Unit 22 Solution

Unit 22 Chlorine and hypochlorite Suggested answers to in-text activities Internet Search and Presentation (page 3) Uses of chlorine Virtually every part of the home benefits from chlorine chemistry. Chlorine contributes to advances in areas as diverse as disinfection, medicine, public safety and enhancing everyday life. Life itself depends on chlorine and its ability to react with other elements. Chlorinated compounds occur naturally in humans and are found in blood, skin and teeth. Even white blood cells need chlorine to enable them to fight off infections. Disinfection Fighting diseases Chlorine was first used in drinking water in the late 19th century to control the spread of waterborne diseases such as typhoid and cholera. Fighting these diseases remains vital today. Chlorine acts as a powerful disinfection agent when used either on its own or as sodium hypochlorite. When added to water, it quickly kills bacteria and other microbes. In addition to purifying water, chlorine helps remove tastes and odours, controls the growth of slime and algae in mains pipes and storage tanks, and helps to remove unwanted nitrogen compounds from water. Many events testify to the importance of chlorine in water purification: • In the US, annual deaths from cholera totalled 25 000 in 1900. By 1960, this figure had fallen to fewer than 20. • In 1986, 4 000 people in Tenerife were hospitalized due to water contamination which followed the withdrawal of chlorine. A range of chlorine compounds - including iron(III) chloride and hydrochloric acid - are used

to purify waste water and sewage.

In the home and public swimming pools In the home, chlorine-based products are used in laundry bleach, dishwasher detergents, scouring powders and paper towels, and as all-purpose disinfectants to kill common household germs. Disinfectants based on chlorine keep swimming pool water safe by killing a range of dangerous microbes which could otherwise threaten health. Medical applications Stops infections In hospitals, chlorine compounds help protect patients from infections through their use in cleaning, disinfection and as antiseptics. Among their many benefits are the abilities to: • prevent bacterial contamination of patients?burns and wounds; • disinfect kidney dialysis machines; • clean and disinfect work surfaces and equipment in medical laboratories; • kill bacteria which can live in hospital water and air conditioning systems. Medicines Some 85% of pharmaceuticals contain or are manufactured using chlorine, including products to treat Aids, allergies, arthritis, cancer, depression, diabetes, heart disease, hypertension, infections, pneumonia and ulcers. Chlorine-containing compounds are also important intermediates in the manufacture of vitamin C. Medical devices One-quarter of medical devices contain chlorine. Examples are: • chlorine-based plastics are used to make intravenous drips and blood bags, sterile tubing and packaging; • silver chloride is used for mammography and x-ray films;

• chlorine is used to make semiconductors for diagnostic instruments; • common salt is the basis of intravenous saline solutions. Public safety In public safety, chlorine is used to make protective equipment for police officers and firefighters: • chlorine-based plastics are used to make protective helmets, face shields and spectacles; • polycarbonate (produced using chlorine) is used to make bullet-resistant glass for police cars, bank teller windows and security vehicles; • communications equipment used to reach emergency services (such as radios, telephones and computer equipment) relies on components made using chlorine (for example, chlorinated solvents are used during their manufacture). Enhancing living standards Plastics made using chlorine include: • PVC - durable, easy to clean, stain resistant, lightweight, corrosion resistant; • Polyurethanes - flexible and rigid; • Polycarbonates used where strength is important; • temperature-resistant, non-stick PTFE for frying pans, and bakery tins; • Polyvinylidene chloride (PVDC) resins used for their barrier coating properties, particularly in food packaging. The following table shows examples of uses of chlorine in different areas. Area House construction

Examples • PVC window frames and plumbing pipes; wire and cable insulation. • Pant (chlorine is commonly used to make

titanium(IV) dioxide, the non-toxic white pigment used in paints.) • Polyurethane seat cushions in cars.

Traffic

Food production

• Chlorinated solvents used as degreasing agents during the manufacture of metal components for aircraft engines and car baking systems. • Help to boost crop yields and quality through its use in the manufacture of crop protection chemicals. • Chlorine-based products are used for sanitation (killing food-borne bacteria) in commercial food preparation.

Check Your Understanding (page 4) 1

Sodium hypochlorite solution is easy to handle. (Chlorine is stored in heavy steel cylinders which are difficult to transport.) 2

Toxic

Discussion (page 11) 1

2

a)

Sodium hypochlorite / hypochlorite ions

b)

The bleach is toxic, irritating to eyes and skin.

c)

Chlorine gas will be given off when the bleach is mixed with acids.

If the concentration of chlorine in the water is too low, it is ineffective in killing germs.

Suggested answers to exercise 1

Chlorine is used: • in the manufacture of bleaching powder and liquid bleaches; • to bleach fabrics (e.g. linen and cotton);

• in the manufacture of a wide range of organic solvents; • in the manufacture of disinfectants, insecticides, paints, plastics and hydrochloric acid; • to sterilize drinking water and water in swimming pools. 2

Sodium hypochlorite is used: • as a bleach to remove stains from clothing; • to disinfect and clean drains, toilets and sinks; • to sterilize water in swimming pools; • to treat sewage to reduce odour and increase the digesting efficiencies of sewage

disposal systems. 3

4

a)

Chlorine gas and sodium hydroxide solution

b)

From the electrolysis of brine

c)

Cl2(g) + 2NaOH(aq)

d)

The colour of the piece of cloth is removed. The bleaching action of chlorine bleach is due to the oxidizing action of the hypochlorite ion. oxidized form

NaCl(aq) + NaOCl(aq) + H2O(l)

OCl-(aq) + dye colourless coloured

Cl-(aq) + (O + dye) colourless colourless

5

• Chlorine bleach irritates our eyes and skin. Improper handling may cause danger. • Chlorine bleach can damage delicate materials such as wool, silk, paper, etc. • Sewage containing chlorine bleach will cause water pollution if discharged directly into the sewage system. • Chlorine bleach must be stored in cool places and away from direct sunlight. This is because hypochlorite is unstable and it gradually decomposes into chloride and oxygen. This decomposition takes place faster if the temperature is higher.

6

D

7

C

Option A - Chlorine is greenish-yellow in colour. Option B - Chlorine has a pungent smell. Option D- Chlorine is very reactive.

8

A

Option B - Its bleaching action lasts for a long time. Option D - Its bleaching action is due to the presence of hypochlorite ion.

9

C

The solution becomes orange due to the formation of bromine. In the reaction, chlorine atoms gain electrons and bromide ions lose electrons. Chlorine oxidizes bromide ions to bromine. Cl2(aq) + 2e2Cl- (aq) 2Br-(aq) Br2(aq) + 2e-

10 B 11 D 12 A

(3) When chlorine is passed into cold and dilute sodium hydroxide solution, sodium chloride and sodium hypochlorite are formed. Cl2(g) + 2NaOH(aq) NaCl(aq) + NaOCl(aq) + H2O(l)

13 D 14 A

Chlorine is produced in the experiment.

15 B

(1) Chlorine bleach can damage delicate materials such as wool, silk, paper, etc. (3) Sewage containing chlorine bleach will cause water pollution if discharged directly into the sewage system.

16 17

a)

The mercury electrode is the cathode because sodium is produced at the mercury electrode. b)

Sodium is produced at the mercury cathode. The sodium dissolves

in mercury to form an amalgam. The amalgam then flows into a second cell and reacts with water to form hydrogen gas and sodium hydroxide solution. Mercury is recovered. 2Na/Hg(l) + 2H2O(l) 2NaOH(aq) + H2(g) + 2Hg(l) sodium amalgam c)

The process produces waste which contains poisonous mercury

compounds. These waste products will cause serious pollution problems if they are discharged into rivers and seas. The marine animals may die of mercury poisoning. 18 19

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