Acid-base reactions  (Page 5/5)

 Page 5 / 5

Method:

1. Pour limewater into one of the test tubes and seal with a rubber stopper.
2. Carefully pour a small amount of hydrochloric acid into the remaining test tube.
3. Add a small amount of sodium carbonate to the acid and seal the test tube with the rubber stopper.
4. Connect the two test tubes with a delivery tube.
5. Observe what happens to the colour of the limewater.
6. Repeat the above steps, this time using sulfuric acid and calcium carbonate.

Observations:

The clear lime water turns milky meaning that carbon dioxide has been produced.

When an acid reacts with a carbonate a salt, carbon dioxide and water are formed. Look at the following examples:

• Nitric acid reacts with sodium carbonate to form sodium nitrate, carbon dioxide and water. $2{\mathrm{HNO}}_{3}+{\mathrm{Na}}_{2}{\mathrm{CO}}_{3}\to 2{\mathrm{NaNO}}_{3}+{\mathrm{CO}}_{2}+{\mathrm{H}}_{2}\mathrm{O}$
• Sulfuric acid reacts with calcium carbonate to form calcium sulfate, carbon dioxide and water. ${\mathrm{H}}_{2}{\mathrm{SO}}_{4}+{\mathrm{CaCO}}_{3}\to {\mathrm{CaSO}}_{4}+{\mathrm{CO}}_{2}+{\mathrm{H}}_{2}\mathrm{O}$
• Hydrochloric acid reacts with calcium carbonate to form calcium chloride, carbon dioxide and water. $2\mathrm{HCl}+{\mathrm{CaCO}}_{3}\to {\mathrm{CaCl}}_{2}+{\mathrm{CO}}_{2}+{\mathrm{H}}_{2}\mathrm{O}$

Acids and bases

1. The compound NaHCO ${}_{3}$ is commonly known as baking soda. A recipe requires 1.6 g of baking soda, mixed with other ingredients, to bake a cake.
1. Calculate the number of moles of NaHCO ${}_{3}$ used to bake the cake.
2. How many atoms of oxygen are there in the 1.6 g of baking soda? During the baking process, baking soda reacts with an acid to produce carbon dioxide and water, as shown by the reaction equation below: ${\mathrm{HCO}}_{3}^{-}\left(\mathrm{aq}\right)+{\mathrm{H}}^{+}\left(\mathrm{aq}\right)\to {\mathrm{CO}}_{2}\left(\mathrm{g}\right)+{\mathrm{H}}_{2}\mathrm{O}\left(\mathrm{l}\right)$
3. Identify the reactant which acts as the Bronsted-Lowry base in this reaction. Give a reason for your answer.
4. Use the above equation to explain why the cake rises during this baking process.
(DoE Grade 11 Paper 2, 2007)
2. Label the acid-base conjugate pairs in the following equation: ${\mathrm{HCO}}_{3}^{-}+{\mathrm{H}}_{2}\mathrm{O}⇋{\mathrm{CO}}_{3}^{2-}+{\mathrm{H}}_{3}{\mathrm{O}}^{+}$
3. A certain antacid tablet contains 22.0 g of baking soda (NaHCO ${}_{3}$ ). It is used to neutralise the excess hydrochloric acid in the stomach. The balanced equation for the reaction is: ${\mathrm{NaHCO}}_{3}+\mathrm{HCl}\to \mathrm{NaCl}+{\mathrm{H}}_{2}\mathrm{O}+{\mathrm{CO}}_{2}$ The hydrochloric acid in the stomach has a concentration of 1.0 mol.dm ${}^{-3}$ . Calculate the volume of the hydrochloric acid that can be neutralised by the antacid tablet. (DoE Grade 11 Paper 2, 2007)
4. A learner is asked to prepare a standard solution of the weak acid, oxalic acid (COOH) ${}_{2}$ 2H ${}_{2}$ O for use in a titration. The volume of the solution must be 500 cm ${}^{3}$ and the concentration 0.2 mol.dm ${}^{-3}$ .
1. Calculate the mass of oxalic acid which the learner has to dissolve to make up the required standard solution. The leaner titrates this 0.2 mol.dm ${}^{-3}$ oxalic acid solution against a solution of sodium hydroxide. He finds that 40 cm ${}^{3}$ of the oxalic acid solution exactly neutralises 35 cm ${}^{3}$ of the sodium hydroxide solution.
2. Calculate the concentration of the sodium hydroxide solution.
5. A learner finds some sulfuric acid solution in a bottle labelled 'dilute sulfuric acid'. He wants to determine the concentration of the sulphuric acid solution. To do this, he decides to titrate the sulfuric acid against a standard potassium hydroxide (KOH) solution.
1. What is a standard solution?
2. Calculate the mass of KOH which he must use to make 300 cm ${}^{3}$ of a 0.2 mol.dm ${}^{-3}$ KOH solution.
3. Calculate the pH of the 0.2 mol.dm ${}^{-3}$ KOH solution (assume standard temperature).
4. Write a balanced chemical equation for the reaction between H ${}_{2}$ SO ${}_{4}$ and KOH.
5. During the titration he finds that 15 cm ${}^{3}$ of the KOH solution neutralises 20 cm ${}^{3}$ of the H ${}_{2}$ SO ${}_{4}$ solution. Calculate the concentration of the H ${}_{2}$ SO ${}_{4}$ solution.
(IEB Paper 2, 2003)

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