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Edexcel IAL Chemistry Practical Skills in Chemistry II

Practical Skills II questions usually ask you to justify a method choice, calculate uncertainty, plot or use a graph and evaluate whether data support a conclusion.

Syllabus
First assessment 2019
Course
Chemistry YCH11
Level
A2

Exam points

  • Plot graphs with labelled axes, suitable scales, accurate points and best-fit lines.
  • Calculate percentage uncertainty from apparatus readings, titres or temperature changes.
  • Explain method choices such as constant volume, cooling, stirring or solvent extraction.

Covered syllabus points

6.P1—Solving practical-context problems6.P2—Controlling variables6.P3—Presenting data6.P4—Evaluating results and conclusions6.P5—Uncertainties and errors6.P6—Evaluating an experimental method6.P7—Plotting and interpreting graphs6.P8—Processing and analysing data6.P9—Significant figures6.P10—Accuracy and precision6.P11—Selecting laboratory apparatus6.P12—Using apparatus and techniques6.P13—Range and resolution of apparatus6.P14—Health and safety6.P15—Tests for ions and gases6.P16—Tests for organic functional groups6.P17—Measurements and data across the specification6.P18—Preparing inorganic and organic compounds

Practical Skills in Chemistry II question 1

[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.

Question (a)

(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 ]

Question (i)

(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 ]

Question (ii)

(ii)

State what would be seen during the reaction.

[ 2 ]

Question (iii)

(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 ]

Question (iv)

(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 cm3250.0 \mathrm{~cm}^{3} of a homogeneous solution before Stage 2.

[ 3 ]

Practical Skills in Chemistry II question 2

[Maximum number: 5]

This question is about the nitration of methyl benzoate.

The equation for the reaction is shown.

Figure for Question Practical Skills in Chemistry II 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 cm350 \mathrm{~cm}^{3} conical flask.
Step 2 Slowly add 5 cm35 \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 cm32.0 \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 cm32.0 \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∘C7^{\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.

Question (a)

(a)

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

Figure for Question (a) — Edexcel A-Level Chemistry A2
Figure for Question (a) — Edexcel A-Level Chemistry A2
[ 2 ]

Question (i)

(i)

State the two hazards.

[ 1 ]

Question (ii)

(ii)

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

[ 1 ]

Question (b)

(b)

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

[ 2 ]

Question (c)

(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, C6H5CO2CH3=136 g mol−1\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CO}_{2} \mathrm{CH}_{3}=136 \mathrm{~g} \mathrm{~mol}^{-1}
methyl 3-nitrobenzoate, C6H4CO2CH3NO2=181 g mol−1\mathrm{C}_{6} \mathrm{H}_{4} \mathrm{CO}_{2} \mathrm{CH}_{3} \mathrm{NO}_{2}=181 \mathrm{~g} \mathrm{~mol}^{-1} ]

[ 1 ]

Question (i)

(i)

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

[ 1 ]

Practical Skills in Chemistry II question 3

[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 cm3250.0 \mathrm{~cm}^{3} in a volumetric flask.

The volumetric flask has an uncertainty of ±0.25 cm3\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

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