Assignment 1: Chemical demonstrations

CHEMISTRY ELECTIVE ASSIGNMENT 1:

USEFUL CHEMICAL DEMONSTRATIONS

Every teacher of chemistry needs a repertoire of chemical reactions that s/he can use to demonstrate some of the basic principles of the subject and also to fulfill the requirements of column 3 of the Stage 6 Chemistry Syllabus – first-hand investigations. University textbooks and coursework in chemistry are not always reliable guides to the kinds of things which are appropriate for or achievable in schools. The legal and economic environments within which a high school teacher of chemistry works impose certain limits on the kinds of chemical reactions that are of practical use, namely that these reactions:

must be affordable (ie must not require expensive chemicals or large amounts of chemicals)
must not use substances or procedures that are banned in schools
should ideally be able to be used in a variety of contexts (eg to show a number of chemical principles)
should, as far as possible, be typical of reactions which occur in the real world
should help the students work out the chemical ideas involved

Among the principles which should therefore guide selection of reactions to use in school chemistry activities and demonstrations are:

effectiveness in helping to develop undestanding
availability, affordability and amounts of reagents required
safety of reactants, products and procedures
ease of preparation, including special equipment needs
visibility and impact
real world applicability
time for observable changes to occur

The list of reactions following are examples of some reactions that could form the basis for a collection of useful chemical reactions.

CLASS PRESENTATIONS

Each member of the class will choose a reaction related to the modules the class is exploring - preferably the module you are working on - to research, trial and present to the class. Your research should include such things as the chemistry of the process (principles, chemical equation, safety aspects etc) and its potential for use in the context of the Stage 6 Chemistry syllabus module (dot points, purpose, outcomes etc). The presentation should take no more than 10-15 minutes and be accompanied by a 2 page handout detailing how the reaction is prepared and carried out, the chemical principles behind the reaction(s), and howe you would use the reaction and results to develop student ideas and understanding.

Due date: Presentation will take place during sessions 5 - 9 (after the MTeach practicum). The roster will be determined in session 4.

The following list contains examples of some reactions that could form the basis for a collection of useful chemical reactions.

SOME USEFUL CHEMICAL REACTIONS FOR TEACHING

	Reaction	Reactant	Reactant	Procedure	Notes
1	Combustion of sulfur	Sulfur	Oxygen	Heat small amount S in deflagrating spoon and place in pure oxygen (in gas jar) in a fume cupboard. See note 1 below for oxygen preparation.	Observe in darkened room Dissolve products and test for pH
2	Decomposition of hydrogen peroxide	Hydrogen peroxide	Manganese dioxide catalyst	Add 6% H₂O ₂ progressively to small amount of MnO ₂	Test oxygen (glowing split) Use in gas generator for bulk amounts
3	Decomposition of copper carbonate	Copper carbonate	.	Heat small amount of copper carbonate	Compare reactions of CuCO₃and CuO with dilute H₂SO₄
4	Decomposition of lead nitrate	Lead nitrate	.	Heat small amount of lead nitrate (in fume cupboard)	Dissolve gas in water and test acidity
5	Precipitation of lead iodide	Lead nitrate	Potassium iodide	Mix equal amounts of 0.1M solutions	.
6	Acid on a (bi)carbonate	Sodium hydrogen carbonate	Ethanoic acid	Add solid NaHCO₃to vinegar and collect gas.	Test gas with lime water
7	Acid on an oxide (neutralisation)	Copper oxide	Sulfuric acid	Heat small amount of copper (II) oxide in 2M sulfuric acid	.
8	Acid on an active metal	Magnesium Calcium Zinc	Hydrochloric acid	Add magnesium ribbon to dilute hydrochloric acid. Collect gas in t-t. Repeat with calcium and zinc metals.	Test gas with lighted splint
9	Precipitation of calcium carbonate (tests for carbon dioxide)	Calcium hydroxide	Carbon dioxide	Bubble carbon dioxide progressively through lime water. Repeat using universal indicator and/or bromothymol blue indicator	Blow exhaled air through a straw placed in lime water. Continue until a further change
10	Formation of copper tetrammine	Copper sulfate	Ammonia	Progressively add 2M ammonia to 0.1M copper sulfate	Observe initial appearance of precipitate which re-dissolves
11	Synthesis of oil of wintergreen	Salicylic acid	Methanol	Warm a small amount of salicylic acid in methanol, catalysed by 0.5mL dilute sulfuric acid	Set aside, lightly stoppered, in a beaker of just boiled water Smell for odour
12	Saponification of oil of wintergreen	Methyl salicylate	Sodium hydroxide	Shake a small amount of methyl salicylate with some 2M sodium hydroxide	.
13	Reduction of permanganate	Potassium permanganate	Oxalic acid	Warm a small amount of 1% acidified KMnO₄ solution with 0.1M oxalic acid	Reaction is self-catalysed.
14	Synthesis of silver (Tollens silver mirror test)	Silver diammine (Tollens reagent)	Glucose	Add small amount of glucose to Tollens' reagent and heat. The reagent is prepared in situ from 2M NaOH, 0.1M silver nitrate and 2M ammonia (see note 2 below)	Silver mirror forms on test-tube walls.
15	Oxidation of glucose	Glucose	(i) Benedict's reagent and (ii) Fehling's reagent	Add small amount of glucose to Benedict's reagent and heat . Repeat test using Fehling's reagent	.
16	Formation of colloidal sulfur	Sodium thiosulfate	Hydrochloric acid	Add 2mL dilute HCl to 50mL 2% sodium thiosulfate solution	Observe against dark background Vary amounts of acid
17	Dichromate equilibrium shifts	Sodium dichromate	Hydrochloric acid and sodium hydroxide	Add 2M NaOH dropwise to 0.1M dichromate soln. Then add 2M HCl dropwise	Observe reversibility of colour changes
18	Synthesis of a Prussian blue	Potassium ferricyanide	Iron (II) sulfate	Mix equal amounts of 0.5% ferricyanide with 0.1M Iron (II) sulfate	Observe precipitate of artists pigment
19	Synthesis of chrome yellow	Lead nitrate	Potassium chromate	Mix 0.1M solutions of CrO₄^2- and Pb²⁺	Observe precipitate of artist's pigment
20	Boric acid neutralisation	Boric acid	Sodium hydroxide	Progressively add, with stirring, 2M NaOH to saturated (3%) soln of boric acid colored with Universal indicator	Indicator colour changes through rainbow hues progressively

Notes:

1: Oxygen is prepared from 6% hydrogen peroxide and manganese dioxide catalyst. For this reaction it is sufficient to put a very small amout (0.25g - 0.5g) of manganese dioxide in the bottom of the gas jar and add 5-10mL of 6% H₂O ₂ just before the oxygen is required.

2: Tollens' reagent. See a book on experimental organic chemistry (eg Vogel's Textbook of Practical Organic Chemistry). The reagent is made by heating 2M sodium hydroxide in a test-tube to clean the inside walls. Discard the solution (but do not rinse the teat-tube). Add 0.5mL of 0.1M silver nitrate to form a precipitate of silver oxide. Then add just enough 2M ammonia, with frequent shaking, dropwise until the brown precipitate dissolves. The solution is Tollens reagent.

References

See list of references for demonstrations and experiments in course outline.

A useful website for elements and their reactions is the Chemicool Periodic Table. Other useful websites can be accessed via the WWW Science links (see the Science Curriculum Index page).