4.1 Physical Chemistry Core Practicals

1 mark

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.

q3-9cho-al-3-june-2018-qp-edexcel-a-level-chem

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.

Measurement	Value
Mass of weighing bottle and carbonate / g	13.247
Mass of empty weighing bottle / g	12.431
Mass of carbonate used / g	...................
Volume of acid used / cm³	100
Volume of gas collected / cm³	225

Table 1

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

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1 mark

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

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4 marks

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.

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1d5 marks

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.

(2)

ii)

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⁻³dilute nitric acid.

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

No calculation is required.

(1)

iii)

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.

(2)

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4 marks

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.

Measurement	Value
Mass of carbonate + test tube / g	20.447
Mass of oxide + test tube / g	20.205
Mass of empty test tube / g	19.996

Table 2

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

(1)

ii)

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

(3)

How did you do?

1 mark

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.

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1g2 marks

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.

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6 marks

The enthalpy change for the decomposition of sodium hydrogencarbonate can be determined indirectly using Hess’s Law.

2NaHCO₃(s) $\overset{∆_{r} H}{\to}$ Na₂CO₃(s) + H₂O (l) + CO₂(g)

A student carried out two experiments.

Experiment 1 involved the reaction between sodium hydrogencarbonate and hydrochloric acid.

The student used the following procedure:

use a measuring cylinder to measure 50 cm³of 2.00 mol dm⁻³hydrochloric acid and pour it into a polystyrene cup
measure the initial temperature of the acid
weigh the test tube containing sodium hydrogencarbonate
tip the sodium hydrogencarbonate into the hydrochloric acid in the polystyrene cup, stir the mixture and record the lowest temperature reached
weigh the empty test tube.

Results

Measurement	Value
Mass of test tube + NaHCO₃ / g	21.23
Mass of empty test tube / g	15.61
Mass of NaHCO₃ used / g
Initial temperature / °C	21.0
Final temperature / °C	14.4
Temperature fall / °C

Complete the table.

(1)

ii)

Show, by calculation, that the hydrochloric acid is in excess.
You must show your working.

NaHCO₃(s) + HCl (aq) → NaCl (aq) + H₂O (l) + CO₂(g)

(2)

iii)

Calculate the enthalpy change for the reaction between sodium hydrogencarbonate and hydrochloric acid, using the results of the experiment.

Include a sign and units in your answer.

[Assume: mass of reaction mixture = 50.0 g specific heat capacity of the reaction mixture = 4.18 J g^-1 ^oC^-1]

(3)

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5 marks

Experiment 2 involved the reaction between sodium carbonate and hydrochloric acid.

The student repeated the procedure for Experiment 1 but used sodium carbonate instead of sodium hydrogencarbonate and measured the maximum temperature rise.

Na₂CO₃(s) + 2HCl (aq) → 2NaCl (aq) + H₂O (l) + CO₂(g)

The student calculated the enthalpy change for this reaction as −29.4 kJ mol⁻¹.

Complete the Hess cycle with appropriate formulae and labelled arrows.

(2)

q7b-9cho-al-3-oct-2021-edexcel-a-level-chem

ii)

Calculate the enthalpy change for the decomposition of sodium hydrogencarbonate.
Include a sign and units in your answer.

2NaHCO₃(s) $\overset{∆_{r} H}{\to}$ Na₂CO₃(s) + H₂O(l) + CO₂(g)

(3)

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8 marks

Another student carried out the same two experiments and obtained a value for the enthalpy change of decomposition of sodium hydrogencarbonate of +74 kJ mol⁻¹.
The data book value for this enthalpy change is +90 kJ mol⁻¹.

Calculate the percentage error in this student’s value.

(1)

ii)

Calculate the percentage uncertainties in measuring 50 cm³ of hydrochloric acid using a burette and using a measuring cylinder.

(1)

Apparatus	Uncertainty	Percentage uncertainty
Measuring cylinder	±0.5 cm³ for each volume measured
Burette	±0.05 cm³ for each reading

iii)

Give a reason why using a burette rather than a measuring cylinder will not improve the accuracy of the experiment.

(1)

iv)

Describe changes to the method and how the data is used that would improve the accuracy of the determination of the temperature change in Experiment 2.
Your description should involve the use of a clock and plotting a graph.

(5)

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1a4 marks

This question is about the synthesis and reactions of butane-1,4-diol.

Butane-1,4-diol can be synthesised from but-2-ene-1,4-diol, by reaction with a reagent, B.

Identify reagent B and state suitable conditions for this reaction.

(2)

ii)

This reaction is best described as

(1)

☐	A	hydrolysis
☐	B	oxidation
☐	C	reduction
☐	D	substitution

iii)

Name one other commercially important product that can be manufactured by this type of reaction with the alkene group.

(1)

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6 marks

Butane-1,4-diol can be oxidised to form butanedioic acid. The molecular formula of butanedioic acid is C₄H₆O₄ and it is a solid at room temperature.

Describe how you would make 250 cm³ of a solution of butanedioic acid with an accurately known concentration of approximately 0.0500 mol dm⁻³.

Butanedioic acid is sufficiently soluble in water to achieve this concentration.

[Molar mass of butanedioic acid = 118 g mol⁻¹]

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2a5 marks

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 1 HOOCCH $═$ CHCOOH

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.

(2)

ii)

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.

(3)

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2 marks

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.

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7 marks

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.

(2)

ii)

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

(2)

iii)

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 ± 0.0 4cm³ for the volume

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

(3)

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2d4 marks

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 1 HOOCCH $═$ CHCOOH

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.

(2)

ii)

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.

(2)

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2e2 marks

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.

Structure	Test with bromine water	Test with acidified potassium dichromate(VI)
HOOCCH $═$ CHCOOH
HOCH₂CH $═$ CHCH₂COOH
CH₃CH₂CH₂CH₂CH₂COOH

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2f4 marks

The structure HOOCCH $═$ CHCOOH has two stereoisomers.

Draw the structures of these stereoisomers.

(2)

E-isomer

Z-isomer

ii)

State why HOOCCH

═

CHCOOH has E / Z isomers.

(2)

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Edexcel AS Chemistry

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4.1 Physical Chemistry Core Practicals