7.5 - Preparation and Properties of Ethanoic Acid

The overall reaction is:

$$3\text{C}_2\text{H}_5\text{OH} + 2\text{Cr}_2\text{O}_7^{2-} + 16\text{H}^+ \rightarrow 3\text{CH}_3\text{COOH} + 4\text{Cr}^{3+} + 11\text{H}_2\text{O}$$

Materials and Apparatus Required

Chemicals

• Dilute sulfuric acid
• Sodium dichromate ($Na₂Cr₂O₇$)
• Concentrated sulfuric acid
• Ethanol ($C₂H₅OH$)
• Magnesium ribbon
• Anhydrous sodium carbonate ($Na₂CO₃$)
• Universal indicator paper

Apparatus

• Round-bottom or pear-shaped flask
• Anti-bumping granules
• Quickfit apparatus for distillation
• Dropping funnel
• Retort stand and clamps
• Water bath and ice bath
• Graduated cylinder
• Thermometer (-10°C to 100°C)
• Bunsen burner
• Test tubes and droppers
• Safety glasses

Safety Precautions

• Wear safety glasses and gloves throughout the experiment.
• Sodium dichromate is toxic and corrosive; handle it with care and avoid skin contact.
• Sulfuric acid is highly corrosive; always add acid to water slowly, never the reverse.
• Ethanol is highly flammable; avoid open flames during the experiment.
• Magnesium ribbon burns with a bright flame; avoid looking directly at it.

Method

Preparation of Ethanoic Acid

1. Add 10 cm³ of dilute sulfuric acid and a few anti-bumping granules to the round-bottomed flask.
2. In a fume cupboard, dissolve 9 g of sodium dichromate in the sulfuric acid by swirling.
3. Slowly add 6 cm³ of concentrated sulfuric acid while cooling the mixture in an ice bath to control the reaction's exothermic nature.
4. In a dropping funnel, mix 2 cm³ of ethanol with 10 cm³ of deionised water.
5. Set up the reflux apparatus for distillation and add the ethanol-water mixture dropwise to the acidified dichromate solution while heating gently using a water bath.
6. Reflux the mixture for 30 minutes.
7. After refluxing, rearrange the apparatus for distillation and collect about 15 cm³ of aqueous ethanoic acid by distilling the mixture.

Testing the Properties of Ethanoic Acid

1. Odour Test: Carefully waft the vapours of the distillate toward your nose and compare its smell to ethanol. Record your observations.
2. Acidity Test: Dip universal indicator paper into the ethanoic acid solution and record the colour change.
3. Reaction with Magnesium: Add a piece of magnesium ribbon to a test tube containing ethanoic acid and observe any effervescence, indicating the production of hydrogen gas.
4. Reaction with Sodium Carbonate: Add 1 g of anhydrous sodium carbonate to another test tube of ethanoic acid and observe any effervescence, indicating the production of carbon dioxide gas.
5. Esterification: Add 1 cm³ of ethanol and 2 drops of concentrated sulfuric acid to ethanoic acid in a test tube. Warm gently and observe any sweet, fruity odour, indicating the formation of ethyl ethanoate.

Results

Test	Observation	Conclusion
Odour	Smells like vinegar	Ethanoic acid is present
Universal Indicator	Changes from green to red	Solution is acidic
Reaction with Magnesium	Effervescence (hydrogen gas evolved)	Acid-metal reaction
Reaction with Sodium Carbonate	Effervescence (carbon dioxide gas evolved)	Acid-carbonate reaction
Esterification	Sweet odour and oily droplets	Ethyl ethanoate (an ester) produced

Example Questions with Answers