Making salts (1)

How do we make salts? A salt is a compound formed when a metal or an ammonium group (NH 4 + ) replaces hydrogen in an acid. Many salts are soluble in water although a few (e.g. lead chloride) are insoluble. There are four ways of making salts: 1. reacting a metal with an acid 2. reacting an insoluble base with acid 3. neutralising an alkali with acid by titration method 4. by precipitation We have to choose the method that best fits the type of salt we want to make.

Salts from metals Salts can be made by reacting acids with metals. This method is only suitable for metals above hydrogen in the reactivity series. We can make salts of Mg, Zn, Al and Fe in this way. For example: to make zinc sulphate, we carry out the following reaction: Zn (s) + H 2 SO 4(aq) → ZnSO 4(aq) + H 2(g) zinc + sulphuric acid → zinc sulphate + hydrogen We cannot use this type of reaction for making salts of Cu, Pb and Ag which are too close to or below hydrogen in reactivity series. It is not a good idea to prepare salts of very reactive metals (such as Na and K) using this method because the reaction of these metals with acid is too violent – a titration method is more suitable in these cases.

This is the procedure: 1. Add the metal to the acid an a flask so that the metal is in the excess. The acid is the limiting agent. 2. Warm the flask gently to complete the reaction. 3. Filter off the excess metal. The filtrate is a solution of the metal salt. 4. Put the filtrate into an evaporating basin and evaporate the water until the crystallisation point is reached. Then you allow the salt to crystallise at room temperature. 5. Filter off the crystals and wash them with a tin amount of water so they don’t dissolve. 6. Dry the crystals between the sheets of filter paper.

Salts from insoluble bases Salts of many metals can be made by reacting an insoluble base with an acid. We use this method for making salts of metals that are low in reactivity series. For example: to make copper(II) sulphate we carry following reaction: CuO (s) + H 2 SO 4(aq) → CuSO 4(aq) + H 2 O (l) copper(II) oxide + sulphuric acid → copper(II) sulphate + water Q: Similarities and differences between this reaction and reaction for preparing zinc sulphate?