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Module 1: Practical chemistry for exams›PAG 5.3 - Oxidation of ethanol

PAG 5.3 - Oxidation of ethanol

An overview of PAG 5.3 - Oxidation of ethanol
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In OCR Chemistry Required Practical 5 (PAG 5.3), ethanol is oxidised to ethanal, and then ethanoic acid, through heating with acidified sodium dichromate solution.

Initially, complete oxidation to ethanoic acid is carried out under reflux. In a separate experiment, ethanol is partially oxidised to ethanal under distillation.

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In OCR PAG 5.3: Oxidation of ethanol, there are several important safety requirements to be aware of (beyond normal lab practice such as wearing goggles).

  • Addition of the ethanol to the acidified sodium dichromate(VI) solution should be carried out dropwise and kept cool in an ice water bath.
  • After distillation, cool the reaction flask in a beaker of cold water for 30 seconds. Pour the green reaction mixture into a large bowl/bucket of water in a fume hood.
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Experimental method for OCR Chemistry Required Practical 5 (PAG 5.3)

Part 1 – Reflux to form ethanoic acid.

EXPERIMENTAL METHOD OCR PAG 5.3 - PART 1. 1. Prepare the acidified dichromate(VI). Add the pre-weighed 4.0 g sodium dichromate(VI) to the pear-shaped/round-bottomed flask using the funnel. Slowly add 20 cm³ of 3.0 mol dm⁻³ sulfuric(VI) acid into the flask using the funnel. Add a few anti-bumping granules and swirl the flask to dissolve the solid. 2. Add the ethanol to the oxidising agent. Cool the mixture by swirling the flask in an ice water bath for 2–3 minutes, until the solution is ice cold (<10 °C). Carefully add 0.5 cm³ of ethanol to the flask dropwise with swirling in the ice water. The reaction will begin and the orange colour will darken. When the temperature is stable, add a second 0.5 cm³ of ethanol drop by drop. 3. Heat under reflux. Clamp the flask by the neck, then position the assembly in the heating apparatus (water bath/electric heater) and arrange the condenser above the flask in a reflux setup. Heat the reaction mixture under reflux for about 20 minutes. Control the heating to maintain a gentle, continuous boiling and condensing process. Allow the apparatus to cool. 4. Dry the product. Heat the reaction mixture gently with a low blue Bunsen flame. Make a note of the boiling point of the distillate. Continue to distil until 2–3 cm³ of clear colourless distillate has been obtained. Stopper the test tube ready for analytical tests. 5. Analysis of ethanoic acid product. Cautiously smell the product – it should have a vinegar-like odour. Add a drop of distillate to universal indicator paper and observe the orange/red colour from the low acidic pH. Add a little sodium carbonate solution to the remaining distillate and observe the effervescence from carbon dioxide production.
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Experimental method for OCR Chemistry Required Practical 5 (PAG 5.3)

Part 2 – Distillation to form ethanoic acid.

EXPERIMENTAL METHOD OCR PAG 5.3 - PART 2. 1 Prepare the acidified dichromate(VI). Add the pre-weighed 2.0 g sodium dichromate(VI) to the pear-shaped/round-bottomed flask using the funnel. Slowly add 20 cm³ of 2.0 mol dm⁻³ sulfuric(VI) acid to the flask using the funnel. Add a few anti-bumping granules and swirl the flask to dissolve the solid. 2 Add the ethanol in two parts. Cool the mixture by swirling the flask in an ice water bath for 2–3 minutes, until the solution is ice cold (<10°C). Carefully add 0.5 cm³ ethanol to the flask dropwise with swirling in the ice water. The reaction will begin and the orange colour will darken. When the temperature is stable, add a second 0.5 cm³ of ethanol drop by drop. 3 Heat the reaction mixture in a distillation set-up. Using Quickfit apparatus, assemble for distillation with collection in a test tube in an ice bath. Heat the reaction mixture gently with a low blue Bunsen flame. Make a note of the boiling point of the distillate. Continue to distil until 2–3 cm³ of clear colourless distillate has been obtained. Stopper the test tube and carry out the analytical tests. 4 Analysis of ethanol product. Add a drop of distillate to universal indicator paper and observe the colour – ethanol is neutral. Add a little sodium carbonate solution to the remaining distillate and observe – ethanol will not produce effervescence. Warm a sample of distillate with acidified potassium dichromate solution – further oxidation of ethanol will occur causing an orange to green colour change.
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The distillation process requires careful setup to avoid loss of product or accidents.

An illustration of a distillation apparatus showing a thermometer, water out, receiver, water in, anti-bumping granules, and distillate. The flask is labeled to heat gently.
Do

Use anti-bumping granules and heat the mixture gently during distillation.

A diagram illustrating a distillation setup. It includes a thermometer, a flask labeled 'Heat vigorously', a receiver, and arrows indicating 'Water in' and 'Water out'. The flask contains blue liquid, and the receiver is labeled 'Distillate'.
Don't

Allow the mixture to boil violently or heat it without anti-bumping granules – this can cause dangerous splashing or bumping.

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Ethanol can be oxidised to ethanal or ethanoic acid depending on the reaction conditions. The orange dichromate(VI) solution turns green, showing that oxidation has occurred.

During partial oxidation, ethanal is distilled from the reaction mixture before it can be further oxidised. The colourless distillate can be tested with acidified dichromate(VI), which changes from orange to green as the ethanal is oxidised further.

During complete oxidation, the mixture is heated under reflux to form ethanoic acid. The product is acidic and reacts with sodium carbonate, producing effervescence.

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