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AQA A-Level Chemistry has 12 Required Practicals (RPs) which together fulfil the practical-endorsement requirement and underpin roughly 15% of the marks across Papers 1, 2 and 3. The RPs are formally assessed two ways: your teacher signs off the Common Practical Assessment Criteria (CPAC 1-5) in the lab (producing the "Pass" on your certificate but no UMS), and the written examination asks you to recall apparatus, justify methodological choices, manipulate data (Ka, Ea, Kc, yield, Rf, mp), and propose specific procedural improvements — and those marks do count. This lesson is the single-page consolidated reference. Each RP has its own deep-dive elsewhere in the AQA suite (cross-referenced below); here we collect the headline method, dominant uncertainty source, characteristic exam angle and deep-dive link so you can revise the practical strand horizontally in the final fortnight before the exam.
Spec mapping (AQA 7405): This lesson is the anchor for the cross-cutting "Practical Skills" strand that the AQA specification embeds at every content section. RP1 (volumetric titration) is developed in detail in atomic-structure L8; RP2 (enthalpy by calorimetry) in energetics L7; RP3 (Arrhenius from temperature-rate data), RP6 (continuous gas-volume monitoring) and RP7 (initial-rate / iodine clock) in kinetics L7; RP8 and RP9 (pH curves and indicator choice) in acids-buffers L7; RP10 (Group 2 and Group 7 reactions) in inorganic L1 and L2; RP11 (cation/anion qualitative analysis scheme) in inorganic L8; RP4 and RP5 (oxidation of ethanol, distillation of cyclohexene) in organic-foundations L9; RP12 (transition-metal complex preparation and TLC) in analytical L9. Refer to the official AQA specification document for the exact wording of each section.
Assessment objectives: Recall of the standard apparatus list, method outline and named indicator/reagent for each RP is AO1. Manipulating supplied data — calculating yield from masses, gradient of an Arrhenius plot, Ka from a half-equivalence pH, Kc from titre data, Rf from a TLC plate — is AO2. Evaluating the dominant error source for a specific RP (e.g. heat-loss in RP2, parallax in RP1, indicator overshoot in RP8) and proposing a specific procedural improvement (not a generic "repeat more times") is AO3. Top-band candidates also bring a quantitative uncertainty budget: which apparatus contributes the largest percentage uncertainty, what the propagated uncertainty in the final answer is, and whether the discrepancy with the literature value is consistent with that uncertainty.
One-paragraph summary. Weigh anhydrous sodium carbonate (Na₂CO₃, Mᵣ = 106.0) on a 2-d.p. balance — typically 2.65 g for a 0.100 mol dm⁻³ solution. Dissolve in distilled water in a beaker, transfer quantitatively to a 250 cm³ volumetric flask via funnel and three beaker-rinses, make up to the calibration line dropwise with the meniscus on the line at eye level, stopper and invert ten times. Pipette 25.0 cm³ of this standard into a conical flask using a Class B 25 cm³ pipette and pipette filler. Add 2-3 drops of methyl orange (for HCl titrant; phenolphthalein for NaOH titrant). Run HCl from a burette swirling continuously until the indicator change is sharp; record final burette reading to the nearest 0.05 cm³. Discard the rough titre and take the mean of concordant titres (within 0.10 cm³). The dominant uncertainty is the burette (±0.05 cm³ per reading, ±0.10 cm³ per titre), giving typically 0.4% percentage uncertainty on a 25 cm³ titre. Common exam angle: justify each piece of apparatus and indicator. Deep dive: atomic-structure L8.
One-paragraph summary. Three standard variants are examined: neutralisation (HCl + NaOH), combustion of an alcohol (spirit-burner under a copper calorimeter) and metal-displacement (Cu²⁺ + Zn). Measure a known volume of acid (e.g. 25.0 cm³, 1.00 mol dm⁻³ HCl) into an insulated polystyrene cup standing inside a beaker. Record temperature every 30 s for 3 minutes (the cooling-baseline). At t = 3 min, add the second reactant (NaOH solution, or weighed Zn powder), continue stirring, and record temperature for a further 7 minutes. Plot T against t, and extrapolate the cooling line backwards to t = 3 min to read off the true ΔT corrected for heat loss. Apply q = mcΔT (m = total solution mass in g, c = 4.18 J g⁻¹ K⁻¹, ΔT in K), then ΔH = −q/n in kJ mol⁻¹ where n is moles of the limiting reagent. The dominant uncertainty is heat-loss to the surroundings (typically underestimates the magnitude of ΔH by 5-15%) and the thermometer (±0.5 K, contributing 5-10% to ΔT). Common exam angle: draw the cooling-curve extrapolation. Deep dive: energetics L7.
One-paragraph summary. Use the iodine clock or Na₂S₂O₃ + HCl "cross-disappearing" reaction at five temperatures spanning ~20-60 °C, with a thermostatted water bath. Mix fixed volumes of reactants, start a stopclock, and stop it when the same visual endpoint is reached (cross obscured by S precipitate, or starch turning blue-black). Take 1/t as proportional to rate. Plot ln(1/t) against 1/T (T in kelvin) and read the gradient, m. Then Ea = −m × R = −m × 8.314 J K⁻¹ mol⁻¹. A typical Ea for the thiosulfate reaction is 50-60 kJ mol⁻¹. The dominant uncertainty is the human-judgement endpoint (when has the cross truly disappeared?), giving up to ±5 s on times of 30-60 s — propagates to ~10% in 1/t. Common exam angle: why is the gradient negative? (Higher T → smaller 1/T → larger 1/t.) Deep dive: kinetics L7.
One-paragraph summary. The exemplar AQA reaction is oxidation of ethanol to ethanal (distil off as formed) or to ethanoic acid (heat under reflux). Place 5 cm³ ethanol in a pear-shaped flask with anti-bumping granules, add dilute H₂SO₄ from a dropping funnel, drip in acidified K₂Cr₂O₇ solution, and heat gently. To obtain ethanal: distil off as it forms (b.p. 21 °C) — this prevents further oxidation. To obtain ethanoic acid: heat under reflux for 20 min before distilling. The colour change Cr₂O₇²⁻ (orange) → Cr³⁺ (green) confirms oxidation occurred. Yield, purity (by re-distillation b.p. range or IR/mass spec) and observation of the dichromate colour change are all common exam targets. Common exam angle: explain why distilling immediately gives the aldehyde but reflux gives the carboxylic acid. Deep dive: organic-foundations L9.
One-paragraph summary. Standard AQA exemplar: cyclohexanol → cyclohexene by acid-catalysed dehydration. Mix 10.0 g cyclohexanol with concentrated H₃PO₄ in a pear-shaped flask with anti-bumping granules; attach a fractionating column packed with glass beads, a Liebig condenser, and a receiver cooled in ice. Distil slowly, collecting fractions in the range 80-85 °C (cyclohexene b.p. 83 °C; cyclohexanol b.p. 161 °C). Wash the crude distillate in a separating funnel with saturated NaHCO₃ (removes acid), then with brine, then dry over anhydrous CaCl₂ (overnight or 15 min with swirling). Redistil to obtain pure cyclohexene; record yield and final b.p. range. Confirm identity by decolourisation of bromine water (alkene C=C test). The dominant uncertainty is product loss at every transfer (separating-funnel, drying, redistillation), giving typical yields of 50-65%. Common exam angle: purpose of each washing step. Deep dive: organic-foundations L9.
One-paragraph summary. Method A: Mg + HCl(aq) with a gas syringe collecting the H₂. Weigh a fixed mass of Mg ribbon, place in a conical flask, add a known volume of HCl, stopper immediately to a gas syringe, start the stopclock, and record syringe reading every 10 s. Plot V(H₂) against t; the initial gradient (tangent at t = 0) gives the initial rate in cm³ s⁻¹, convertible to mol s⁻¹ using the molar gas volume. Method B: H₂O₂ decomposition catalysed by MnO₂, monitored similarly. Method C: BrO₃⁻ + Br⁻ + H⁺ → Br₂ monitored by colorimeter at λ ≈ 400 nm. Vary one initial concentration at a time (keeping total volume constant by adding water) to determine partial orders. The dominant uncertainty is the initial portion of the curve, where mixing is still incomplete — drawing the tangent at t = 0 is judgement-limited. Common exam angle: how to read the initial rate from a curved graph. Deep dive: kinetics L7.
One-paragraph summary. The AQA exemplar is esterification: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O. Prepare several sealed boiling tubes containing different known initial moles of ethanoic acid, ethanol, ester and water (often with HCl as catalyst). Leave for a week at constant temperature to reach equilibrium. Then titrate each mixture against standardised NaOH to determine the moles of ethanoic acid present at equilibrium, subtracting the contribution from the HCl catalyst (which you titrated separately). Construct an ICE (Initial, Change, Equilibrium) table for each tube, calculate equilibrium moles of all four species, divide by total volume to give concentrations, and compute Kc = [ester][water] / [acid][alcohol]. For this reaction Kc is dimensionless and approximately 4 at room temperature. The dominant uncertainty is the long equilibration time and the assumption that titration captures the instantaneous equilibrium without disturbing it. Common exam angle: explain why Kc is dimensionless here. Deep dive: kinetics L7.
One-paragraph summary. This RP extends RP1 by systematically running the four combinations: strong-strong (HCl + NaOH), strong-weak (HCl + NH₃), weak-strong (CH₃COOH + NaOH) and weak-weak (CH₃COOH + NH₃). For each, plot the indicator-based titre against expected stoichiometry, and qualitatively predict the equivalence pH (7, ~5, ~9, ~7 respectively). For weak-weak combinations the equivalence point is too gradual to detect by indicator and a pH meter must be used (overlaps with RP9). Indicator choice matters: methyl orange (pKa ~3.7) works for strong-weak only; phenolphthalein (pKa ~9.3) for weak-strong only; both work for strong-strong; neither works for weak-weak. Common exam angle: match an indicator to a titration combination and justify by reference to the pH range of its colour transition versus the vertical region of the pH curve. Deep dive: acids-buffers L7.
One-paragraph summary. Calibrate a pH probe at pH 4, 7 and 10 using buffer tablets. Place 25.0 cm³ of the analyte in a beaker on a stirrer; lower in the probe. Add titrant from a burette in 1 cm³ increments (0.1 cm³ within ±2 cm³ of the equivalence point), waiting for the reading to stabilise after each addition. Plot pH against volume. Key features: initial pH (determined by analyte concentration and Ka if weak); buffer region (gentle gradient, half-equivalence point); equivalence point (steepest gradient, vertical or near-vertical region); final pH plateau. The half-equivalence point on a weak-acid + strong-base curve gives pH = pKa directly — this is the standard route to determining pKa of a weak acid experimentally. The dominant uncertainty is probe drift (recalibrate at the end and accept any drift > 0.1 unit as systematic error). Common exam angle: sketch the curve for a weak diprotic acid (e.g. carbonic acid) and identify two equivalence points and two pKa values. Deep dive: acids-buffers L7.
One-paragraph summary. Two parallel investigations. Group 2: examine the solubility of Group 2 hydroxides (Mg(OH)₂ to Ba(OH)₂ — solubility increases down the group) and of Group 2 sulfates (MgSO₄ to BaSO₄ — solubility decreases down the group). Add NaOH to solutions of Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺: white precipitate of M(OH)₂ in each case, with the Mg(OH)₂ appearing most readily because it is the least soluble. Add Na₂SO₄ to the same set: white BaSO₄ precipitates first (least soluble), MgSO₄ not at all (highly soluble). Group 7: halogen-displacement reactions. Add aqueous Cl₂, Br₂ and I₂ to solutions of KCl, KBr and KI, and observe colour changes in the cyclohexane layer after shaking (Cl₂ pale green/yellow, Br₂ orange, I₂ purple in cyclohexane). The order of oxidising power is Cl₂ > Br₂ > I₂. Common exam angle: explain the trend in oxidising power down Group 7 in terms of atomic radius, shielding and ease of electron-gain. Deep dive: inorganic L1 (Group 2) and L2 (Group 7).
One-paragraph summary. This is the systematic "unknown salt" identification practical. Step 1: appearance (colour, crystalline form). Step 2: flame test (lithium crimson, sodium yellow, potassium lilac, calcium brick-red, barium apple-green, copper blue-green). Step 3: add NaOH(aq) and observe precipitate colour and behaviour in excess; then repeat with NH₃(aq) — diagnostic for Al³⁺ (amphoteric, dissolves in excess NaOH), Cu²⁺ (deep-blue solution in excess NH₃), Fe²⁺ (green precipitate, oxidises to brown over time) and Fe³⁺ (red-brown precipitate, no change in excess). Step 4: add dilute HCl — effervescence indicates CO₃²⁻; pass the gas through limewater to confirm CO₂. Step 5: for SO₄²⁻ add dilute HCl first (removes carbonate) then BaCl₂ — white precipitate insoluble in acid confirms sulfate. Step 6: for halides add dilute HNO₃ (removes carbonate) then AgNO₃: white (Cl⁻), cream (Br⁻), yellow (I⁻); confirm with NH₃ solubility (Cl⁻ soluble in dilute, Br⁻ in concentrated, I⁻ insoluble). Step 7: for NH₄⁺ warm with NaOH; ammonia evolved turns damp red litmus blue. Common exam angle: given a sequence of observations, identify the unknown salt. Deep dive: inorganic L8.
One-paragraph summary. Standard AQA preparation: tetraamminecopper(II) sulfate monohydrate, [Cu(NH₃)₄]SO₄·H₂O, from CuSO₄·5H₂O. Dissolve 5 g CuSO₄·5H₂O in 10 cm³ water, add 10 cm³ concentrated NH₃(aq) dropwise with stirring — solution turns deep blue. Add 10 cm³ ethanol and chill in an ice bath to crystallise. Filter under vacuum (Buchner funnel), wash with cold ethanol and ether to remove water, and air-dry. Determine percentage yield from product mass / theoretical mass. To verify purity and identity, run thin-layer chromatography (TLC): spot a solution of product alongside a reference [Cu(NH₃)₄]²⁺ solution on a silica plate, develop with a polar mobile phase (e.g. butan-1-ol/water/acetic acid mix), and visualise under UV or with iodine vapour. Calculate Rf = (distance spot moved)/(distance solvent front moved). Compare with the reference Rf to confirm identity. Common exam angle: why is the product washed with ethanol rather than water? (Lower solubility of the complex in ethanol minimises product loss while removing soluble impurities.) Deep dive: analytical L9.
The twelve RPs share a common set of cross-cutting skills that surface repeatedly in the written papers and which an A* candidate handles consciously.
Quantitative uncertainty budgets. For every measured quantity, identify the dominant uncertainty source, express it as a percentage, propagate it through the calculation, and compare against the literature discrepancy. If the discrepancy exceeds the uncertainty budget, explain via systematic error (heat-loss, indicator overshoot, evaporation, parallax). Worked examples: burette ±0.05 cm³ per reading × 2 = ±0.10 cm³ per 25 cm³ titre = 0.4% uncertainty; thermometer ±0.5 K on a typical ΔT of 5 K = 10% uncertainty (usually the limiting uncertainty in calorimetry); pH probe ±0.1 unit after calibration (critical near equivalence); gas syringe ±0.5 cm³ on 50 cm³ = 1.0% uncertainty but dominated by tangent-drawing error at t = 0.
Apparatus precision summary table (memorise for AO3 evaluative questions):
| Apparatus | Typical precision | Use in RPs |
|---|---|---|
| Burette (50 cm³) | ±0.05 cm³ per reading (±0.10 per titre) | RP1, RP7, RP8, RP9 |
| Bulb pipette (25 cm³) | ±0.06 cm³ | RP1, RP7, RP8 |
| Graduated pipette | ±0.10 cm³ | RP3, RP6 |
| Volumetric flask (250 cm³) | ±0.30 cm³ | RP1 |
| Measuring cylinder (25 cm³) | ±0.50 cm³ | RP2, RP10 (less precise — use only where indicated) |
| Balance (2 d.p.) | ±0.005 g | RP1, RP5, RP10, RP12 |
| Balance (3 d.p.) | ±0.0005 g | High-precision RP1 preparations |
| Thermometer (1 °C) | ±0.5 K | RP2, RP3 |
| Digital thermometer (0.1 °C) | ±0.05 K | Improvement over standard RP2 |
| Gas syringe (100 cm³) | ±0.5 cm³ | RP6 |
| pH probe (calibrated) | ±0.1 pH unit | RP9 |
| Stopclock (digital) | ±0.5 s reaction-time | RP3, RP6 |
CPAC 1-5 mapping (the criteria your teacher signs off in the lab — but worth knowing because exam questions assess the same skills):
In the written exam an AO3 question that asks "how could the student improve the experiment?" is really asking you to identify which CPAC criterion was poorly applied and propose a specific fix — not a generic "do more repeats". For instance: "the student's mean titre is 24.65 cm³ but their concordant range was 24.40 to 24.90 cm³ — the spread of 0.50 cm³ is too wide. Suggest two specific improvements." Acceptable answers: (i) use a more precise burette (e.g. Class A, ±0.025 cm³); (ii) add titrant dropwise within 1 cm³ of the endpoint; (iii) use a white tile to detect the colour change earlier; (iv) repeat until three titres concordant rather than two. Unacceptable: "do more experiments" without specifying what or why.
Common error patterns shared across RPs. Five themes recur across the practical strand and are worth a single revision pass:
Practical-skills questions across the three papers. Paper 1 (physical/inorganic): expect RP1, RP2, RP3, RP6, RP7, RP10 as ~6-mark integrated questions. Paper 2 (organic/analytical): RP4, RP5, RP10 (Group 7), RP11, RP12. Paper 3 (synoptic + practical-focused) carries ~30 marks of practical skills sampling all 12 RPs, often as unfamiliar variants — e.g. dehydration of pentan-2-ol rather than cyclohexanol — that ask you to transfer the underlying RP methodology.
The RP strand is the connective tissue of the entire course. Each RP has a dedicated deep-dive lesson elsewhere in the AQA A-Level Chemistry suite — use this lesson as the horizontal revision pass and the deep-dives for vertical mastery.
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