Transcript Document

The Chinese University of Hong Kong
Faculty 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
Learning Objectives
(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.
Curriculum Links
For Secondary 4-5 Chemistry
Section 8.2 Detergents
For Sixth Form Chemistry
Section 5.6 Catalysis
Section 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 Part
Topic 13 Industry Chemistry
- Catalysis and industrial process
- Green chemistry
The Story
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.
Suggested Lesson Plan
The case can be completed in approximately 100 minutes.
(1) Briefing and Grouping
(i)
(ii)
(iii)
20 minutes
To introduce the case
To split the class into groups of 3-5
To assign a sub-task to each of these groups
(2) Working for the Sub-Task
not in the class
Students are asked to find related information and solve the sub-task with
discussion inside the group.
(3) Presentation and Discussion
60 minutes
Each group is given about 5 minutes to present their finding or solution of the
sub-task, followed by an open discussion.
(4) Conclusion
20 minutes
Sum 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 ž 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.
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.
The
traditional
(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:
CH3CHCH2CHCH2CHCH2CH
CH3
CH3
CH3
SO3- Na +
CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2CH
CH3
ABS detergent
SO3- Na +
CH3
LAS detergent
- 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.
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