown in Table 1.

Complete Table 1 to show the mass of the carbonate used.

Calculate the amount, in moles, of carbon dioxide collected in the measuring cylinder at r.t.p.

Calculate the molar mass of the Group 2 carbonate to an appropriate number of significant figures and hence deduce the identity of the Group 2 metal.

Student 2 carried out the same experiment as Student 1, using the same mass of the Group 2 carbonate. Student 2 made no errors in their measurements or calculations but obtained a value for the molar mass which was 10 g mol⁻¹ greater than the value obtained by Student 1.

Explain one procedural error which could have resulted in Student 2 obtaining a molar mass greater than that of Student 1.

It was later discovered that Student 2 had used 110 cm³ of 0.200 mol dm⁻³ dilute nitric acid, instead of 100 cm³ of 0.200 mol dm⁻³ dilute nitric acid.

Give a reason why this mistake would not have affected Student 2 ’s result.

No calculation is required.

The teacher noticed that Student 2 had used the Group 2 carbonate in powdered form rather than in lumps. Explain how, if at all, this would affect the rate of reaction and the final volume of gas produced in the reaction.

Student 3 suggested a different experiment. Student 3 realised that, by heating the carbonate, carbon dioxide would be lost and an oxide would remain. Student 3 decided to measure the change in mass of the carbonate and to use this information to calculate its molar mass.

Student 3 weighed an empty test tube.

Using a spatula, Student 3 added some of the carbonate to the test tube.

The test tube containing the carbonate was then weighed.

The test tube and its contents were heated to constant mass.

The results obtained by Student 3 are shown in Table 2.

Write an equation, including state symbols, for the thermal decomposition of a Group 2 carbonate, MCO₃, where M represents the metal.

Using Student 3 ’s results, calculate the molar mass of the Group 2 carbonate.

Student 3 used the same balance as Student 1.

Give a reason why the mass of the carbonate measured by Student 3 has a greater percentage uncertainty than that measured by Student 1.

Student 3 noticed that on heating the test tube some solid was lost. Explain how this would affect the calculated value for the molar mass of the Group 2 carbonate.

This question is about the use of NMR spectroscopy to distinguish between isomers of C₆H₁₂O₂.

Tetramethylsilane (TMS) is a compound used as a standard when recording both ¹H and¹³C NMR spectra.

Give the structural formula of TMS.

TMS is an inert and non-toxic compound. State two other reasons why TMS is suitable for use as a standard when recording NMR spectra.

Draw the structural formulae of the two esters with formula C₆H₁₂O₂ that each have only two peaks, both singlets, in their high resolution proton NMR spectra. The relative peak areas are 3:1 for both esters.

The high resolution proton NMR spectrum of another isomer of C₆H₁₂O₂ is shown.

The ratios of the number of protons for the five sets peaks in the spectrum are given in the table.

Show that all these data are consistent with the displayed formula shown. Refer to the five chemical shifts and explain two of the splitting patterns.

H H H H O H

H H H H H

There are three other isomers of C₆H₁₂O₂ which are carboxylic acids with five peaks in their carbon-13 NMR spectra.

Draw the structural formula of two of these isomers.

Draw the skeletal formula of a cyclic diol isomer of C₆H₁₂O₂ that has only two peaks in its carbon-13 NMR spectrum.

This question is about the properties of transition elements, their ions and their complexes.

Give the oxidation state of vanadium in the compound NH₄VO₃.

Excess zinc powder is added to an acidified solution of the compound NH₄VO₃. Using the data in the table, explain the sequence of reactions that takes place.

ionic equations with their calculated values. State symbols are not required.

Explain how vanadium(V) oxide acts as a catalyst in one step of the contact process. The equation for this step is

*(d) Describe the reactions of separate portions of aqueous copper(II) ions with aqueous sodium hydroxide solution, with excess aqueous ammonia solution and with concentrated hydrochloric acid.

In your answer you should link observations with equations which include the formulae of any copper-containing complex ions. Include state symbols.

This question is about the analysis of an unknown carboxylic acid X by three students.

The students analyse the mass spectrum of X and find that it has a molecular ion peak at m / z = 116.

The three students each propose a different structural formula for compound X.

Structure 2 HOCH₂CH CHCH₂COOH

Structure 3 CH₃CH₂CH₂CH₂CH₂COOH

The students are given the infrared spectrum of X.

State two wavenumber ranges of the infrared absorptions providing evidence that compound X is a carboxylic acid. Include the bonds responsible.

One of the students suggests that this infrared spectrum and the data in the Data Booklet alone could be used to identify which of the three proposed structures is X.

Show that this student’s suggestion is correct. Include relevant infrared data in your answer.

The students decide to carry out an acid-base titration to obtain further information about compound X. Each student uses solid sodium hydroxide, NaOH, to prepare a solution of concentration 0.140 mol dm⁻³.

Calculate the mass, in grams, of solid sodium hydroxide that each student should weigh out to prepare 250.0 cm³ of a 0.140 mol dm⁻³ solution.

Each of the students makes up 250.0 cm³ of 0.140 mol dm⁻³ sodium hydroxide solution in a volumetric flask and titrates this solution with the same solution of X of known concentration.

Student A

correctly prepares the 0.140 mol dm⁻³ sodium hydroxide solution and pipettes a volume of 10.0 cm³ of the solution into a conical flask

fills a burette with the solution of X and carries out a titration

repeats the procedure until obtaining concordant results

obtains a mean titre of 10.20 cm³.

Student B

dissolves the sodium hydroxide in distilled water and transfers the solution to a volumetric flask

adds more distilled water to the volumetric flask and mixes the solution

notices that the volumetric flask has been filled with distilled water several cm³ beyond the graduation mark

realises the mistake, removes the extra solution and discards it

pipettes 10.0 cm³ of the sodium hydroxide solution into a conical flask and titrates this with the solution of X.

Student C

correctly prepares the 0.140 mol dm⁻³ sodium hydroxide solution

washes a conical flask thoroughly with distilled water and pipettes 10.0 cm³ of the sodium hydroxide solution into the wet conical flask

titrates the contents of the conical flask with the solution of X.

Explain how, if at all, Student B ’s mistake affects the value of the titre.

Explain how, if at all, Student C ’s use of a wet conical flask affects the value of the titre.

Student A uses three pieces of apparatus to measure volumes in this experiment.

The burette has an uncertainty of ±0.05 cm³ for each volume reading

The volumetric flask has an uncertainty of ±0.30 cm³ for the volume

The pipette has an uncertainty of ±0.04 cm³ for the volume

Show by calculation which volume measurement has the lowest percentage uncertainty.

Student A calculates the correct value for the molar mass of compound X, using the mean titre of 10.20 cm³. The results indicate that X has structure 1.

Structure 2 HOCH₂CH CHCH₂COOH

Structure 3 CH₃CH₂CH₂CH₂CH₂COOH

Write the equation for the reaction between structure 1 and sodium hydroxide solution. State symbols are not required.

Deduce the value that would have been obtained for the mean titre if the structural formula of X had been structure 2. Justify your answer.

The students could have identified the three structures using chemical tests.

Complete the table to show whether or not the suggested structures react with bromine water and when heated with acidified potassium dichromate(VI).

Use a tick (✓) if a reaction occurs.

Use a cross (✗) if no reaction occurs.

The structure HOOCCH CHCOOH has two stereoisomers.

Draw the structures of these stereoisomers.

E-isomer

Z-isomer

State why HOOCCH CHCOOH has E/Z isomers.

This question is about weak acids.

A weak acid, HX, has a K_a value of 5.25 × 10⁻⁵mol dm⁻³. A solution was formed by mixing 10.5 cm³ of 0.800 mol dm⁻³ dilute sodium hydroxide with 25.0 cm³ of 0.920 mol dm⁻³ HX(aq). Calculate the pH of the solution formed, showing all your working.

Propanoic acid, CH₃CH₂COOH, is a weak acid. On the grid below, sketch the change in pH during the addition of 50.0 cm³ of 0.100 mol dm⁻³ sodium hydroxide solution to 25.0 cm³ of 0.100 mol dm⁻³ propanoic acid solution.

Volume of sodium hydroxide added / cm³

Explain how you would use the graph in (b)(i) to obtain the value of the acid dissociation constant, K_a, for propanoic acid. You are not expected to calculate this value.

State the role of the hydrochloric acid in the esterification reaction.

Calculate the total amount, in moles, of acid present in the volumetric flask in the equilibrium mixture.

The 2.0 cm³ of dilute hydrochloric acid contained 0.00400 mol of H⁺(aq) ions. Use this and your answer to part (b)(i) to calculate the amount, in moles, of ethanoic acid present in the equilibrium mixture.

The initial mixture in the boiling tube contained 0.105 mol of ethanoic acid. Use your answer to (b)(ii) to calculate the amount, in moles, of ethanoic acid that reacted to form the ester in the equilibrium mixture.

Use information given in the method, and your answer to (c)(i), to calculate the amounts, in moles, of propan-1-ol, propyl ethanoate and water that are present in the equilibrium mixture.

Moles of propan-1-ol at equilibrium_........

Moles of propyl ethanoate at equilibrium_........

Moles of water at equilibrium_........

Write the expression for the equilibrium constant, K_c, for this reaction.

Explain why it is possible, in this case, to calculate K_c using equilibrium amounts in moles, rather than equilibrium concentrations.

Calculate the value of K_c. Give your answer to an appropriate number of significant figures.

The pink colour of the phenolphthalein fades after the end-point of the titration has been reached. Give a possible explanation for this observation.

Explain what you could do to confirm that one week is sufficient time for the mixture to reach equilibrium.

A student repeated the experiment, but left the mixture in a water bath at 40 °C until equilibrium was reached.

Deduce the effect, if any, on this student’s value for K_c compared with that obtained in part (d)(iii).