Uses of Halogens - Bromine & Chlorine

Melissa Lamb
Note by Melissa Lamb, updated more than 1 year ago
Melissa Lamb
Created by Melissa Lamb about 4 years ago


AS - Level Chemistry (Group 7 - Halogens) Note on Uses of Halogens - Bromine & Chlorine, created by Melissa Lamb on 06/06/2017.

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Uses of BromineBromine is extracted from seawater with the displacement reaction between bromine and chlorine:Cl₂ (aq) + 2Br⁻ (aq) --> 2Cl⁻ (aq) + Br₂ (aq)

Uses of ChlorineChlorine is manufactured by the electrolysis of brine (sodium chloride solution).When chlorine dissolves in water, it undergoes disproportionation - forms a mixture of hydrochloric acid and chloric (I) acid. Adding universal indicator to chlorine in water will turn it red due to the presence of strong (fully ionised) hydrochloric acid. Red colouration then disappears, leaving a colourless solution of chloric (I) acid. Chloric (I) acid is a weak acid (partially ionised in solution) - it is very effective as bleach.

There are two important aspects of this reaction to consider, equilibrium and redox.Equilibrium: Equilibrium mixture is called chlorine water - pale green in colour shows presence of chlorine. Addition of alkali removes hydrogen ions and makes more water molecules which shifts equilibrium to right and green colour fades. Addition of acid increases number of hydrogen ions and shifts equilibrium to left so green colour intensifies. Redox: Chlorine is both oxidised and reduced simultaneously. This is a disproportionation reaction. Cl₂ (aq) + H₂O (l) ⇄ 2H⁺ (aq) + Cl⁻ (aq) + ClO⁻ (aq) In sunlight, chlorine can decompose water to form chloride ions and oxygen.Cl₂ (aq) + H₂O (l) ⇄ 2H⁺ (aq) + 2Cl⁻ (aq) + 1/2O₂ (g)

Testing for ChlorineIf chlorine is tested with blue litmus paper: Turns red first due to solution being acidic due to H⁺ ions. Then turns white as chloric (I) acid is a bleach.

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Water Treatment Chlorine and its compounds are used to sterilise drinking water and water in swimming baths. Kills cholera, typhus bacteria and E. coli. Chlorine is toxic so can only be used in small amounts. Some chlorine persists in water - prevents re-infection further down the supply. Also inhibits the growth of algae, eliminating bad tastes and smells. Usually chloric (I) acid or sodium chlorate (I) is added to water but calcium chlorate (I) may also be used as it is less hazardous. Using chlorine compounds reduces the risks of using elemental chlorine. Chlorine gas is very harmful - irritates the respiratory system. Liquid chlorine on skin or eyes can cause severe burns. Amount of chlorine added is monitored - due to toxicity and to prevent excess chlorine reacting with organic waste products in water to form organochlorine compounds that may be toxic / carcinogenic. Failure to chlorinate water can cause serious health risks for potentially thousands of people. Risks and benefits need to be weighed up when making decisions about whether to add chemicals to drinking water supplies.

Making BleachChlorine in water produces an equilibrium mixture:Cl₂ (g) + H₂O (l) ⇄ 2H⁺ (aq) + Cl⁻ (aq) + ClO⁻ (aq)If chlorine is reacted with an excess of cold, dilute sodium hydroxide solution at room temperature: Equilibrium shifts to far right so goes to completion. Due to hydroxide ions reacting with the hydrogen ions. Cl₂ (aq) + 2NaOH (aq) ⇄ H₂O (l) + NaCl (aq) + NaClO (aq) This reaction used industrially to make bleach - contains a mixture of sodium chloride and sodium chlorate (I).Chlorine both oxidised and reduced - it is a disproportionation reaction.

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Group 7 of period table is made up of the elements fluorine, chlorine, bromine and iodine. Group 7 are called the halogens. Boiling point increases down the group and electronegativity decreases down the group. Ability of halogens to act as oxidising agents decreases down the group.

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