WJEC Chemistry for AS: Student Bk

The reaction of halide ions with silver ions in dilute nitric acid is important in qualitative analysis in both organic and inorganic chemistry. The general reaction is: Ag + (aq) + X − (aq) → AgX(s) The precipitate colours are chloride (white), bromide (pale cream) and iodide (pale yellow) and only the silver chloride dissolves in dilute ammonia. This gives a simple way of identifying the halogen present. Uses of chlorine and fluoride in water treatment Chlorine is widely used in water treatment and makes water safe to drink and use by killing pathogenic bacteria and viruses and preventing the outbreak of serious diseases such as typhoid and cholera. It is commonly added as the gaseous element and sets up the equilibrium below in water: Cl 2 + H 2 O → HOCl + HCl There appear to be only beneficial effects below one part per million (ppm). Fluoride is generally added to water and to toothpaste and again appears to have only beneficial effects below 1ppm, both on reducing tooth decay caused by cavity formation and in strengthening bones, thus reducing osteoporosis. It is added as fluorosilicic acid, sodium fluorosilicate or sodium fluoride. Although effective, there has been opposition to its addition to public water supplies on ethical grounds. However, the gains outweigh the losses, especially in the case of chlorine, where the once deadly disease of cholera has been largely eliminated. Practical work Soluble salt formation For example, copper(II) sulfate can be formed by neutralising sulfuric acid with the insoluble base copper(II) oxide, H 2 SO 4 (aq) + CuO(s) → CuSO 4 (aq) + H 2 O(l) These are the steps in the formation of the salt. 1. Some copper(II) oxide is added to dilute sulfuric acid. More is added until no more dissolves. (Warming might be necessary.) The solution turns blue. 2. All the acid has been used up. The excess solid is removed by filtering. This leaves a blue solution of copper(II) sulfate in water. 3. The solution is heated to evaporate some of the water. 4. It is left to cool. Blue crystals of copper(II) sulfate start to form. The water is not fully evaporated because if this happens, a powder will form rather than crystals. If copper(II) carbonate is used the method is exactly the same but effervescence (fizzing) is seen when the carbonate is added to the acid because carbon dioxide is given off. When no more effervescence is seen, all the acid has been used up. Knowledge check Name the reactants required to form copper(II) chloride, water and carbon dioxide. 42 ▲ Common silver halide precipitates 60 WJEC AS Level Chemistry: Unit 1