Edexcel International A Level Chemistry: Practical Skills in Chemistry II (YCH11) – Revision Notes

These notes condense Unit 6: Practical Skills in Chemistry II from the Pearson Edexcel International Advanced Subsidiary/Advanced Level in Chemistry specification (YCH11), Issue 1, September 2017. The unit is International A2 (IA2). Its paper tests the practical work of Units 4 and 5, including Core Practicals 9 to 16, and can also ask about the ion, gas and functional-group tests from Units 1 and 2. For full explanations and worked examples, use the Unit 6 study guide.

Then test yourself with the Unit 6 practice questions. The Unit 3 revision notes cover the AS practical skills. Track progress on the Edexcel Chemistry checklist, browse the course hub or take a free diagnostic.

The paper in brief

• One written paper: 1 hour and 20 minutes, 50 marks, all questions compulsory.
• Short-open, open-response and calculation questions; at least 6 marks of Level 2 mathematics.
• Calculators may be used. Contexts can be familiar or new.
• Skills tested: solving practical problems, controlling variables, presenting and evaluating data, uncertainty, graphs, significant figures, accuracy and precision, and choosing and using apparatus safely.

Core practicals at a glance

CP	What you measure	What you calculate
9a	Thiosulfate titre of quenched samples over time	[I₂] at each time; order in iodine from the graph shape
9b	Time for the blue-black colour to appear	Rate ∝ 1/t; order from how 1/t changes with volume
10	Time for a fixed amount of reaction at several temperatures	Ea from the gradient of ln(1/t) against 1/T
11	pH at half-neutralisation	Ka = 10^(−pH)
12	Cell emf with a high-resistance voltmeter	E⦵cell from data; compare with measured value
13a	Titre of KMnO₄ against Fe²⁺	Moles and mass of iron; percentage purity
13b	Titre of Na₂S₂O₃ against I₂	Moles of the oxidising agent that released the I₂
14	Mass of dry complex	Percentage yield
15	Observations	Identity of ions and functional groups
16	Mass and melting range of aspirin	Percentage yield; purity from melting range

Key equations and ratios

Situation	Relationship
Iodine and thiosulfate	I₂ : S₂O₃²⁻ = 1 : 2
Manganate(VII) and iron(II)	MnO₄⁻ : Fe²⁺ = 1 : 5
Iodate(V) releasing iodine	IO₃⁻ + 5I⁻ + 6H⁺ → 3I₂ + 3H₂O, so IO₃⁻ : S₂O₃²⁻ = 1 : 6
Clock reactions	Initial rate ∝ 1/t
Arrhenius (given if needed)	ln k = −Ea/(RT) + constant; Ea = −gradient × R
Half-neutralisation	pH = pKa; Ka = 10^(−pH)
Weak acid of known concentration	Ka ≈ [H⁺]² ÷ [HA]
Cell emf	E⦵cell = E⦵(positive electrode) − E⦵(negative electrode)
Percentage uncertainty	(uncertainty ÷ value) × 100; a titre or difference carries twice the single-reading uncertainty
Percentage yield	(actual mass ÷ theoretical mass) × 100

Method in steps

Core Practical 9a, quench and titrate

1. Start the clock when the iodine is added.
2. Pipette a sample into sodium hydrogencarbonate solution; note the time.
3. Titrate with thiosulfate; add starch when pale yellow.
4. Plot titre (or [I₂]) against time. A straight line means zero order in iodine.

Core Practical 9b, clock reaction

1. Fix every volume except one; make up the total with water.
2. Mix quickly, start timing, stop at the blue-black colour.
3. Compare 1/t values: ×2 concentration with ×2 rate is first order; ×4 rate is second order.

Core Practical 10, activation energy

1. Bring the solutions to temperature separately.
2. Mix, time, and record the mixture’s temperature.
3. Plot ln(1/t) (y) against 1/T in K⁻¹ (x). Ea = −gradient × 8.31, then divide by 1000 for kJ mol⁻¹.

Core Practical 11, Ka

1. Calibrate the pH meter with buffers.
2. Find the end-point volume, V, for one portion of acid.
3. Add V/2 of alkali to an identical portion; read the pH.

Core Practical 12, cells

1. Clean metals with emery paper.
2. Use a potassium nitrate salt bridge (it forms no precipitate with metal ions).
3. Read the emf on a high-resistance voltmeter.

Core Practicals 13a and 13b, redox titrations

1. Rinse the burette with the titrant and the pipette with the sample solution.
2. Manganate(VII): acidify with dilute sulfuric acid; end point is the first permanent pale pink.
3. Thiosulfate: titrate to pale yellow, add starch, continue until the blue-black goes.
4. Repeat until titres agree within 0.10 cm³; average only those.

Core Practicals 14 and 16, preparations

1. Use the limiting reagent for the theoretical yield.
2. Collect crystals by filtration under reduced pressure; wash with a small volume of cold solvent.
3. Recrystallise: minimum hot solvent, filter hot, cool, filter, dry.
4. Check purity by melting temperature: sharp and close to the data value means pure.

Worked reminder: standardising thiosulfate

A standard solution holds 0.642 g of KIO₃ (Mr 214.0) in 250.0 cm³. Each 25.0 cm³ aliquot is treated with excess KI and dilute acid; the iodine released needs a mean of 22.50 cm³ of thiosulfate.

• KIO₃ per aliquot = (0.642 ÷ 214.0) ÷ 10 = 3.00 × 10⁻⁴ mol
• I₂ released = 3 × 3.00 × 10⁻⁴ = 9.00 × 10⁻⁴ mol; thiosulfate used = 2 × 9.00 × 10⁻⁴ = 1.80 × 10⁻³ mol
• c(S₂O₃²⁻) = 1.80 × 10⁻³ ÷ 0.02250 = 0.0800 mol dm⁻³

Test results to know (Core Practical 15)

Test	Positive result	Shows
NaOH(aq), then excess	Pale blue precipitate, no change	Cu²⁺
NH₃(aq), then excess	Pale blue precipitate, then deep blue solution	Cu²⁺
NaOH(aq)	Green precipitate, browning in air	Fe²⁺
NaOH(aq)	Red-brown precipitate	Fe³⁺
NaOH(aq), then excess	White precipitate, then colourless solution	Zn²⁺ (also dissolves in excess NH₃)
NaOH(aq), then excess	Grey-green precipitate, then dark green solution	Cr³⁺
2,4-DNPH	Orange precipitate	Aldehyde or ketone
Tollens’ reagent, warm	Silver mirror	Aldehyde
Fehling’s or Benedict’s, warm	Blue to red precipitate	Aldehyde
Iodine and alkali	Pale yellow precipitate	CH₃CO– group

For all eight aqua ions, see the full aqua-ion table.

Must-know distinctions

• Equivalence point vs half-neutralisation: pKa is the pH at half the end-point volume, not at the end point.
• Quenching vs slowing: removing the acid catalyst stops the iodination; dilution with cold water only slows it.
• Accuracy vs precision: a measured E⦵cell close to the calculated value is accurate; repeat readings close together are precise.
• Random vs systematic error: repeats reduce random error; air oxidation of iron(II) before titration is systematic and lowers every result.
• Yield vs purity: a high mass of wet or impure product is not a high yield of pure product.

Quick self-test

1. A 10.0 cm³ quenched sample needs 12.50 cm³ of 0.0200 mol dm⁻³ thiosulfate. Find [I₂].
2. In a clock reaction, doubling [X] cuts the time from 64 s to 16 s. What is the order in X?
3. At half-neutralisation the pH is 5.12. Find Ka.
4. Use E⦵: Mg²⁺/Mg = −2.37 V and Cu²⁺/Cu = +0.34 V. Find E⦵cell.
5. A titre of 18.00 cm³ of 0.0150 mol dm⁻³ KMnO₄ reacts with how many moles of Fe²⁺?
6. The burette is read to ±0.05 cm³ at the start and at the end. Find the percentage uncertainty in a 22.50 cm³ titre.
7. The Arrhenius graph (ln k on y, 1/T on x) slopes at −6200 K. Find Ea (R = 8.31 J mol⁻¹ K⁻¹).
8. What is seen when NaOH(aq) is added to Fe³⁺(aq), then in excess?
9. A crude aspirin sample melts over 126 to 131 °C. What does this show?
10. Why is a potassium chloride salt bridge unsuitable with a silver half-cell?
11. Why is the sample in Core Practical 9a run into sodium hydrogencarbonate?
12. Why is ethanol added when preparing tetraamminecopper(II) sulfate?

Answers

1. n(S₂O₃²⁻) = 2.50 × 10⁻⁴ mol; n(I₂) = 1.25 × 10⁻⁴ mol; [I₂] = 0.0125 mol dm⁻³.
2. Rate rises 4 times when [X] doubles: second order.
3. Ka = 10^(−5.12) = 7.59 × 10⁻⁶ mol dm⁻³.
4. 0.34 − (−2.37) = +2.71 V.
5. 5 × 0.0150 × 0.01800 = 1.35 × 10⁻³ mol.
6. 0.10 ÷ 22.50 × 100 = 0.44%.
7. Ea = 6200 × 8.31 = 51 500 J mol⁻¹ = 51.5 kJ mol⁻¹.
8. Red-brown precipitate; no change in excess.
9. It is impure: impurities lower the melting temperature and widen the range.
10. Chloride ions would precipitate silver chloride, changing [Ag⁺].
11. It neutralises the acid catalyst, stopping the reaction at a known time.
12. The complex is less soluble in ethanol, so it crystallises out.

Where marks are usually lost

• Writing “colour change” without giving both the starting and final colours.
• Recording the time a sample was drawn into the pipette rather than the time it was quenched.
• Plotting ln t instead of ln(1/t), then reporting a negative activation energy.
• Leaving Ea in J mol⁻¹ when the question asks for kJ mol⁻¹, or using T in °C.
• Taking pKa from the pH at the end point.
• Writing the cell diagram with the positive electrode on the left.
• Missing the 1 : 5 or 1 : 2 ratio in a redox titration, or the scale-up from portion to flask.
• Giving a general hazard (“chemicals are dangerous”) instead of a named hazard with a matching precaution.
• Quoting a percentage yield to more significant figures than the masses justify.

Official syllabus

Pearson Edexcel International Advanced Subsidiary/Advanced Level in Chemistry (XCH11/YCH11) specification, Issue 1, September 2017, Pearson Education Limited. Unit 6: Practical Skills in Chemistry II, with Core Practicals 9 to 16 from Units 4 and 5.