1. Planning an Experiment and Recording Data
Unit 5 questions often ask you to plan an experiment, choose equipment or improve a results table. Use the steps below.
Variables
- Independent variable: the variable you change.
- Dependent variable: the variable you measure.
- Control variables: the variables you keep the same. Then only the independent variable affects the dependent variable.
Sodium thiosulfate reacts with hydrochloric acid. The student times how long it takes for a cross under the flask to disappear.
- Independent variable: the temperature of the solutions.
- Dependent variable: the time for the cross to disappear.
- Control variables: the volumes and concentrations of both solutions, the same flask and the same cross.
Choosing equipment
- Match the equipment to the size of the measurement. A small quantity needs equipment with a fine scale, or the percentage uncertainty is large.
- Use a pipette, burette or volumetric flask when a volume must be accurate. A measuring cylinder is fine when the volume does not need to be exact, for example an excess of acid.
- Use a gas syringe or a balance (mass loss) to follow a gas produced in a reaction.
- Plan at least five values of the independent variable across a wide range. Repeat each reading so that you can spot anomalies.
Recording data
- Put the independent variable in the first column and the dependent variable in the next column.
- Give each column heading a quantity and a unit, separated by a slash, for example temperature / °C. Do not write units in the body of the table.
- Record every raw reading in a column to the same number of decimal places. This matches the resolution of the equipment. For example, record burette readings to 2 decimal places, ending in 0 or 5 (23.45 cm³).
- Give calculated values to a sensible number of significant figures, usually the same as the least precise raw data.
| Temperature / °C | Time for cross to disappear / s | 1/time / s⁻¹ |
|---|---|---|
| 20.0 | 64.0 | 0.0156 |
| 30.0 | 32.5 | 0.0308 |
| 40.0 | 17.0 | 0.0588 |
Key terms
Precision: how close repeated measurements are to each other.
Repeatability: the same person, with the same method and equipment, gets similar results.
Reproducibility: a different person, or a different method or equipment, gets similar results.
Validity: the experiment measures what it is meant to measure, because the other variables are controlled.
Resolution: the smallest change in a quantity that the equipment can show, for example 0.01 g on a 2-decimal-place balance.
2. Laboratory Apparatus Selection
In OxfordAQA Unit 5, candidates are regularly evaluated on selecting the correct piece of apparatus for a specific experimental measurement. Choosing inappropriate apparatus increases measurement uncertainty or creates severe safety hazards.
| Measurement Task | Recommended Apparatus | Unsuitable Alternative | Chemical & Physical Justification |
|---|---|---|---|
| Accurate aliquot transfer (e.g. 25.0 cm³) | Volumetric pipette (±0.06 cm³) | Graduated measuring cylinder (±0.5 cm³) | Pipettes deliver fixed calibrated volumes with very low percentage uncertainty (0.24% vs 2.0%). |
| Accurate variable delivery (titration) | Burette graduated in 0.10 cm³ (±0.05 cm³) | Dropping pipette or beaker | Burettes permit fine dropwise control to observe the exact colour change at the equivalence point. |
| Volumetric standard solution (250.0 cm³) | Volumetric flask (±0.20 cm³) | Conical flask or beaker | Narrow neck calibration line allows precise eye-level alignment with the bottom of the meniscus. |
| Measuring gas evolution over time | Gas syringe (100 cm³) | Inverted measuring cylinder in water | Gases soluble in water (such as SO₂ or CO₂) dissolve into water troughs, causing severe systematic negative errors. Gas syringes collect all non-reactive gases dry and accurately. |
| Heating flammable organic mixtures | Electric heating mantle or water bath | Direct naked Bunsen burner flame | Volatile organic compounds (ethanol, propanone, cyclohexane) have low flash points; naked flames risk vapor ignition. |
| Filtering fine crystalline precipitates | Buchner funnel with vacuum suction | Gravity filtration through filter funnel | Vacuum suction drastically speeds up filtration and draws liquid out of crystals, leaving them far drier than gravity filtration. |
3. Quickfit Ground-Glass Glassware Rules
Quickfit apparatus consists of borosilicate glass components with standardized ground-glass joints (typically 14/23 or 19/26 sizes). Ground glass provides snug, leak-resistant seals without rubber bungs that would dissolve in organic solvents.
2. Clamping: Clamp the round-bottomed flask firmly at its neck. Do not clamp the condenser tightly; support it lightly to prevent torque and strain on the ground joint.
3. Lubrication: Avoid silicon grease in organic synthesis unless specifically instructed, because organic solvents dissolve grease, contaminating the final product.
4. Never Heat a Sealed System: Heated vapors expand. A sealed closed system will experience rapid pressure rise and shatter violently.
- Rule 1: Water In at the Bottom, Out at the Top: In any Liebig condenser, cooling water must enter through the lower hose nozzle and exit through the upper nozzle. This guarantees the cooling jacket fills completely against gravity, expelling all air bubbles.
- Rule 2: Add Anti-Bumping Granules Before Heating: Anti-bumping granules contain microscopic pores that provide nucleation sites for small vapor bubble formation. This prevents superheating and violent boiling (bumping) that can propel boiling liquid up the column. Never add granules to a hot or boiling liquid, as this causes instantaneous flash boiling.
- Rule 3: Open Vent to Atmosphere: In distillation setups, the receiver adapter must have an open side-arm or the collection flask must be loosely fitted so expanding air can escape safely.
4. Reflux vs Distillation Setups
A frequent Unit 5 question asks candidates to distinguish between heating under reflux and simple distillation, identify errors in provided diagrams, or sketch the correct apparatus.
Heating Under Reflux
Purpose: To heat a reaction mixture to its boiling point for an extended period without losing volatile reactants, reagents, or solvents.
Setup: A vertical Liebig condenser fitted directly into the neck of a round-bottomed flask. Vapors rise, condense on the water-cooled inner wall, and drip back down into the flask.
Safety: The top of the vertical condenser must remain open to the air (no stopper). Clamped at flask neck.
Simple Distillation
Purpose: To separate a liquid from a solution or separate liquids with significantly different boiling points (e.g. collecting ethanal as soon as it forms, preventing further oxidation to ethanoic acid).
Setup: Pear-shaped or round-bottomed flask connected to a still head, thermometer, downward-sloping Liebig condenser, receiver adapter, and collection flask.
Safety: Thermometer bulb must sit directly opposite the side-arm entry to measure the true temperature of condensing vapor. Vent must remain open to atmosphere.
5. Suction Filtration & Recrystallisation Drying
When purifying synthesised organic solids (such as aspirin in RP10), separation of solid crystals from the mother liquor requires suction filtration using a Buchner funnel and vacuum pump or water aspirator.
Advantages of Buchner Suction Filtration
- Rapid Separation: Atmospheric pressure pushes liquid rapidly through the porcelain plate and filter paper, reducing filtration time from 30 minutes to under 2 minutes.
- Drier Product: The partial vacuum pulls residual solvent out of the crystal matrix, leaving the cake substantially drier than gravity filtration.
- Ease of Washing: Ice-cold solvent can be poured over the crystal cake under vacuum to wash away soluble impurities without dissolving the solid.
Drying to Constant Mass Protocol
To confirm that all moisture or solvent has been removed without decomposing the sample:
- Place the dry solid on a pre-weighed watch glass.
- Dry in an oven set well below the melting point of the compound (or in a desiccator containing silica gel).
- Weigh the sample, return to desiccator/oven for 15 minutes, and reweigh.
- Repeat until two consecutive weighings agree within 0.01 g (constant mass achieved).
6. Yield Optimisation & Loss Minimisation
In quantitative laboratory synthesis, calculated percentage yields rarely reach 100%. In Unit 5, candidates are expected to identify specific practical causes of yield loss and explain techniques used to maximize recovery.
| Source of Material Loss | Practical Manifestation | Method to Minimise Loss / Optimise Yield |
|---|---|---|
| Transfers between vessels | Solution clings to beaker walls and glass stirring rods. | Rinse beaker and rod multiple times with small portions of distilled solvent and add all washings to the receiver. |
| Equilibrium reactions | Reversible reactions (e.g. esterification) do not proceed to completion. | Use an excess of one cheap reactant (e.g. excess alcohol) or continuously remove product (e.g. water or ester) to shift equilibrium right. |
| Side reactions | Competing pathways form unexpected by-products. | Strictly control reaction temperature (e.g. maintain below 10 °C during nitration to prevent dinitration). |
| Recrystallisation losses | Some desired solid remains dissolved in cold solvent after cooling. | Use the minimum volume of hot solvent to dissolve solid, cool thoroughly in an ice-water bath before filtering, and wash with ice-cold solvent. |
| Separating funnel partition | Slight solubility of organic liquid in aqueous wash layers. | Wash aqueous layer with fresh organic solvent ("salting out" with NaCl to decrease organic solubility in water). |
7. Chemical Hazards, PPE & Risk Control
Safety evaluation is an essential component of Unit 5. Candidates must identify hazards associated with specific reagents and prescribe appropriate risk mitigation strategies beyond generic "wear lab coat".
| Reagent / Gas | Hazard Classification | Specific Danger | Mandatory Control Measure & PPE |
|---|---|---|---|
| Concentrated H₂SO₄ / HNO₃ | Corrosive, Oxidising | Causes severe skin burns and permanent eye damage; highly exothermic dilution. | Wear chemical splash goggles and nitrile gloves; add acid slowly to water, never water to acid; work in fume cupboard. |
| Sulfur dioxide (SO₂) / Chlorine (Cl₂) | Toxic, Corrosive gas | Severe respiratory irritant; triggers acute asthmatic attacks. | Generate strictly inside an active fume cupboard; in RP8 (thiosulfate), place reaction vessel into a sodium carbonate stop bath immediately upon completion. |
| Volatile Organics (Ethanol, Cyclohexene) | Highly Flammable | Vapours travel along bench and ignite on contact with naked flame. | Strictly no open flames; heat using an electrically heated water bath or heating mantle. |
| Acyl chlorides | Corrosive | React vigorously with water, releasing HCl gas. | Use a fume cupboard and dry apparatus; wear nitrile gloves and eye protection. |
| Ethanoic anhydride | Corrosive | Its vapour irritates the eyes and lungs; reacts with water to form ethanoic acid (not HCl). | Use a fume cupboard and gloves; wear eye protection. |
| Potassium dichromate (K₂Cr₂O₇) | Carcinogenic, Mutagenic, Toxic | Harmful by inhalation and skin contact; toxic to aquatic life. | Avoid powder inhalation; handle solution wearing gloves; dispose of hexavalent chromium waste in designated heavy metal container. |
8. Worked Experimental Design Problem
Problem: A student intends to synthesize ethyl ethanoate by reacting ethanoic acid with ethanol in the presence of concentrated sulfuric acid. The student proposes the following procedure:
"Mix 10 cm³ ethanol, 10 cm³ ethanoic acid, and 2 cm³ conc. H₂SO₄ in a round-bottom flask. Assemble Quickfit apparatus for distillation with a Bunsen burner. Heat strongly for 30 minutes with a stopper tightly sealed in the top of the still head and receiver flask to prevent smelly vapors escaping."
Identify three major errors in the student's experimental design and provide the correct laboratory procedure for each.
Solution:
- Error 1: Using distillation instead of reflux for the initial reaction.
Correction: Esterification is a slow, reversible reaction that requires prolonged heating. Distillation would boil off volatile ethanol (b.p. 78 °C) before it reacts. The mixture must be heated under reflux for 30 minutes with the condenser mounted vertically so vapors condense and return to the flask. - Error 2: Heating a sealed apparatus with stoppers in place.
Correction: Heating an airtight sealed container causes expanding gases and vapors to build up extreme internal pressure, resulting in an explosive apparatus rupture. The top of the reflux condenser must remain open to the atmosphere. - Error 3: Using a naked Bunsen burner flame with flammable organics.
Correction: Ethanol and ethyl ethanoate are highly flammable with low flash points. Heating with an open flame risks serious laboratory fire. The flask must be heated using an electric heating mantle or a water bath.
9. Practice Exam Questions
Question 1: Why must cooling water enter at the bottom of a Liebig condenser and exit at the top?
Show Answer & Explanation
Correct Answer: B
Introducing water at the lowest point forces water to flow upward against gravity, driving all air bubbles out through the top exit and keeping the condenser jacket completely flooded for maximum thermal heat exchange.
Question 2: What is the primary operational function of adding anti-bumping granules to a reaction flask prior to heating?
Show Answer & Explanation
Correct Answer: C
Anti-bumping granules contain trapped air in porous silica cavities. As temperature increases, these cavities release tiny bubble streams that break surface tension, preventing dangerous superheating and liquid surging.