The acidic reactions of ethanoic acid

This practical can be carried out by the students in about 30 minutes, or 40 minutes if the extension is included.

There are a lot of colourless solutions involved, so students will have to be organised. It may be useful to issue labels so that the test tubes can be labelled with their contents.

The extension with copper carbonate is a colourful additional reaction, or it could be used as an alternative to sodium carbonate.

Students could be asked to write equations for the reactions that occur. The most tricky area is the formulas of salts of divalent ions with ethanoate.

The experiments illustrated are the four classical tests for acidity (pH, action with alkalis, carbonates and reactive metals). The need for water so that acids can show acidity is dealt with in What makes a substance acidic? which could follow this experiment.

Another extension is making salts, for example copper sulfate, as in Reacting copper(II) oxide with sulfuric acid.

Equipment

Apparatus

• Eye protection
• Test tube rack
• Test tubes x6
• Beakers, 100 cm³, x2
• Glass rod
• Dropping pipettes x2
• Access to a flame for testing hydrogen (but keep away from magnesium)
• Access to a universal indicator pH colour chart (pH 1–14)

For extension:

• Spatula
• Filter paper
• Test tubes x2

Chemicals

• Magnesium ribbon, two 1 cm strips
• Ethanoic acid solution, 0.05 M, 15 cm³
• Hydrochloric acid solution, 0.05 M, 15 cm³
• Sodium hydroxide solution, 0.4 M (IRRITANT), 5 cm³
• Sodium carbonate solution, 0.4 M, 5 cm³
• Universal indicator solution (full range, pH 1–14) (HIGHLY FLAMMABLE), a few drops

For extension:

• Copper carbonate (HARMFUL), 0.5 g

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection.
• Magnesium ribbon, Mg(s) – see CLEAPSS Hazcard HC059A. The magnesium should be scraped with a spatula, or rubbed with sandpaper, to remove any oxide coating. Magnesium ribbon is attractive to students who might try to remove it from the laboratory and light it. Any subsequent fires will be very hot and hard to extinguish. It is advisable therefore to keep it under teacher control. Cutting is best done with scissors, as attempts to tear with the fingers can result in cuts.
• Ethanoic acid solution, CH₃ COOH(aq) – see CLEAPSS Hazcard HC038a and CLEAPSS Recipe Book RB039.
• Hydrochloric acid solution, HCl(aq) – see CLEAPSS Hazcard HC047a and CLEAPSS Recipe Book RB043.
• Sodium hydroxide solution, NaOH(aq), (IRRITANT at concentration used) – see CLEAPSS Hazcard HC091a and CLEAPSS Recipe Book RB085.
• Sodium carbonate solution, Na₂ CO₃ (aq) – see CLEAPSS Hazcard HC095A and CLEAPSS Recipe Book RB080.
• Universal indicator solution (HIGHLY FLAMMABLE) – see CLEAPSS Hazcard HC032 and CLEAPSS Recipe Book RB000. 

For extension:

• Copper carbonate, CuCO₃.Cu(OH)₂ (s), (HARMFUL) – see CLEAPSS Hazcard HC026. 

Procedure

1. Set up a test tube rack containing six test tubes.
2. Into three of the test tubes place 2 cm depth of ethanoic acid solution (0.05 M).
3. Into the other three tubes place 2 cm depth of hydrochloric acid (0.05 M).
4. Add three drops of full-range universal indicator solution to one of the ethanoic acid tubes and note the pH.
5. Add three drops of full-range universal indicator solution to one of the hydrochloric acid tubes and note the pH.
6. Place about 0.5 cm depth of sodium carbonate (0.4 M) solution in a small beaker.
7. Use a dropping pipette to add one drop of sodium carbonate solution to the ethanoic acid tube which contains the indicator. Stir the tube with a glass rod and note any observations. Continue to add drops until the pH is neutral. Count the number of drops you have used.
8. Repeat the procedure in step 7 but adding the sodium carbonate to the tube containing hydrochloric acid and indicator.
9. Add sodium hydroxide solution (0.4 M) to ethanoic acid and hydrochloric acid. Do this by following the procedure in steps 5, 6, 7 and 8, but using sodium hydroxide instead of sodium carbonate.
10. Add a small piece of magnesium ribbon to the remaining hydrochloric acid tube. Try to identify the gas given off.
11. Repeat step 10 using the third ethanoic acid tube. Compare the rate of reaction with that of hydrochloric acid.

Extension, or instead of using sodium carbonate solution:

12. Take a small amount (one-quarter spatula measure) of solid copper carbonate on a filter paper.
13. Pour 1 cm depth of hydrochloric acid and ethanoic acid respectively into two more test tubes.
14. Add the copper carbonate to each tube in very small quantities, stirring with a glass rod, until nothing further happens. Record your observations.

Teaching notes

The hydrochloric acid will show pH = 1, and the ethanoic acid will show pH = 3.

Only a few drops of sodium carbonate solution and sodium hydroxide solution will be needed to neutralise the acids as the alkaline solutions are eight times more concentrated than the acids. The hydrochloric acid will fizz strongly with the sodium carbonate solution, whereas for ethanoic acid the fizzing will be detectable but less vigorous.

The hydrochloric acid will fizz with the magnesium. Hydrogen is given off. (Collect in a tube inverted over the reaction tube and transfer the open end quickly to a flame – a squeaky pop should be heard.) Hydrogen is given off more slowly with ethanoic acid.

Both acids will fizz with the copper carbonate, but the reaction with hydrochloric acid will be more vigorous. When any excess copper carbonate has settled, the colours of copper chloride (green) and copper ethanoate (blue) will be seen.

The discussion of weak and strong acids is probably more suited to advanced rather than intermediate students.

Ethanoic acid is a weak acid which means it does not fully dissociate into ions in water.

CH₃COOH ⇌ H⁺ + CH₃COO^-

Hydrochloric acid is a strong acid and dissociates fully.

HCl → H⁺ + Cl^-

This means that the concentration of H⁺ ions in 0.4 M HCl is higher than that in 0.4 M ethanoic acid, so its pH is lower and its acid reactions are faster. When alkalis are added, however, the ethanoic acid equilibrium position moves to the right, so eventually all the acid reacts, as shown in the equations below:

NaOH + HCl → NaCl + H₂O
NaOH + CH₃COOH → CH₃COONa + H₂O

Na₂CO₃ + 2HCl → 2NaCl + CO₂ + H₂O
Na₂CO₃ + 2CH₃COOH → 2CH₃COONa + CO₂ + H₂O

Mg + 2HCl → MgCl₂ + H₂
Mg + 2CH₃COOH → (CH₃COO)₂Mg + H₂

CuCO₃ + 2HCl → CuCl₂ + CO₂ + H₂O
CuCO₃ + 2CH₃COOH → (CH₃COO)₂ Cu + CO₂ + H₂O