Bleach 21 What is a bleach? What has caused this colour change? Why is chlorine added to water in a swimming pool?

Bleach 21 21.1 What are household bleaches? 21.2 Production of bleach 21.3 Preparation and properties of chlorine 21.4 The bleaching action of chlorine 21.5 Sulphur dioxide - an alternative bleach 21.6 Comparing bleaches 21.7 Advantages and problems with the use of bleaches 21.8 Use of chlorine to sterilize water 21.9 Setting up a chlorine plant in the Hong Kong SAR

IT on the net Summary Misconception analysis

Bleach is used in two ways: 21.1 What are household bleaches? Bleach is used in two ways: 1 to clean and bleach fabrics 2 to kill germs Class Practice Activity 21.1 B Testing the bleach solution.

21.2 Production of bleach The bleach we use at home smells like chlorine gas. Thus, we call it a ‘chlorine bleach’. It is made by dissolving chlorine gas in a solution of sodium hydroxide. Cl2(g) + 2NaOH(aq)  NaOCl(aq) + NaCl(aq) + H2O(l) bleaching solution

The laboratory preparation of bleach Bleach is prepared during the electrolysis of brine (a concentrated solution of sodium chloride). During electrolysis, three products are formed: 1 chlorine gas 2 hydrogen gas 3 sodium hydroxide solution Activity 21.1 C How can we prepare bleach solution?

The ionic equation for the reaction is: The chlorine then reacts with the sodium hydroxide solution to form a dilute solution of bleach. The ionic equation for the reaction is: Cl2(g) + 2OH-(aq)  Cl-(aq) + OCl-(aq) + H2O(l) Thus, the bleaching solution contains the hypochlorite ion OCl-. IT ON THE NET Class Practice

Preparation and properties 21.3 Preparation and properties of chlorine Obtaining chlorine from bleach Chlorine is conveniently obtained from a chlorine bleach by the action of a dilute acid. For example: Cl-(aq) + OCl-(aq) + 2H+(aq)  Cl2(g) + H2O(l) from bleaching solution from acid Class Practice Activity 21.2 A How can chlorine be prepared?

Properties of chlorine is a greenish-yellow gas is denser than most gases has a choking smell is poisonous is slightly soluble in water turns moist blue litmus paper red then bleaches it IT ON THE NET

Cl2(g) + H2O(l) HCl(aq) + HOCl(aq) Chlorine water When chlorine dissolves in water, it reacts to form hydrochloric acid HCl and hypochlorous acid HOCl: Cl2(g) + H2O(l) HCl(aq) + HOCl(aq) ‘chlorine water’ Cl2(aq) Activity 21.2 B Investigationg chlorine water.

Chlorine water is a convenient source of chlorine. Hypochlorous acid is unstable. It slowly decomposes into oxygen and hydrochloric acid: 2HOCl(aq)  2HCl(aq) + O2(g) This decomposition is faster in sunlight. Class Practice

Chlorine as an oxidizing agent When chlorine molecules react, they gain electrons and form stable chloride ions: reduction -1 Cl2 + 2e-  2Cl- Therefore, chlorine is an oxidizing agent. Activity 21.2 C Is chlorine an oxidizing or a reducing agent?

For example, when chlorine water is added to a solution of potassium bromide, the colourless solution changes to pale brown. Cl2(aq) + 2e-  2Cl-(aq) reduction - gain of electrons very pale green colourless 2Br-(aq)  Br2(aq) + 2e- oxidation - loss of electrons colourless pale brown The overall ionic change is: Cl2(aq) + 2Br-(aq)  2Cl-(aq) + Br2(aq)

Extraction of bromine Seawater contains a small percentage of sodium bromide NaBr. Bromine is extracted from seawater using chlorine gas.

Chlorine turns moist blue litmus paper red and then bleaches it. Test for chlorine Chlorine turns moist blue litmus paper red and then bleaches it. Class Practice

The bleaching action of chlorine 21.4 The bleaching action of chlorine Both chlorine water and household bleach are bleaching agents. They are used to bleach dyes from plants and fabrics made from plant fibres. Therefore they bleach: flowers and litmus (a dye obtained from plants)

stains made by fruit juice, tea and wine cotton, linen and other fabrics made from plant fibres

How chlorine bleach works Chlorine water contains hypochlorous acid HOCl. Household bleach contains sodium hypochlorite NaOCl. Both solutions contain the hypochlorite ion OCl-. The hypochlorite ion gives up its oxygen to the coloured dye, thus making it colourless as follows:

OCl-(aq) + dye(aq)  Cl-(aq) + (dye + O) coloured colourless As oxygen is added to the dye, chlorine bleaches by oxidation.

21.5 Sulphur dioxide - an alternative bleach An alternative to chlorine bleach is sulphur dioxide bleach. This bleach is mainly for industrial use. Bleaching delicate materials such as wool, silk and paper that are damaged by the more powerful chlorine bleaches. Paper making To give foods such as flour and some cheeses a white appearance.

How sulphur dioxide bleach works Sulphur dioxide only bleaches in moist conditions. The gas dissolves in the water to form sulphurous acid: SO2(g) + H2O(l) H2SO3(aq) sulphurous acid The acid contains the sulphite ion SO32-(aq). This ion removes oxygen from the coloured dye, leaving it colourless: SO32-(aq) + dye  SO42-(aq) + (dye - O) coloured colourless Activity 21.3 A How does sulphur dioxide affect coloured substances?

As oxygen is removed from the dye, sulphur dioxide bleaches by reduction. The bleaching action of sulphur dioxide is not permanent. On exposure to air and light, the original colour slowly returns. This is because the air supplies the oxygen removed during bleaching. Class Practice

Sulphur dioxide as a reducing agent Sulphur dioxide in the presence of water is a good reducing agent. oxidation +4 +6 SO32- (aq)  SO42- (aq) Activity 21.3 B Investigating reactions of aqueous sulphur dioxide.

SO32- (aq) + H2O(l)  SO42- (aq) + 2H+(aq) +2e- The half equation is written: SO32- (aq) + H2O(l)  SO42- (aq) + 2H+(aq) +2e- The oxidation number of sulphur increases from +4 to +6. Here are three examples.

1 Iron(III) ions SO2(aq) Fe3+(aq) Fe2+(aq) Reaction of aqueous sulphur dioxide with a solution of iron(III) ions. Fe3+(aq) + e-  Fe2+(aq) reduction pale green yellow/brown

2 Acidified potassium permanganate solution SO2(aq) MnO4-(aq) Mn2+(aq) Reaction of aqueous sulphur dioxide with a solution of acidified potassium permanganate. MnO4-(aq) + 8H+(aq) + 5e-  Mn2+(aq) + 4H2O(l) colourless purple

3 Bromine solution SO2(aq) Br-(aq) Br2(aq) Reaction of aqueous sulphur dioxide with bromine water. Br2(aq) + 2e-  2Br-(aq) brown colourless

Tests for sulphur dioxide Sulphur dioxide has a characteristic choking smell. Filter paper moistened with acidified potassium dichromate solution changes from orange to green in the presence of sulphur dioxide. Class Practice

Sulphur dioxide bleach 21.6 Comparing bleaches Sulphur dioxide bleach Chlorine bleach involves the sulphite ion SO32-(aq) involves the hypochlorite ion OCl-(aq) bleaches by reduction bleaches by oxidation bleaching action is slow and mild bleaching action is faster and more vigorous bleaching action is not permanent and the original colour will slowly be restored bleaching action can last for a longer time used with wool, silk, paper, some foods used with cotton, linen, some food stains A comparison of sulphur dioxide and chlorine bleaches.

Advantages and problems with the use of bleaches 21.7 Advantages and problems with the use of bleaches Advantages The use of chlorine in drinking water has prevented the spread of certain diseases. Bleaches have improved hygiene in the home. Bleaches have helped to improve the quality of food and clothes.

Chlorine and sulphur dioxide are toxic substances. Some problems Chlorine and sulphur dioxide are toxic substances. Household bleaches can harm the eyes and skin. Sulphur dioxide in food can cause severe breathing difficulties in some people. Class Practice

Chlorine in water can kill bacteria. 21.8 Use of chlorine to sterilize water Chlorine in water can kill bacteria. In the Hong Kong SAR, chlorine gas is added to our drinking water at water treatment plants. It is also added to the water in swimming pools IT ON THE NET Class Practice

Setting up a chlorine plant in the Hong Kong SAR 21.9 Setting up a chlorine plant in the Hong Kong SAR Class Practice

IT on the net 21.2 Production of bleach More on sodium hypochlorite and its uses. http://www.chlor-chemicals.com 21.3 Preparation and properties of chlorine Uses of chlorine http://c3.org/newsroom/brochures/enhancing.html http://www.chlor-chemicals.com/

IT on the net 21.8 Use of chlorine to sterilize water How does chlorine kill harmful micro-organisms http://www.sciam.com/askexpert/environment/environment22/environment22.html The use of sodium hypochlorite for the treatment of swimming pool water. http://www.chlor-chemicals.com/ Basics of water purification - main steps http://c3.org/aol/newsroom/factsheets/watertreat.html

Summary 1 Make a written summary of this chapter by answering these questions: What is household bleach used for? How is household bleach manufactured? How is chlorine prepared in the laboratory? What are some properties of chlorine?

How do the bleaching action of sulphur dioxide bleach and chlorine bleach differ? What are some properties of sulphur dioxide? What are some industrial uses of bleaches? 2 Using the answers to your questions, prepare a table to compare chlorine and sulphur dioxide bleaches.

Misconception Analysis Think carefully about the following statements and then mark them TRUE or FALSE. Check your answers at the back of the book to see whether or not you have any misconceptions. 1Q: Domestic bleach is usually prepared from chlorine and sodium hydroxide obtained from electrolysis of brine. 1A: True

2Q: The active ingredient in household bleach is hypochlorous acid HOCl. 2A: False 3Q: A piece of red litmus paper will turn blue then white when dipped into a solution of sodium hypochlorite. 3A: False

4Q: Chlorine water is a solution of chlorine molecules in water. 4A: False 5Q: Bleaches work by oxidation. 5A: False