Acid-base reactions  (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: ${\mathrm{HCO}}_3^{-}+{\mathrm{H}}_2\mathrm{O}\leftrightharpoons {\mathrm{CO}}_3^{2-}+{\mathrm{H}}_3{\mathrm{O}}^{+}$
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)