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The AQA A-Level Chemistry course awards a separate Practical Endorsement based on twelve specified Required Practicals. Three of those — RP4, RP5 and RP6 — sit naturally inside the organic foundations module and are the focus of this lesson. RP4 is a test-tube reaction with simultaneous distillation, typically the partial oxidation of ethanol to ethanal. RP5 is the distillation of a product from an organic preparation, used to isolate and purify the target compound. RP6 is the measurement of reaction rate by gas-volume, mass-loss or colorimetric methods. For each practical we set out the apparatus, the protocol, the safety hazards, the data-handling expected at A-Level, the CPAC criteria the practical evidences, and the principal sources of error with realistic improvements. The lesson cross-references lesson 4 (alkene tests), lesson 6 (halogenoalkane hydrolysis rate) and lesson 7 (alcohol oxidation) where the underlying chemistry is developed in detail.
Spec mapping (AQA 7405): RP4 maps to §3.3 — preparation of a pure organic liquid (the AQA prescription is "a test-tube reaction with distillation", and the worked example here is oxidation of ethanol to ethanal). RP5 maps to §3.3 — distillation of a product, used to isolate a pure organic liquid from a reaction mixture (worked example: cyclohexene from cyclohexanol dehydration). RP6 maps to §3.1.9 — measuring rate of reaction by an initial-rate (clock) method, by continuous monitoring of gas volume or by mass loss. Cross-reference lesson 4 of this course (alkene addition and bromine-water test, where the colour change underpins RP6 colorimetric tracking), lesson 6 (halogenoalkane hydrolysis, where reflux and silver-halide precipitate timing illustrate RP6), and lesson 7 (alcohol oxidation, the reference reaction for RP4). Refer to the official AQA Required Practical handbook and the AQA specification document for the exact wording.
Assessment objectives: AO1 — recall of apparatus, technique and safety (e.g. why a Liebig condenser must be water-cooled counter-current; why an ice-bath is needed for volatile aldehyde receivers; why anti-bumping granules are used). AO2 — applying numerical methods to practical data: calculating percentage yield, taking a gradient from a V–t plot to extract a rate, propagating uncertainty through n = m/M, computing percentage uncertainty for each reading. AO3 — evaluating error sources, distinguishing systematic from random uncertainty, proposing realistic improvements to a given protocol, and judging whether a measured value is consistent with a literature value at the stated precision. Practical questions on Paper 3 (and within Papers 1 and 2) routinely combine all three AOs in the same item.
The AQA prescription for RP4 is "preparation of an organic liquid by a test-tube reaction with simultaneous distillation". The canonical worked example is the partial oxidation of a primary alcohol to an aldehyde: ethanol is oxidised to ethanal (acetaldehyde) using acidified potassium dichromate(VI). Because the aldehyde would itself be oxidised further to a carboxylic acid (ethanoic acid) under reflux, the aldehyde must be distilled out of the reaction mixture as soon as it forms — its boiling point of 21 °C is well below that of ethanol (78 °C) and water (100 °C), so a vertical condenser angled for distillation collects it preferentially. The associated colour change — orange dichromate(VI) to green chromium(III) — provides immediate visual confirmation that oxidation has occurred.
The half-equation for dichromate reduction:
Cr₂O₇²⁻(aq) + 14H⁺(aq) + 6e⁻ → 2Cr³⁺(aq) + 7H₂O(l)
Ethanol oxidation to ethanal (lose two electrons per molecule):
CH₃CH₂OH → CH₃CHO + 2H⁺ + 2e⁻
Combined (multiply the alcohol half-equation by three so electrons balance):
3CH₃CH₂OH + Cr₂O₇²⁻ + 8H⁺ → 3CH₃CHO + 2Cr³⁺ + 7H₂O
The shorthand "[O]" notation used by AQA:
CH₃CH₂OH + [O] → CH₃CHO + H₂O
Each "[O]" represents an oxygen atom delivered by the oxidising agent.
If the same reagents are heated under reflux (vertical condenser, vapour returned to the flask) rather than distilled, the aldehyde is held in contact with the oxidising agent and is oxidised further to ethanoic acid:
CH₃CH₂OH + 2[O] → CH₃COOH + H₂O
The reflux product gives a positive carboxylic-acid test (e.g. effervescence with sodium hydrogencarbonate, producing CO₂) but a negative Fehling's and Tollens' test because no aldehyde remains. This contrast — distillation gives the aldehyde, reflux gives the carboxylic acid — is the central principle of RP4 and is frequently examined.
RP5 is the purification of a synthesised liquid product by distillation. The worked example used by most AQA centres is the dehydration of cyclohexanol to cyclohexene with concentrated phosphoric acid (or concentrated sulfuric acid) as catalyst. The product (cyclohexene, b.p. 83 °C) is distilled out of the reaction mixture as soon as it forms; any unreacted starting material (cyclohexanol, b.p. 161 °C) and the catalyst are left behind in the flask.
C₆H₁₁OH → C₆H₁₀ + H₂O
(With concentrated H₃PO₄ as catalyst at ~160 °C.)
Mechanism (covered in lesson 7): E1 elimination — protonation of OH, loss of water to give a secondary carbocation, loss of H⁺ from an adjacent carbon to give the alkene.
Theoretical yield is calculated from the stoichiometry. Worked example:
Typical A-Level yields for this reaction are 50-70%. Below 30% suggests either insufficient heating (incomplete reaction) or losses during the brine wash and drying steps. Above 80% may indicate that water or unreacted cyclohexanol has co-distilled, contaminating the product — a careful purity check is then essential.
When the product and a contaminant boil within ~25 °C of each other, simple distillation does not give a clean separation. The remedy is fractional distillation, in which a fractionating column is interposed between the flask and the still-head. The column is packed with glass beads, ceramic saddles or a vigreux-style indentation pattern that gives a high surface area on which vapour can repeatedly condense and re-evaporate. Each cycle (a "theoretical plate") enriches the rising vapour in the lower-boiling component. A column with several plates can separate liquids differing by as little as 5 °C.
RP6 is the continuous measurement of reaction rate using one of three monitoring techniques: gas volume, mass loss, or colorimetric absorbance. Each is appropriate to a different chemistry. The unifying skill is producing a property-versus-time curve and extracting a rate (= gradient) from it.
Suitable reactions: any that produces a gas at a manageable rate, e.g.
Apparatus. Conical flask sealed with a bung carrying a delivery tube; gas syringe (0–100 cm³, precision ±0.5 cm³ or ±1.0 cm³ depending on model) clamped horizontally to minimise friction; stopwatch (±0.1 s, but human reaction time dominates at ~±0.3 s); thermometer in the reaction mixture (rate doubles approximately every 10 K — temperature control is critical).
Method. Weigh out the solid reactant (e.g. 0.10 g of magnesium ribbon, polished with emery paper to remove the oxide layer). Measure 25 cm³ of HCl into the flask. Start the stopwatch the instant the magnesium is added; immediately replace the bung. Read the gas-syringe volume at fixed time intervals (every 15 s for a slow reaction; every 5 s for a fast one). Continue until the volume becomes constant (reaction complete).
Analysis. Plot V (cm³ H₂) on the y-axis against t (s) on the x-axis. The initial rate is the gradient of the tangent at t = 0; the rate at any later time is the gradient at that point. Convert to mol s⁻¹ via n = V/24000 (RTP) if a molar rate is wanted, but for kinetic order determination the gradient itself is usually sufficient because only ratios matter.
Suitable when the gas produced is not collected but allowed to escape, e.g.
Apparatus. Conical flask on a top-loading balance (0.01 g precision, ±0.005 g per reading). Cotton-wool plug in the mouth of the flask to prevent loss of acid spray (acid loss would over-estimate mass loss and inflate the apparent rate). Stopwatch.
Method. Tare the balance with the flask plus acid in place. Add solid CaCO₃; immediately replace the plug; start the stopwatch. Record mass at fixed time intervals. Subtract the reading from the initial value to give total mass lost = mass of CO₂ released.
Analysis. Plot Δm against t. Gradient (in g s⁻¹) divided by M(CO₂) = 44.01 g mol⁻¹ gives rate in mol s⁻¹.
The mass-loss method is only valid when the gas has a substantial molar mass: hydrogen (M = 2) escaping into the atmosphere produces a barely-detectable mass change relative to balance precision, whereas CO₂ (M = 44) gives a clearly resolved signal.
Suitable when a reactant or product is coloured. Classic examples:
Apparatus. Colorimeter (or visible spectrophotometer), precision ±0.001 absorbance units. Filter or monochromator set to the absorption maximum of the coloured species (a complementary-colour filter — e.g. blue for orange Br₂). Cuvettes of fixed path length (1.00 cm). Calibration curve A versus [coloured species] prepared from standards (Beer-Lambert law: A = εcl).
Method. Mix the reagents in the cuvette, immediately close it and place in the colorimeter. Record absorbance every 10-30 s. Convert A to [species] using the calibration; plot [species] against t; extract the gradient.
| Instrument | Typical range | Precision | Percentage uncertainty (typical reading) |
|---|---|---|---|
| Gas syringe (100 cm³) | 0–100 cm³ | ±0.5 cm³ | 0.5/50 × 100 = 1.0% (mid-scale) |
| Top-pan balance | 0–200 g | ±0.005 g per reading; ±0.01 g for a difference | 0.01/2.00 × 100 = 0.5% for a 2 g mass change |
| Stopwatch | — | ±0.3 s (human reaction) | 0.3/60 × 100 = 0.5% for a 60 s reading |
| Colorimeter | 0–2 A | ±0.001 A | 0.001/0.500 × 100 = 0.2% at mid-range |
| Thermometer (digital) | −10 to 110 °C | ±0.1 °C | negligible for 25 °C ± 0.1 |
| Volumetric pipette (25 cm³) | — | ±0.06 cm³ | 0.06/25 × 100 = 0.24% |
| Burette (50 cm³) | — | ±0.10 cm³ for a difference | 0.10/25 × 100 = 0.4% for a 25 cm³ titre |
The dominant uncertainty in RP6 is usually the timing of t = 0: there is an inevitable lag of 1-3 s between mixing the reagents and starting the stopwatch. This lag is systematic (always under-estimates t) but small compared with the 30-300 s total run length.
Percentage yield = (actual mass of pure product / theoretical mass from stoichiometry) × 100%.
For RP4 (ethanol → ethanal): 5 cm³ ethanol × 0.789 g cm⁻³ = 3.95 g; n(ethanol) = 3.95 / 46.07 = 0.0857 mol; theoretical n(ethanal) = 0.0857 mol; theoretical m(ethanal) = 0.0857 × 44.05 = 3.78 g. A typical actual yield (2 cm³ distillate, ρ ≈ 0.788 g cm⁻³) = 1.58 g, so yield ≈ 1.58/3.78 = 42%. Aldehydes give low yields because the small amount of contact time with the oxidising agent in the flask is balanced by the loss of volatile aldehyde during the distillation transfer.
Suppose at t = 30 s, V(H₂) = 12.0 cm³ and at t = 60 s, V(H₂) = 22.0 cm³.
In an order-of-reaction experiment the gradient is taken at t = 0 (initial rate) for each of several different starting concentrations, and a log(rate) vs log([A]) plot extracts the order.
If percentage uncertainty in the volume of gas is 1.0% and in the time is 0.5%, the combined uncertainty in the rate (a quotient) is √(1.0² + 0.5²) = 1.12% — quadrature addition because the two uncertainties are independent. A measured rate of 0.333 cm³ s⁻¹ should therefore be quoted as 0.333 ± 0.004 cm³ s⁻¹ to one significant figure of uncertainty.
The Common Practical Assessment Criteria (CPAC) are five categories that must each be demonstrated at least once across the twelve Required Practicals. Below, each RP is mapped against the CPACs it most clearly evidences.
| CPAC | Description | RP4 evidence | RP5 evidence | RP6 evidence |
|---|---|---|---|---|
| CPAC 1 | Follows written procedures | Set up still and ice bath; sequence acid addition | Heat to correct temperature range; collect 80-85 °C fraction | Time-zero start; consistent reading intervals |
| CPAC 2 | Applies investigative approaches | Choose distillation over reflux to trap aldehyde | Choose simple vs fractional based on b.p. gap | Choose gas syringe / balance / colorimeter for each chemistry |
| CPAC 3 | Safely uses range of practical equipment | Heating mantle, condenser, ice bath, fume cupboard | Heating mantle, separating funnel, drying agent | Gas syringe, colorimeter, balance, stopwatch |
| CPAC 4 | Makes and records observations | Record orange → green; record Fehling's red ppt | Record distillation T; record yield | Record V or m or A at fixed intervals |
| CPAC 5 | Researches, references and reports | Cite literature b.p. and yield; reference AQA RP handbook | Identify literature b.p. for purity check | Quote literature rate / order; reference data book |
A student must demonstrate each CPAC at least once across the twelve Required Practicals to be awarded the Practical Endorsement. Teachers tick the matrix as evidence accumulates; the endorsement is reported separately from the A-Level grade.
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