a white salt-like compound – what is it? - simulation of laboratory exercises jens josephsen...

A white salt-like compound – what is it?

-simulation of laboratory exercises

Jens JosephsenScience Department (Institut for Natur, Systemer og Modeller)

Roskilde UniversityDenmark

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Roles of experimental and practical work:

• to get experience with phenomena

• to practice academic processes

• to practice skills

(White 1996)

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to get experience with inorganic compounds

• laboratory investigations

• written problems

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Possible elements of an ”unknown” inorganic compoundTypically a simple salt (solid or in solution)

Li+, Na+, K+, H (in HX, H2O, OH-)

B (in borates), O (in oxyanions and H2O)

C +4 (as HxCO3(2-x)- )

N +5, +3, and -3 (as NO3-, NO2

-, NH4+)

P +5 (as HxPO4(3-x)-)

S +6, +4, +2, and -2 (as HxSO4(2-x)-, HxSO3

(2-x)-, S2O32-, S2-)

Cl +5, +3, +1, and -1 (as ClOx-)

Br, I +7, +5, +3, +1, and -1 (as BrOx- , IOx

-)20 hours lab-investigation, students working in pairs.28.11.2011 IYC Nordic Chemistry Teachers Conference 4

Standard laboratory procedures

Laboratory manuals for •Qualitative analysis (semi micro or micro)•Quantitative analysis (mainly volumetric)

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to get experience with inorganic compounds

• laboratory investigations

• written problems

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Written problems example: (formulated mainly at the macroscopic level, following lab-procedures)

• A white crystalline compound A is soluble in water. The conductivity is proportional to concentration. Flame colour is violet. Air passed through an acid solution of A, and then directed through a solution of barium hydroxide gives a white precipitate B. B is isolated and treated with aqueous potassium triiodide until a brown colour appears and the white residue C is insoluble in hydrochloric acid (4M).

• The component giving rise to the violet flame colour is quantified: Its concentration is found as 18mM in a solution of 43.7 mg A in 25 ml of water.

• 81.6 mg of A is dissolved in 10 ml of sulphuric acid (.125M) and the solution boiled while air is passed through. After cooling the solution is titrated with 16.6 ml sodium hydroxide (.100M) while added phenolphthalein turns red.

• What is the composition of the white salt A?(K2SO3 2H∙ 2O)

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to get experience with inorganic compounds

• laboratory investigations

• written problems

• computer simulation

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http://milne.ruc.dk/kemikurser/kemiopgaver/KCO2e (eller ../KCO2)

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What did we learn ?

• Interactive self-instructive IT-resources may function as a valuable supplement if properly co-ordinated with other teaching and learning activities

• A tailored simulation programme may support teaching and scaffold student learning - it leaves capacity in the short time memory for thinking.

• Co-working between pairs of students support learning.

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Simulation of laboratory exercises -extends laboratory time

Jens JosephsenScience Department (NSM)Roskilde UniversityDenmarkphjens@ruc.dk

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Thank you for being here

Please comment on: in your opinion•does it fit with upper sec. school A-level?•do your students know the chemistry of simple salts like NH4NO3 (acid-base, redox, solubility)

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What did we learn ?

• Interactive self-instructive IT-resources may function as a valuable supplement if properly co-ordinated with other teaching and learning activities.

• A tailored simulation programme may support teaching and scaffold student learning - it leaves capacity in the short time memory for thinking.

• Co-working between pairs of students support learning.

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Written problems example: (formulated mainly at the macroscopic level)

960 mg of the reducing component (A) in ”Cowboy-matches” was burned off in an atmosphere of pure oxygen (1 atm) in a closed glass container, equipped with a little water. The formation of a white compound (B) was observed. When shaking the container and then opening it, air was obviously sucked in. The aqueous solution was strongly acidic. The white compound B was dissolved in the solution, which was made up to 100.00 ml (solution C).

To a drop of solution C was added aqueous barium chloride (0.1 M), and a white precipitate was formed.

To a hot, acidic solution of ammonium molybdate was added a drop of solution C. A yellow precipitate was formed.

No other components was identified

Potentiometric titration of 4.00 of solution C with 1.00 M sodium hydroxide revealed a jump in pH (3-6) at 1.76 ml and another jump (8-11) at 2.46 ml. (numbers not given in the text, but taken from the original titration curve shown in the text)

Find the formula and name of the reductor A. (Found:P4S3)

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