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The acidic reactions of ethanoic acid

Class practical

Students test ethanoic acid with full-range universal indicator solution, magnesiumsodium hydroxide solution and sodium carbonate solution. (Extension: copper carbonate). They compare these reactions with those of hydrochloric acid of the same concentration. This shows that ethanoic acid is a weak acid.
 

Lesson organisation


This experiment can be carried out by the students in about 30 min, or 40 min 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 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 formulae 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? and could follow this experiment.

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

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, each group of students will need:

Copper carbonate (HARMFUL), 0.5 g

Refer to Health & Safety and Technical notes section below for additional information.

Apparatus

Eye protection

Each group of students will need:

Test-tube rack

Test-tubes, 6

Beakers (100 cm3), 2

Glass rod

Dropping pipettes, 2

Access to

A flame for testing hydrogen (but keep away from magnesium)

Universal indicator pH colour chart (Full range pH 1-14)

For extension, each group of students will need:

Spatula

Filter paper

Test tubes, 2

Health & Safety and Technical notes


Read our standard health & safety guidance

Wear eye protection. 

Magnesium ribbon, Mg(s) - see CLEAPSS Hazcard. 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, CH3COOH(aq) - see CLEAPSS Hazcard and CLEAPSS Recipe Book.

Hydrochloric acid solution, HCl(aq) - see CLEAPSS Hazcard and CLEAPSS Recipe Book. 

Sodium hydroxide solution, NaOH(aq), (IRRITANT at concentration used) - see CLEAPSS Hazcard and CLEAPSS Recipe Book. 

Sodium carbonate solution, Na2CO3(aq) - see CLEAPSS Hazcard and CLEAPSS Recipe Book. 

Universal indicator solution (HIGHLY FLAMMABLE) - see CLEAPSS Hazcard and CLEAPSS Recipe Book. 

For extension:
Copper carbonate, CuCO3.Cu(OH)2(s), (HARMFUL) - see CLEAPSS Hazcard

 

Procedure


a Set up a test-tube rack containing six test-tubes.

b Into three of the test-tubes place 2 cm depth of ethanoic acid solution (0.05 M).

c Into the other three tubes place 2 cm depth of hydrochloric acid (0.05 M).

d Add three drops of full-range Universal indicator solution to one of the ethanoic acid tubes and note the pH.

e Add three drops of full-range Universal indicator solution to one of the hydrochloric acid tubes and note the pH.

f Place about 0.5 cm depth of sodium carbonate (0.4 M) solution in a small beaker.

g 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.

h Repeat the procedure in g but adding the sodium carbonate to the tube containing hydrochloric acid and indicator.

i Add sodium hydroxide solution (0.4 M) to ethanoic acid and hydrochloric acid. Do this by following the procedure in efg and h but using sodium hydroxide instead of sodium carbonate.

j Add a small piece of magnesium ribbon to the remaining hydrochloric acid tube. Try to identify the gas given off.

k Repeat j using the third ethanoic acid tube. Compare the rate of reaction with that of hydrochloric acid.
 

Extension, or instead of using sodium carbonate solution:
l Take a small amount (one-quarter spatula measure) of solid copper carbonate on a filter paper.

m Pour 1 cm depth of hydrochloric acid and ethanoic acid respectively into two more test tubes.

n 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)2Cu + CO2 + H2O

Health & Safety checked February 2008

 

Weblinks


Chemguide - offers details of these and other properties of ethanoic acid.

S-cool - gives simple acid-alkali facts

 

Page last updated on 09 December 2011