Revise the theory
This practical links to these notes pages:
- Alcohols: Oxidation, Elimination and Tests (Unit 2 · Spec 3.3.5)
1. Oxidation Reactions & Practical Aim
Required Practical 4 focuses on the preparation and separation of a volatile organic product. The classic reaction studied is the oxidation of primary alcohol ethanol (CH₃CH₂OH) using acidified potassium dichromate(VI) (K₂Cr₂O₇ / H₂SO₄):
- Reagents: Excess ethanol, limited acidified potassium dichromate(VI).
- Apparatus: Simple distillation (product is distilled off immediately to prevent further oxidation).
- Observation: Orange dichromate (Cr₂O₇²⁻) is reduced to green chromium(III) (Cr³⁺).
- Reagents: Excess acidified potassium dichromate(VI).
- Apparatus: Heating under reflux (condenser returned vertically to boiling flask to ensure complete oxidation before subsequent distillation).
2. Distillation vs Reflux Strategy: Boiling Point Hierarchy
The success of the synthesis depends on understanding the intermolecular forces and boiling points of the three compounds:
| Compound | Structure & Mr | Predominant Intermolecular Forces | Boiling Point | Volatility |
|---|---|---|---|---|
| Ethanal | CH₃CHO (Mr = 44) | Permanent dipole-dipole forces & London dispersion (no O-H hydrogen bonding) | 21 °C | Very high (boils just below room temperature) |
| Ethanol | CH₃CH₂OH (Mr = 46) | Hydrogen bonding (due to polar O-H group) | 78 °C | Moderate |
| Ethanoic Acid | CH₃COOH (Mr = 60) | Extensive hydrogen bonding (forms stable cyclic dimers) | 118 °C | Low |
Because ethanal lacks an O-H bond, it cannot form intermolecular hydrogen bonds with itself. Its boiling point (21 °C) is far lower than that of ethanol (78 °C). In a simple distillation apparatus with gentle heating, ethanal vaporises the moment it forms, travels down the condenser, and is collected in an ice-cooled receiver before it can be further oxidised to ethanoic acid.
3. Quickfit Assembly Rules & Examiner Checklist
In Unit 5, candidates are regularly asked to identify errors in student sketches of distillation apparatus. The four critical assembly rules are:
1. Thermometer Bulb Alignment
The bulb of the thermometer must be placed level with the entrance to the side-arm of the still head. If placed too high, it reads low temperatures of stagnant air; if placed too deep in the flask, it measures superheated boiling liquid rather than the true boiling point of vapor entering the condenser.
2. Condenser Water Flow (In at Bottom, Out at Top)
Cold water must enter at the lowest nozzle (nearest the receiver) and exit at the highest nozzle (nearest the still head). This ensures the water jacket completely floods with no air pockets and provides maximum counter-current cooling efficiency.
3. Anti-Bumping Granules
Add 2 to 3 porcelain chips to the boiling flask before heating. They provide nucleation sites for smooth bubble formation, preventing explosive superheating (bumping) of liquid into the condenser.
4. Open Vent Safety (Never Seal the System)
The receiving adapter must have an open vent to the atmosphere. A completely sealed apparatus creates rapid pneumatic pressure buildup during heating, leading to violent glassware explosion.
4. Step-by-Step Practical Protocol
- Carefully add 20 cm³ of acidified potassium dichromate(VI) solution into a 50 cm³ round-bottom flask.
- Cool the flask in an ice-water bath. Slowly add 5 cm³ of ethanol dropwise using a dropping pipette, swirling continuously to disperse heat (the reaction is highly exothermic).
- Add 2 to 3 anti-bumping granules to the flask.
- Clamp the flask securely and assemble the distillation apparatus: attach still head, thermometer, Liebig condenser, and receiving adapter.
- Connect the water hoses: cold tap water to bottom inlet, drain hose from top outlet to the sink. Turn tap on to achieve a steady, slow flow.
- Immerse the receiver flask into an ice-water bath. Ensure the system is open to the atmosphere via the adapter vent.
- Gently heat the boiling flask using an electric heating mantle or warm water bath.
- Collect the distillate boiling below 35 °C (pure ethanal boils at 21 °C).
5. Chemical Hazards & Laboratory Safety
| Chemical Reagent | Hazard Classification | Precautionary Safety Control |
|---|---|---|
| Potassium dichromate(VI) (K₂Cr₂O₇) | Carcinogenic, mutagenic, toxic to reproduction, strong oxidiser. | Wear nitrile gloves and safety goggles; avoid all skin contact; handle solid inside a fume cupboard. |
| Concentrated sulfuric acid (H₂SO₄) | Severely corrosive; causes severe chemical burns. | Add acid slowly to water/solution with cooling; never add water directly to concentrated acid. |
| Ethanol & Ethanal | Highly flammable liquids and vapours. | Eliminate all naked flames; use an electric heating mantle or hot water bath; keep ice bath cold. |
6. Worked Yield Calculation Problem
Problem: A student oxidises excess ethanol with 9.80 g of potassium dichromate(VI), K₂Cr₂O₇ (Mr = 294.2), in dilute sulfuric acid, and distils off the ethanal (Mr = 44.0) as it forms. The mass of pure ethanal collected is 2.42 g.
Equation: Cr₂O₇²⁻ + 3CH₃CH₂OH + 8H⁺ → 2Cr³⁺ + 3CH₃CHO + 7H₂O
Calculate the theoretical yield and the percentage yield of ethanal obtained in this preparation.
Step 1: Calculate moles of potassium dichromate(VI), the limiting reagent
Moles of K₂Cr₂O₇ = mass / Mr = 9.80 / 294.2 = 0.03331 mol (limiting, because ethanol is in excess)
Step 2: Calculate theoretical yield of ethanal
Stoichiometry: 1 mol Cr₂O₇²⁻ → 3 mol ethanal
Theoretical moles of ethanal = 3 × 0.03331 = 0.09993 mol
Theoretical mass = moles × Mr = 0.09993 × 44.0 = 4.40 g
Step 3: Calculate percentage yield
Percentage Yield = (actual mass / theoretical mass) × 100
Percentage Yield = (2.42 / 4.40) × 100 = 55.0%
Final Answer: Percentage Yield = 55.0%
7. Practice Exam Questions
Question 1: In a simple distillation apparatus, why must water enter the condenser jacket at the bottom nozzle and leave from the top nozzle?
Show Answer & Explanation
Correct Answer: B
Introducing water at the bottom forces water to fill the entire cylindrical jacket before overflowing out the top, eliminating air bubbles that would leave sections of the condenser uncooled.
Question 2: Why is a Bunsen burner NEVER used to heat the reaction flask during the oxidation of ethanol to ethanal?
Show Answer & Explanation
Correct Answer: B
Ethanol and ethanal produce flammable vapours that can ignite upon exposure to open flames. An electric heating mantle or thermostated water bath must always be used.
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Exam-style question
2 marksWhen propan-1-ol is oxidised to propanal, explain why the aldehyde is distilled off as soon as it forms.
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- Propanal has a lower boiling point than propan-1-ol, because there is no hydrogen bonding between aldehyde molecules, so it distils off first [1]
- Removing it immediately prevents further oxidation to propanoic acid [1]