The color change is caused by the production of more FeSCN 2+. Although the Fe(NO 3) 3solution may have been slightly colored, the deep color of the mixtureof Fe(NO 3) 3 and KSCN solutions cannot be explainedby dilution of Fe(NO 3) 3 solution with the colorlessKSCN solution.Ħ. Some FeSCN 2+ product forms when Fe(NO 3 ) 3solution is added to the KSCN solution. The evidence for a chemical reaction between Fe(NO 3) 3andKSCN is the observed color change.ĥ. Fe 3+ (aq) + SCN (aq) FeSCN 2+(aq) (NOTE: Students will probably not know to include the double arrow.This symbol can be introduced at the appropriate time in the post-laboratorydiscussion. Students will write their observations in making the comparisons.General consensus should be obtained during the class discussion.Ģ. Adding AgNO 3or NaF causes the color of the solution to fade.Īnswers to Data Analysis and Concept Developmentġ. Adding either Fe(NO 3) 3solution or KSCN crystals deepens the color of the solution. The colorless KSCN solution and nearly colorless Fe(NO 3 ) 3solutionshould form a brownish-red product. Assure students that there will be a class discussionof this laboratory activity after they have finished it. Your questioning serves to focus studentsÕ thoughts on thesystem being observed. This is not the time to explain the observationsto students. While students are performing the activity, walk around the laboratorycorrecting errant procedures, and asking students for oral interpretationsof their observations. No procedural instructionsare needed except the location of the solutions and the solid KSCN. Students should be told that although the procedure is short and relativelysimple, their detailed observations are essential in answering the questionsand in understanding the concept being studied. Small, centrally located containers of solid KSCN and NaFwill be sufficient. Check the solution before using.)ĪgNO 3solution: 1.7 g solid AgNO 3 in 100mL solution. Includeenough concentrated nitric acid (a few drops) to make the solution slightlyacidic. KSCN solution: 0.05 g solid KSCN in 100 mL solution. (NOTE: The acid stabilizesthe solution and represses the formation of a yellow color. Carefully add 2-3 drops of concentratednitric acid, HNO 3, to the solution. Materials (For 24 students working in pairs)Ġ.2 M Iron(III) nitrate, Fe(NO 3 ) 3, small droppingbottleĠ.005 M Potassium thiocyanate, KSCN, 50 mLĠ.1 M Silver nitrate, AgNO 3, small dropper bottleįe(NO 3 ) 3 solution: 8 g solid Fe(NO 3) 3.9H 2 O in 100 mL solution. Some students may be concernedwith the superficial resemblance between SCN and CN .Although SCN should be treated with the same respect asother chemicals, it does not have the level of toxicity of CN .Soluble chemicals should be washed down the drain with plenty of water.Solid chemicals should be disposed of in a solid waste jar. No safety precautions beyond those that apply to normal chemical laboratorywork need to be stressed for this activity. Some knowledge of kinetic molecular theory.Ĭompleting the activity and answering questions requires one 50-minclass period. Experience with double replacement reactions.Ĥ. Knowledge of common ions and ionic representations of equations.ģ. Familiarity with observational evidence for chemical reactions.Ģ. The activity fits wellin either a general or honors level course.ġ. While the laboratory activity itself is quite easily performed, theunderlying notions are not basic level concepts. Dynamic equilibriumdoes not necessarily (or usually) occur when equal concentrations of reactantsand products exist in the chemical system. The ideas developed in this activity are: (1) chemicalreactions do not always go to completion, and (2) a state of dynamic equilibriumcan be established in a chemical system by the simultaneous formation ofproducts from reactants and reactants from products. This is intended as an introductory activity involving the concept ofdynamic equilibrium.
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