Edexcel IAL Chemistry A2 Practical Skills II

[Maximum number: 8]

Brass is an alloy of copper. The percentage by mass of copper in a sample of brass can be determined by a three-stage process.

Stage 1 Oxidation of the copper to copper(II) ions with excess concentrated nitric acid.
Stage 2 Reduction of the copper(II) ions to copper(I) ions with excess iodide ions.
Stage 3 Titration of the iodine produced in Stage 2 against a standard solution of sodium thiosulfate.

(a)

In Stage 1, 2.53 g of brass is carefully warmed with 25 cm³ concentrated nitric acid in a 250 cm³ beaker.

The equation for the oxidation of copper metal by nitric acid is shown.

Cu(s) + 4H⁺(aq) + 2NO₃⁻(aq) → Cu²⁺(aq) + 2NO₂(g) + 2H₂O(l)

[ 8 ]

(i)

State two hazards in this experiment, giving the precautions you would take to minimise the risk associated with each hazard.
Assume safety spectacles and a laboratory coat are worn.

[ 2 ]

(ii)

State what would be seen during the reaction.

[ 2 ]

(iii)

Excess nitric acid must be neutralised before the iodide ions are added in Stage 2.

Sodium carbonate solution is slowly added to the cooled solution. The solution is neutralised when a faint precipitate appears.

Suggest why the solution is cooled before the addition of the sodium carbonate solution.

[ 1 ]

(iv)

Ethanoic acid solution is added, drop by drop, to the neutralised mixture until the precipitate redissolves.

Describe how the solution in the beaker should then be made into $250.0 \mathrm{~cm}^{3}$ of a homogeneous solution before Stage 2.

[ 3 ]

[Maximum number: 5]

This question is about the nitration of methyl benzoate.

The equation for the reaction is shown.

Figure for Question 2 — Edexcel A-Level Chemistry A2

Procedure

Step 1 Weigh between 1.9 g and 2.1 g of methyl benzoate in a $50 \mathrm{~cm}^{3}$ conical flask.
Step 2 Slowly add $5 \mathrm{~cm}^{3}$ of concentrated sulfuric acid to the methyl benzoate with swirling and place the flask in an ice-water bath to cool.

Step 3 Place $2.0 \mathrm{~cm}^{3}$ of concentrated nitric acid into a test tube.
Cool the nitric acid by immersing the test tube in an ice-water bath before slowly adding $2.0 \mathrm{~cm}^{3}$ of concentrated sulfuric acid.
Allow this nitrating mixture to cool.
Step 4 Using a teat pipette, add the nitrating mixture very slowly to the conical flask, ensuring the temperature does not exceed $7^{\circ} \mathrm{C}$ .

Step 5 Allow the flask to stand at room temperature for about 15 minutes and then pour the contents into a beaker containing some crushed ice. Impure methyl 3-nitrobenzoate will form.

Step 6 Recrystallise the methyl 3-nitrobenzoate using methanol as the solvent.
Step 7 Weigh the dry crystals and determine their melting temperature.

(a)

A bottle of concentrated nitric acid has two hazard warning signs.

Figure for Question (a) — Edexcel A-Level Chemistry A2

[ 2 ]

(i)

State the two hazards.

[ 1 ]

(ii)

Give a precaution to reduce the risk when using concentrated nitric acid. Assume that safety goggles and a laboratory coat are used.

[ 1 ]

(b)

Explain why the nitrating mixture is added slowly in Step 4.

[ 2 ]

An answer that makes reference to the following points:

- the reaction is (highly) exothermic

- (if the temperature gets too hot) other reactions may take place / multiple substitutions may take place / multiple nitration may take place/the ester may be hydrolysed

Standalone marks

Ignore mixture gets hot/ (heat) energy given off/ heat is produced Comment: the question implies that the reaction gets hot so M1 is only for exothermic

Allow mixture will boil and reactants will be lost Allow reactants will evaporate Allow other products may be formed

Ignore products will evaporate Ignore splash / spray /spill/spit Ignore to keep the temperature below $7^{\circ} \mathrm{C} /$ low Ignore prevents decomposition Ignore violent reaction

Do not award phenol may be formed

(c)

1.95 g of methyl benzoate reacted with an excess of nitric acid to form
1.51 g of methyl 3-nitrobenzoate.
[Molar mass values: methyl benzoate, $\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CO}_{2} \mathrm{CH}_{3}=136 \mathrm{~g} \mathrm{~mol}^{-1}$
methyl 3-nitrobenzoate, $\mathrm{C}_{6} \mathrm{H}_{4} \mathrm{CO}_{2} \mathrm{CH}_{3} \mathrm{NO}_{2}=181 \mathrm{~g} \mathrm{~mol}^{-1}$ ]

[ 1 ]

(i)

Give one possible reason why the yield in (d)(i) is less than 100\%.

[ 1 ]

[Maximum number: 1]

Sodium thiosulfate can be used to determine the concentration of an iodine solution by titration using starch indicator.

5.00 g of sodium thiosulfate was dissolved in deionised water and the solution made up to $250.0 \mathrm{~cm}^{3}$ in a volumetric flask.

The volumetric flask has an uncertainty of $\pm 0.25 \mathrm{~cm}^{3}$ .
What is the minimum uncertainty of the balance required to match the uncertainty of the volumetric flask?

Assume two weighings are needed.

A

±0.0025 g

B

±0.005 g

C

±0.01 g

D

±0.05 g

[Maximum number: 2]

Brass is an alloy of copper. The percentage by mass of copper in a sample of brass can be determined by a three-stage process.

Stage 1 Oxidation of the copper to copper(II) ions with excess concentrated nitric acid.
Stage 2 Reduction of the copper(II) ions to copper(I) ions with excess iodide ions.
Stage 3 Titration of the iodine produced in Stage 2 against a standard solution of sodium thiosulfate.

In Stage 1, 2.53 g of brass is carefully warmed with 25 cm³ concentrated nitric acid in a 250 cm³ beaker.

The equation for the oxidation of copper metal by nitric acid is shown.

Cu(s) + 4H⁺(aq) + 2NO₃⁻(aq) → Cu²⁺(aq) + 2NO₂(g) + 2H₂O(l)

State what would be seen during the reaction.

[Maximum number: 1]

This question is about the nitration of methyl benzoate.

The equation for the reaction is shown.

Figure for Question 5 — Edexcel A-Level Chemistry A2

Procedure

Step 1 Weigh between 1.9 g and 2.1 g of methyl benzoate in a $50 \mathrm{~cm}^{3}$ conical flask.
Step 2 Slowly add $5 \mathrm{~cm}^{3}$ of concentrated sulfuric acid to the methyl benzoate with swirling and place the flask in an ice-water bath to cool.

Step 3 Place $2.0 \mathrm{~cm}^{3}$ of concentrated nitric acid into a test tube.
Cool the nitric acid by immersing the test tube in an ice-water bath before slowly adding $2.0 \mathrm{~cm}^{3}$ of concentrated sulfuric acid.
Allow this nitrating mixture to cool.
Step 4 Using a teat pipette, add the nitrating mixture very slowly to the conical flask, ensuring the temperature does not exceed $7^{\circ} \mathrm{C}$ .

Step 5 Allow the flask to stand at room temperature for about 15 minutes and then pour the contents into a beaker containing some crushed ice. Impure methyl 3-nitrobenzoate will form.

Step 6 Recrystallise the methyl 3-nitrobenzoate using methanol as the solvent.
Step 7 Weigh the dry crystals and determine their melting temperature.

1.95 g of methyl benzoate reacted with an excess of nitric acid to form
1.51 g of methyl 3-nitrobenzoate.
[Molar mass values: methyl benzoate, $\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CO}_{2} \mathrm{CH}_{3}=136 \mathrm{~g} \mathrm{~mol}^{-1}$
methyl 3-nitrobenzoate, $\mathrm{C}_{6} \mathrm{H}_{4} \mathrm{CO}_{2} \mathrm{CH}_{3} \mathrm{NO}_{2}=181 \mathrm{~g} \mathrm{~mol}^{-1}$ ]

Give one possible reason why the yield in (d)(i) is less than 100\%.

[Maximum number: 2]

Ethyl butanoate reacts with water.

\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COOCH}_{2} \mathrm{CH}_{3}+\mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COOH}+\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}

A student investigated the kinetics of this reaction.

Procedure

Step 150 cm^3 of ethyl butanoate was heated under reflux with a very large excess of water.

Step 2 After 30 minutes a sample of known volume was removed from the reaction mixture and placed in a conical flask.

Step 3 This sample was titrated using aqueous sodium hydroxide solution.
Step 4 Further samples were removed from the refluxing mixture at 30-minute intervals. Each sample was titrated until the titres of three consecutive samples were the same.

The student suggested placing ice in the conical flask before carrying out each titration.

Explain whether or not this suggestion would improve the validity of the data collected.

[Maximum number: 1]

This question is about copper and some of its compounds.

Brass is an alloy of copper and zinc.

A student determined the percentage of copper in a sample of brass.

Procedure
- weigh the sample of brass
- place the brass in a beaker and add concentrated nitric acid until all the brass dissolves
- transfer the solution and washings to a $250.0 \mathrm{~cm}^{3}$ volumetric flask
- make the solution up to the mark with distilled water and mix well
- pipette $25.0 \mathrm{~cm}^{3}$ of the solution into a conical flask
- neutralise the excess nitric acid in the solution
- add $10 \mathrm{~cm}^{3}$ of potassium iodide solution (an excess) to the conical flask
- titrate the iodine produced with $0.100 \mathrm{~mol} \mathrm{dm}^{-3}$ sodium thiosulfate solution using starch indicator
- repeat the titration until concordant titres are obtained.

Name the most suitable piece of apparatus to measure the $10 \mathrm{~cm}^{3}$ of potassium iodide solution.

[Maximum number: 3]

Brass is an alloy of copper. The percentage by mass of copper in a sample of brass can be determined by a three-stage process.

Stage 1 Oxidation of the copper to copper(II) ions with excess concentrated nitric acid.
Stage 2 Reduction of the copper(II) ions to copper(I) ions with excess iodide ions.
Stage 3 Titration of the iodine produced in Stage 2 against a standard solution of sodium thiosulfate.

In Stage 1, 2.53 g of brass is carefully warmed with 25 cm³ concentrated nitric acid in a 250 cm³ beaker.

The equation for the oxidation of copper metal by nitric acid is shown.

Cu(s) + 4H⁺(aq) + 2NO₃⁻(aq) → Cu²⁺(aq) + 2NO₂(g) + 2H₂O(l)

Ethanoic acid solution is added, drop by drop, to the neutralised mixture until the precipitate redissolves.

Describe how the solution in the beaker should then be made into $250.0 \mathrm{~cm}^{3}$ of a homogeneous solution before Stage 2.

[Maximum number: 3]

Which test can be used to distinguish between each of the pairs of compounds shown?

(a)

and

A

warm each compound with Tollens' reagent

B

add sodium carbonate solution to each compound

C

warm each compound with iodine under alkaline conditions

D

add phosphorus(V) chloride to each compound

[ 1 ]

(b)

Figure for Question (b) — Edexcel A-Level Chemistry A2

and

A

warm each compound with Tollens' reagent

B

add sodium carbonate solution to each compound

C

warm each compound with iodine under alkaline conditions

D

add phosphorus(V) chloride to each compound

[ 1 ]

(c)

Figure for Question (c) — Edexcel A-Level Chemistry A2

and

A

warm each compound with Tollens' reagent

B

add sodium carbonate solution to each compound

C

warm each compound with iodine under alkaline conditions

D

add phosphorus(V) chloride to each compound

[ 1 ]

[Maximum number: 1]

A group of students prepared methyl 3-nitrobenzoate by the nitration of methyl benzoate.

Figure for Question 10 — Edexcel A-Level Chemistry A2

Procedure

Step 1 Measure $9 \mathrm{~cm}^{3}$ of concentrated sulfuric acid into a small, dry conical flask. Label the flask A and place it in an ice bath.

Step 2 Add $4.0 \mathrm{~cm}^{3}$ of methyl benzoate to flask A.
Gently swirl the flask.
Step 3 Mix $3 \mathrm{~cm}^{3}$ of concentrated nitric acid and $3 \mathrm{~cm}^{3}$ of concentrated sulfuric acid in a test tube to form the nitrating mixture. Place this test tube in the ice bath.

Step 4 Place a thermometer in flask A. Add the nitrating mixture very slowly to flask A using a dropping pipette.
Take care to ensure that the temperature of the flask contents does not rise above $15^{\circ} \mathrm{C}$ .

Step 5 Remove flask A from the ice bath and allow it to stand at room temperature for about 10 minutes.
Pour the reaction mixture into a small beaker containing crushed ice. Stir the contents of the beaker with a glass rod.

Step 6 Allow the ice to melt. Separate the solid methyl 3-nitrobenzoate by suction filtration. Wash the solid with a small amount of deionised water and then with a little ice-cold ethanol.

Step 7 Recrystallise the methyl 3-nitrobenzoate using ethanol as the solvent.
Step 8 Determine the melting temperature of the purified crystals of methyl 3-nitrobenzoate.

An ice bath is a mixture of ice and water in a beaker.

Suggest an advantage of using an ice bath in Steps 1 and 3 rather than a beaker containing only ice cubes. Justify your answer.

Edexcel A-Level Chemistry A2 Practical Skills in Chemistry Ii Questions

Exam points

Covered syllabus points

Covered syllabus points

Question 1

Question (a)

Question (i)

Question (ii)

Question (iii)

Question (iv)

Question 2

Question (a)

Question (i)

Question (ii)

Question (b)

Question (c)

Question (i)

Question 3

Question 4

Question 5

Question 6

Question 7

Question 8

Question 9

Question (a)

Question (b)

Question (c)

Question 10