Acids, bases and salts – Part 2
QUICK POINTS
Dibasic and tribasic acids can form both normal and acid salts. A dibasic acid dissolves in solution to produce 2 moles of H+ ions (e.g., sulphuric acid (H2SO4) and carbonic acid (H2CO3)). Tribasic acids ionise to produce 3 moles of H+ ions in solution (e.g., phosphoric acid (H3PO4)).
A salt is formed when all or part of the hydrogen in an acid is replaced by a metal ion or the ammonium ion. Salts generally consist of a negative ion from an acid and a positive ion from a base or metal.
If all the replaceable hydrogen ions are removed, a normal salt is formed, e.g., NaCl. If only part of the replaceable hydrogen ions is removed, then an acid salt is formed, e.g., NaHCO3.
Salts can be soluble or insoluble. Soluble salts can be prepared from the action of an acid on a metal, carbonate and/or base. The solution is heated to remove excess water, then filtered and evaporated to a small quantity to form the hydrated salt.
Hydrated salts have their water of crystallization in the crystals and become anhydrous when heated (losing all the water of crystallization).
Insoluble salts are obtained from ionic precipitation reactions between two soluble salt solutions. A precipitate (salt) is formed, which is filtered, washed and dried. This is also called a double decomposition reaction.
Anhydrous salts must be prepared by direct combination of the elements involved, for example, with the preparation of iron III chloride.
QUESTION 1
Barium sulfate contains barium ions, Ba2+ and sulfate ions, SO42
1. Barium sulphate is normally taken in the form of a barium meal before patients do an Xray. Why is it safe to swallow barium sulphate although it is toxic?
2. Give the formula of barium sulfate.
3. Describe how you would prepare a pure, dry sample of barium sulfate in the laboratory.
ANSWERS
1. Barium sulphate is an insoluble salt and when swallowed by patients, it will not be absorbed into the body but will be passed out of the digestive tract.
2. The formula for barium sulphate is BaSO4. The 2+ charge on the barium ion and the 2charge on the sulphate ion cancel to give zero (2 + 2 = 0) when added, so only one of each ion is needed.
3. Barium sulphate can be prepared by mixing soluble salt solutions of barium chloride and sodium sulphate (or sulphuric acid). The insoluble barium sulphate is formed as a precipitate and can be filtered off then left to dry on the filter paper.
Equation: BaCl2 (aq) + Na2SO4 (aq) BaSO4 (s) + 2NaCl (aq)
Ionic equation: Ba2+ (aq) + SO42- (aq) BaSO4 (s) precipitate
COMMENT: All insoluble salts can be prepared by mixing two soluble salts that contain the ions of the salt being prepared. In the ionic equation only the ions involved in the formation of the precipitate are written. The other ions are called spectator ions.
QUESTION 2
Salts of some metals can be prepared by reacting one of the metal’s compounds with an appropriate acid.
1. Describe how you can prepare pure, dry crystals of magnesium sulphate by reacting magnesium and sulphuric acid.
2. Identify another suitable magnesium compound which could be reacted with the acid to form magnesium sulphate.
3. A sample of magnesium sulphate MgSO4.7H2O was heated in a test tube. Describe what would be observed and how the substance could be identified.
ANSWERS
1. Magnesium sulphate can be prepared by reacting magnesium ribbon (or powder) with dilute sulphuric acid. Excess magnesium powder is reacted with the dilute acid, then filtered. The solution is heated to a small quantity to remove water, then left to evaporate to form hydrated magnesium sulphate crystals.
2. Magnesium sulphate could also be prepared by reacting magnesium oxide or magnesium hydroxide or magnesium carbonate with dilute sulphuric acid.
COMMENT: Magnesium sulphate is a soluble salt and can, therefore, be prepared by reacting the magnesium metal or MgO, Mg(OH)2 , MgCO3 with dilute sulphuric acid.
3. When hydrated magnesium sulphate is heated, a colourless liquid will be seen forming on the mouth of the test tube. The liquid formed turns cobalt chloride paper blue, indicating the presence of water.
COMMENT: The presence of water is caused by the loss of the water of crystallization from the magnesium sulphate crystals. This forms anhydrous magnesium sulphate.
Francine Taylor-Campbell is an independent contributor. Send Feedback to kerry-ann.hepburn@gleanerjm.com.