1
5 ml 6 d4' HC1 in C-2. Add a few granules of Zn. When react,ion ceases, add more granules. Continue uutil the solution becomes clear and projects well. Observations: Zn + Sn++ - Sn + Zn++; and Zn + 2H+ - Zn++ + HI. AS the bubbles of Hz form, they pull the tin to the top as a spongy mass. After about four additions of Zn the solution clears and projects well. Dem. 632-Amphoteric Sn(OH), Experiment developed by S. 8. Dunham and L. P. John- son (Princeton-63). To shozu: The amphoteric properties of stannous hy- droxide. Materials: SnC12-2H20dil. HC1, NaOH-aq, stirrer, C-3. Procedure: Dissolve 1 g SnCI2.2H20 in 50 ml water. Project C-3 with each cell half-full of this solution. (a) Now add NaOH dropwise with stirring, to cell 2 until a precipitate persists, and to cell 3 until the precipitate which first forms, redissolves. (b) Reverse the process by adding HC1 drop-wise, with stirring, until both cells 2 and 3 are clear. (c) Leaving these same solutions in the cells, repeat cycle (a, b). Observations: What occurs is explained by the follow- ing expressions: NaOH NaOH SnCb Sn(0H). ?jc NalSn01 HCL HCl Dem. 633-Amphoteric Ph(OHJ2 Experiment developed by Alphonse Palermo. To show: The amphoteric properties of lead hydroxide. Materials: Pb(NO&aq, NaOH-aq, dil. HN03-aq, stirrer, C-3. Procedure: Project C-3 with each cell half-full of water. Into each put 5 drops Pb(NO&aq. (a) Now add NaOH dropwise with stirring to cell 2 until the precipitate persists. To cell 3 add NaOH-aq. until the precipitate which first forms re-dissolves. (b) Re- verse the process by adding dil. HN03 to cells 2 and S until they are clear. (c) Repeat cycle by adding NaOH-aq, then dil. HNOl to cells 2 and 3. Observations: What occurs is explained by the follow- ing expressions: OH- OH- Pb'+ = Pb(OH)> ~5 PbOi H + H * Dem. 634-Lead Chromate versus Barium Chromate To show The formatiou of chromates of lead and barium, and the difference in their solubilities in acid. Materials: Pb(NO&aq, BaCL-aq, I<,CrzO~-aq, dil. HOAc-aq, stirrer, C-2. Procedure: Project C-2 with cells half-full of water. To cell 1 add a drop of Pb(N03)z-aq; to cell 9 add a drop of BaClp-aq. To each add a drop of IC,Cr20raq. Stir. Now add 20 ml HOAc-aq to each. Stir. Obsewations: Yellow PbCrOa and yellow BaCrOP prB- cipitate. Only the latter is soluble in the acid. Th~s difference is made use of in routine qualitative analysis of cations. Dem. 635-Lead Iodide Crystals To shozu: Formation of large PbL crystals by slow diffusion of iodide ions into agar containing lead ions. Materials: Test tube, agar, lead acetate-aq, KI-aq. Procedure: Pour a layer of ICI-aq upon a 2% agar gel containing a dropperful of lead acetate-aq. After a week, project. Observations: Large yellow crystal plates of PhIz form, scattered throughout the gel. B. Group V. (Vanadium) Dem. 636-Valences of Vanadium Experiment developed by Robert Barnard. To show: Colors of different.vanadium compounds. Materials: Vanadyl nitrate-aq, Zn-amalgam, dil. HCI, 4-02, narrow neck stoppered bottle, C-4. Procedure: Place vandyl nitrate-aq in the bottle + 10 g Zn + 10 ml dil. HCI. Project C4. Pour some original solution into cell 1. Now shake the bottle with the remaining solution to first color change, and fill cell 2. Continue to the second color change, pour into cell 4, fill cell 3. Continue to the third color change, pour into cell 4. Observations: Colors - are: violet vanadous (V+%), emerald green vanadic (V+a), deep blue vanadyl (V0+2),and yellow vanadate (VOa-). Reference: HAIGHT, G. P., J. CHEM. EDUC., 29, 296 (1952); PEACOCK, P. A. H., ibid., 36, A415 (1959). C. Group VI. (Chromium) Dem. 637-Chromium Compounds from the Ore Experiment developed by Edwin H. Cooper. To show: Preparation of various chromium com- pounds from chromite ore. Materials: Chromite ore (CrzOa), NaOH, ICNO,, H-3, funnel and filter paper; dil. HCI. Procedure: Mix by shaking 0.5 g chromite ore + 1 g NaOH + 1 g I<N03; project in H-3 and heat ti1 molten. Cool. Add 30 ml H20, shake, filter equal amounts into the two cells of C-2. Project. Add dil. HC1 to cell 2. Observations: The Cr+++ in chromite is oxidized by RNOl to yellow chromate, NazCr04. Upon adding acid, orange dichromate is formed. Reactions: (0) Crta -+ Cr0,- 2Cr0,- + H+ e Cr90i- + OH- Reference: HARPIN, R. E., J. CHEM. EDUC., 36, A685, 1959. Note: Industrially the reaction is 4FeCrzOl + 8NalC0, + 70p(air) - 2Fe203 + 8COz + 8NazCr04. A452 / Journal of Chemical Education
