5.1 Soap and Detergent

Chapter 5 Chemicals for Consumers

A. SOAP AND DETERGENT

Definition:

Soap

A sodium or potassium salt of long-chain fatty acid

General formula of soap:

RCOO-Na+ or RCOO-K+

Detergent

A sodium or potassium salt of alkyl sulphonic acids or alkylbenzene sulphonic acid

Soap preparation process

• Soap is prepared by hydrolyzing fats or oils under alkaline condition. This reaction is called SAPONIFICATION

• The process involves boiling fats or oils with concentrated NaOH or concentrated KOH to produce glycerol and the salts of fatty acids which are the soap

Oil/Fat

Glycerol Soap

+

PREPARATION A SOAP IN LAB

Describe how soap can be prepared in laboratory

Procedure: 1. Place 10 g of palm oil in a beaker. 2. Slowly and carefully add 15 cm3 of 6 mol dm-3

concentrated sodium hydroxide solution. 3. Gently heat the mixture, stirring with glass rod until the oil

layer becomes invisible. 4. Add 20 ml of water and 12 g of sodium chloride into the

mixture and stir. 5. Cool the mixture. 6. Filter the mixture. 7. Rinse the solid soap with distrilled water and dry the solid

soap.

Why is sodium chloride added to the mixture?

Sodium chloride, NaCl is added to:

• Reduces the solubility of soap in water

• To precipitate the soap produced

Detergent preparation process

• During the preparation of detergent, a long-chain hydrocarbon obtained from petroleum fraction is converted into an organic acid through a series of steps.

• The organic acid is then neutralized with sodium hydroxide, NaOH solution to produce a detergent.

Structure of soap

• Soap consists of sodium/potassium cations and soap anions

• When a sodium/potassium soap dissolve in water, it will form soap anions and sodium ion, Na+ or potassium ion, K+

What is soap anions?

Hydrophobic (water-hating)

Hydrophilic (water-loving)

Cleaning action of soap & detergent

• When soap/detergent is added to water, soap/detergent molecule dissolve in water to form free moving anions and cations

• Soap reduces surface tension of water

• The hydrophobic part dissolve in grease

• The hydrophilic part attracted to water molecule

• Mechanical agitation during scrubbing helps the hydrophobic part pull away the grease from the surface

• The grease is broken up into small droplet

• The droplet does not coagulate due to the repulsion between negative charges

• The droplets are suspended in water to form an emulsion

• When rinsed, the droplet will be removed

Explain why detergent is more effective in hard water

• Hard water contain calcium ion, Ca2+ and magnesium ion, Mg2+

• Soap anions react with Ca2+ ion and Mg2+ ion to form scum

• Scum is insoluble in water

• Detergent does not form scum in hard water

• Detergent is more effective than soap in hard water

5.1 Soap and Detergent

Describe a laboratory experiment to prepare a soap by using a namely oil and alkali.

State how to verify the product formed is soap.

Terangkan suatu experiment makmal untuk menyediakan sabun dengan menggunakan minyak dan alkali yang dinamakan.

Nyatakan bagaimana anda mengesahkan hasil yang terbentuk itu adalah sabun.

[10 marks]