Reactions Of Acids

E XPE RIME NT 4 . 5

Reactions of acids
Aim
To investigate and compare some reactions of a strong acid, hydrochloric acid, and a weak acid, ethanoic acid
(common name, acetic acid)
Equipment
Dropper bottles containing:
• 0.1 M hydrochloric acid, HCl
• 0.1 M ethanoic acid (acetic acid), CH3COOH
• 0.1 M sodium hydroxide, NaOH
• 1 M hydrochloric acid, HCl
• 1 M ethanoic acid (acetic acid), CH3COOH
• universal indicator solution
• limewater (calcium hydroxide, Ca(OH)2)
Marble chips (calcium carbonate, CaCO3)
Copper(II) oxide, CuO
Small pieces of magnesium, Mg
Measuring cylinder
10 clean, dry test tubes
Test tube rack
Two spatulas
Plastic forceps
Dropping pipette
2 × 100 mL conical flask
One-holed rubber stopper with bent glass delivery tube
Retort stand
2 × Clamp and bosshead
Long taper
Box of matches pH colour chart
Residue bottles
Put on your lab coat, safety glasses and plastic gloves.
Most of the substances used in this experiment need to be handled with care.
Method
1 Draw up a table to record each test and the results for each acid
2 Place 2 mL of 0.1 M hydrochloric acid in one test tube and add two drops of universal indicator solution. Record the colour of the indicator and the corresponding pH from the colour chart.
3 Repeat step 2 with the 0.1 M ethanoic acid, using a fresh test tube.
4 To the first test tube add 0.1 M sodium hydroxide drop by drop, counting the drops, until the solution is neutral (i.e. the pH is 7).
5 Repeat step 5 with ethanoic acid.

1.
6 Add a rice grain amount of copper(II) oxide to each of two fresh test tubes. To one, add 10 drops of 1 M hydrochloric acid and to the other 10 drops of 1 M ethanoic acid.
Record your observations at the time of mixing and then about 10 minutes later.
7 Using a clean spatula, transfer 4–5 marble chips to a conical flask. Set up the apparatus shown in Figure 4.20, but do not seal the flask with the rubber stopper yet.
Add about 2 cm deep of limewater to a fresh tube. Now cover the marble chips with the 1 M hydrochloric acid and quickly seal the flask. The gas produced in the flask should bubble though the limewater. Has the limewater turned milky? If so, you have shown that carbon dioxide was produced. Record your observations of the changes that occur in both the flask and the test tube.
8 Repeat step 7 with 1 M ethanoic acid, using a fresh conical flask and fresh tube of limewater. Compare the rate of this reaction with that of the hydrochloric acid.
9 Set up the retort stand, bossheads and clamps and then clamp a fresh test tube in place, as shown in Figure 4.20.
Add about 2 mL of 1 M hydrochloric acid to the test tube.

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15 Dispose of all solutions into the correct residue bottles according to your teacher’s instructions. Wash the test tubes and flasks out and leave to drain.
Discussion
• In the first tests, you found the pH of the two acids. Both acids were at the same concentration (0.1 M).
• Why were they compared at the same concentration?
• Why did they have a different pH?
• What can be concluded about the strength of ethanoic acid compared with the strength of hydrochloric acid?
Discuss.
• Following is the balanced equation for the reaction of ethanoic acid with water. The CH3COO– ion is termed the ethanoate ion (also known as the acetate ion). Copy the equation and insert the correct type of arrow in the space.
Then explain your reasoning.
CH3COOH(aq) + H2O(l)

2.
†† Fig 4.20 Experiment set-up.
10 Invert, then clamp, another clean, dry test tube above it, leaving a small gap so you can add the magnesium ribbon to the lower test tube. Have your teacher check your apparatus. 11 Next drop two small pieces of magnesium ribbon into the lower test tube, using the forceps. Then quickly lower the top test tube so that there is now only a small gap between the test tubes, as shown in Figure 4.20. Record your observations of the reaction.
12 Lightly touch the bottom of the lower test tube. What has happened to the temperature of the mixture?
13 When the reaction has ceased, raise the inverted test tube and clamp it to a new position. Light a taper and hold the lighted wick just inside the inverted test tube. Do you hear a loud popping sound? If so, then you have shown that hydrogen gas has been produced. But if you do not, it may be that the mixture of hydrogen and air in the test tube is not in the right proportions.
14 Repeat steps 19–13 with the 1 M ethanoic acid. This time your teacher should not need to check your apparatus.

CH3COO–(aq) + H3O+(aq)

• Compare the number of drops of sodium hydroxide used to neutralise each acid. Is this what you expected?
Explain.
• Compare the reactions of the two acids with the metal oxide, copper(II) oxide. Which, if any, appeared to react more vigorously with the metal oxide? Write balanced equations for the reactions.
• The limewater test is the standard test for carbon dioxide gas. It goes milky because the carbon dioxide reacts with the limewater to produce a precipitate of calcium carbonate. This is the main constituent of marble chips and chalk. The equation for the reaction is:
Ca(OH)2(aq) + CO2(g) ➝ CaCO3(s) + H2O(l)
• D id your tests confirm that carbon dioxide gas was produced? Was there a difference in the rate of its production? If so, suggest why. Write balanced equations for the reactions of the two acids with marble. • The ‘pop test’ is the standard test for hydrogen gas.
The ‘pop’ sound you hear is a mini-explosion due to the combustion of hydrogen gas in air, which is a very exothermic reaction. The equation for the reaction is:
2H2(g) + O2(g) ➝ 2H2O(l) + energy
• D id your tests confirm that hydrogen gas was produced? Was there a difference in the rate of its production? If so, suggest why.
• Was the reaction between the acids and magnesium endothermic or exothermic? Justify your answer.
• Write balanced equations for the reactions of the two acids with magnesium.
• Summarise your findings about the reactions of the two acids in your conclusion.