C7 - Investigate the reactions of carboxylic acids

• A carboxylic acid is an organic compound with a -COOH functional group.

• Carboxylic acids can carry out the same reactions as other acids, for example the reactions with metals and metal carbonate.

• Carboxylic acids are weak acids, and will react more slowly than other, stronger acids like hydrochloric acid.

A carboxylic acid is an organic compound with a -COOH functional group.

Carboxylic acids can carry out the same reactions as other acids such as hydrochloric acids.

However, carboxylic acids are weak acids, so will react more slowly than other acids and have a less acidic pH value.

Remember:

• All acids are soluble in water.

• They release H⁺ ions in water.

• They release hydrogen gas when reacted with a metal.

• They release carbon dioxide gas when reacted with a metal carbonate.

• They have pH values less than 7.

In this practical you will compare the reactions of a carboxylic acid (ethanoic acid) with the reactions of hydrochloric acid.

In this investigation you use universal indicator paper to compare the pH of ethanoic acid with hydrochloric acid.

• Test-tubes (x2)

• Measuring cylinder (10 cm³)

• Universal indicator or pH meter

• Ethanoic acid 0.1 mol/dm³

• Hydrochloric acid 0.1 mol/dm³

1. Using a measuring cylinder, measure 5 cm³ of hydrochloric acid and add to a test-tube.

2. Repeat step 1 with ethanoic acid into a separate test-tube.

3. Add 5 drops of universal indicator to each test-tube and record the pH or use a pH meter to record the pH.

What observations can be made about this experiment?

• Hydrochloric acid has a low pH value (1-2).

• Ethanoic acid has a higher pH value (3-5).

This tells us that the hydrochloric acid released more H⁺ ions than the ethanoic acid, even though they are both the same concentration.

We can say that ethanoic acid is a weaker acid than hydrochloric acid as it only partly ionises in water.

Hydrochloric acid is a strong acid and completely ionises in water.

Ethanoic acid and hydrochloric acid will both react with magnesium, though hydrochloric acid will react more vigorously.

The reactions that take place are:

magnesium + hydrochloric acid → magnesium chloride + hydrogen

magnesium + ethanoic acid → magnesium ethanoate + hydrogen

• Boiling tubes

• Delivery tube

• Test tube

• Basin

• Boiling tube rack

• Measuring cylinder (25 cm³) safety glasses, wooden splint

• 1 mol/dm³ hydrochloric acid

• 1 mol/dm³ ethanoic acid

• 2 x 1 cm strips of magnesium ribbon

1. Fill the basin and test-tube with water, let the test tube rest on the bottom of the basin

2. Measure 20 cm³ of ethanoic acid using the measuring cylinder and add to the boiling tube

3. Repeat step 2 with hydrochloric acid, place both boiling tubes in the boiling tube rack

4. Add the magnesium strip to each boiling tube – ensuring that the Mg is fully immersed in the acid, allow the reaction to proceed for 10 seconds and record your observations.

5. The gas produced can be collected using the delivery tube and the test-tube in the basin of water.

Observations

For both hydrochloric acid and ethanoic acid the following is observed:

• Gas produced.
• Solid magnesium disappears.
• Colourless solution formed.
• Heat produced.

The reaction of magnesium takes place more quickly with hydrochloric acid, and more heat is released.

If the gas produced is tested, for both acids, it will cause a lit splint to burn with a squeaky pop, indicating the presence of hydrogen gas.

Ethanoic acid and hydrochloric acid will both react with calcium carbonate, though hydrochloric acid will react more vigorously.

The reactions that take place are:

calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide

calcium carbonate + ethanoic acid → calcium ethanoate + water + carbon dioxide

• Measuring cylinder (25 cm³)

• Boiling tube

• Test-tube

• Disposable pipette/dropper

• Test-tube rack

• Ethanoic acid 0.5 mol/dm³

• Calcium carbonate (3 g)

• Universal indicator

• Limewater (5 cm³)

1. Using a measuring cylinder, measuring 15 cm³ of ethanoic acid and place into the boiling tube

2. Add 5 drops of universal indicator to the acid

3. Using a clean measuring cylinder, measure 5 cm³ of limewater and place into a test-tube. Place the test-tube and boiling tube side by side in a test-tube rack.

4. Carefully add the calcium carbonate to the acid – record your observations, especially any colour changes to the indicator.

5. Using the disposable pipette/dropper, collect the gas produced by opening and closing the dropper above the reaction in the boiling tube.

6. Once the gas has been collected in the disposable pipette/dropper, bubble the gas through the limewater and record your observations.

Observations

For both hydrochloric acid and ethanoic acid the following is observed:

• Gas produced

• Solid calcium carbonate disappears

• Colourless solution formed

• Limewater turns cloudy when the gas is bubbled through it.

The reaction of calcium carbonate takes place more quickly with hydrochloric acid than ethanoic acid.