The Chinese University of Hong KongFaculty of Science

Case-based Learning of High School Science Subjects to Support Learning to Learn

Chemistry Workshop

May 7, 2005

Chemistry in the Laundry(For Secondary 5-7 Students)

By Dennis K. P. Ng

(1) To provide learning experiences for students to recognize the impacts of chemistry in society;

(2) To train students to think rationally and critically, and to apply the knowledge of chemistry in making judgments and solving problems;

(3) To become aware of the social, economical, environmental, and technological implications of chemistry, and show concern for the environment and society;

(4) To consolidate the chemical knowledge in the areas such as action of detergents, oxidation and its relation with bleaching, and the concept of catalysis.

Learning Objectives

Curriculum Links

For Secondary 4-5 ChemistrySection 8.2 Detergents

For Sixth Form Chemistry

Section 5.6 CatalysisSection 12.7 Redox Reactions

– development of environmentally benign oxidizing agents

Section 13.3 Green Chemistry – the use of H2O2 in the presence of manganese based catalyst as bleaching agent

Curriculum Links (cont’d)

For the New Senior Secondary Chemistry (under consultation)I. Compulsory Part

Topic 7 Redox Reactions, Chemical Cells and Electrolysis- Redox reactions- Common oxidizing and reducing agents

Topic 9 Rate of Reaction- Factors affecting rate of reaction

Topic 11 Chemistry of Carbon Compounds- Structure and properties of soaps and detergents

II. Elective PartTopic 13 Industry Chemistry

- Catalysis and industrial process- Green chemistry

Patrick Wong is the General Manager of a detergent company. He is very serious about the quality of his products and believes that this is the key to making profit. One day when he talks to John Chan, the Head of the R&D Department, he knows that Oliver Power, the existing detergent formulation of the company, has a major drawback. It employs hydrogen peroxide as the bleaching agent. This oxidizing agent normally loses its bleaching activity as the temperature falls below 60oC, which is a little too hot for washing silk and synthetic fabrics. Therefore, Patrick would like to improve the product and develop a new formulation that can bleach at room temperature. He is optimistic that this new product, if developed, will bring in a great profit as the market currently does not have this kind of product. He asks John to find a way to solve this problem. Since Oliver Power was developed several years ago, it would also be a good time to review the current formulation to see if there is a room for improvement. Being a chemist working in the field of detergents for many years, John believes that catalysis may be a solution for the hydrogen peroxide bleaching problem. To develop a new detergent formulation, many factors in the technological as well as the environmental and economical aspects should also be considered. To speed up the process, he groups the researchers in his Department and asks them to deal with these concerns separately. Then he can collect the information and prepare a proposal to Patrick. You are one of the researchers in John’s Department and are involved in this venture of product development.

The Story

Suggested Lesson Plan

The case can be completed in approximately 100 minutes.

(1) Briefing and Grouping 20 minutes(i) To introduce the case(ii) To split the class into groups of 3-5(iii) To assign a sub-task to each of these groups

(2) Working for the Sub-Task not in the classStudents are asked to find related information and solve the sub-task with discussion inside the group.

(3) Presentation and Discussion 60 minutesEach group is given about 5 minutes to present their finding or solution of the sub-task, followed by an open discussion.

(4) Conclusion 20 minutesSum up the above information and propose a formulation for such a detergent.

Proposed Sub-tasks

(1) Understand the principle of bleaching process both in an oxidative and reductive manner.

Bleaching in this context involves oxidizing stains. The fundamental problem in bleaching textiles is how to oxidize the color without oxidizing and thereby weakening the cloth. Both the color and the cloth are organic materials. In an industrial bleaching process during textile manufacture, a variety of oxidants and reductants can be used. But on the domestic scene, for incorporation into a detergent formulation, the only currently acceptable oxidants are those based on hydrogen peroxide. Other oxidants such as sodium hypochlorite (NaOCl), which releases chlorine rapidly, can be quite damaging to fabrics.

(2) Study the temperature-dependent oxidizing power of hydrogen peroxide. Write equations to show the changes.

Hydrogen peroxide is a viscous liquid that has strong oxidizing properties and is therefore a powerful bleaching agent. Although pure hydrogen peroxide is fairly stable, it decomposes into water and oxygen when heated above 80oC.

2H2O2 � 2H2O + O2 + Energy

The rate of decomposition is dependent on the temperature and concentration of the peroxide, as well as the presence of impurities and stabilizers. The use of a catalyst (such as manganese dioxide, calcium permanganate, silver, or the enzyme catalase) vastly increases the rate of decomposition of hydrogen peroxide.

see: http://en.wikipedia.org/wiki/Hydrogen_peroxide#Decomposition

(3) Find out substitutes of hydrogen peroxide which can be used in solid form.

The traditional European powdered detergent contains sodium peroxoborate or sodium carbonate peroxohydrate, which liberates hydrogen peroxide when added to water. The formulations also contain the additive tetraaacetyl ethylenediamine, which converts the liberated peroxide to peroxycarboxylic acids. These acids maintain acceptable bleaching activity at washing temperatures down to 40oC.

(4) Understand the concept of catalysis. State the characteristics of catalysts.

(5) Find a suitable catalyst which can promote the bleaching action of hydrogen peroxide and can be used in a detergent formulation. Comment on the cost of this substance.

The actual catalyst used by Unilever, known as Accelerator, is a manganese complex with a cyclic ammine ligand. This is the active ingredient in the detergent formulation called Persil Power in the UK and Omo Power in the Netherlands. The discovery of a series of related manganese complexes was reported in Nature (1994, vol. 369, pp. 637-639) and highlighted in Chemical & Engineering News (1994, June 27, pp. 5-6) and Nature (1994, vol. 369, pp. 609-610). Teachers can ask students to read these articles so as to gain a better understanding on the structures and catalytic bleaching activity of these manganese complexes.

(6) Surfactants are one of the most important ingredients in detergents. Find some common surfactants used in detergents and understand their cleaning action. Select with justification the “best” surfactant for your new formulation.

Some typical surfactants include the anionic sodium alkylcarboxylate (RCO2

-Na+) and sodium alkylbenzenesulfonate (RC6H4SO3-

Na+), cationic alkyl trimethylammonium chloride (RNMe3+Cl-), and the

nonionic alkyl polyethoxylate [R(CH2CH2O)nH]. Anionic surfactants make up the great bulk of all synthetic surfactants. They are particularly effective at cleaning fabrics that absorb water readily such as those made of natural fibers of cotton, silk, and wool. Nonionic surfactants are useful in cleaning synthetic fabrics such as polyesters. Most cationic surfactants are effective germicides. They are used in fabric softeners since their positive charges

adhere to many fabrics that normally carry negative electrical charges.

Two common anionic surfactants:

- In hard water, Ca2+, Mg2+, or Fe2+ ions replace the Na+ ions in alkylbenzenesulfonate (ABS) or linear alkylbenzene sulfonate (LAS) surfactant, and soluble products that do not precipitate out as scum are formed.

- The branched-chain structure of ABS molecules is not readily broken down by the microorganisms in natural waters and in sewage treatment plants.

- Microorganisms (e.g. Escherichia coli) can break down LAS molecules by producing enzymes that degrade the molecule two (and only two) carbon atoms at a time. The branched chain of ABS molecules blocks this enzyme action preventing their degradation.

- So LAS seems to be a very good choice.

SO3 Na SO3 NaCH3CHCH2CHCH2CHCH2CH

CH3 CH3 CH3 CH3

- +

ABS detergent

CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2CH

CH3

- +

LAS detergent

The cleaning action of surfactants:

When greasy, oily dirt is vigorously mixed with soapy water, oily particles become surrounded by soap molecules. A grease-soap droplet called a micelle is formed as the nonpolar end of the soap enters the oily material and the polar end remains dissolved in water. The micelles cannot coalesce into larger droplets because the negative charges on the outer surfaces repel one another. They are washed away during rinsing, leaving behind a clean, grease-free surface.

(7) Apart from surfactants, check the other common ingredients of detergents and their functions.

(8) Discuss the environmental issues associated with the use of detergents.

See: http://www.ul.ie/~childsp/CinA/Issue45/what_in_deterg.htm

(9) Linear alkylbenzene sulfonate (LAS) is a biodegradable surfactant. Find the industrial preparation method of this material and the basic principle of its biodegradation.

See: http://www.lasinfo.org/ff_pro.html http://www.lasinfo.org/life_environ_biod.html

Assessment Guidelines