Heat of solution of Na2S2O3(5H2O) - Journal of Chemical Education


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+ Sugar (2)

T o show: The formation of carbon from the d i o n of conc. H2S01on solid sugar, coke cap.

plugging them lightly with cotton. Project. Into one drop a crystal of NaCI, into the other a crystal of N&S203 5H?O. Place the Polaroid sheet.; on each side of thc tubes and rotate one sheet.

Materials: H-4, plastic bottle cap about l'/i-in. diam '14-in. high; conc. H2SOn,granulated sugar. Procedure: Half-fill a coke cap with granulated sugar, moisten i t with water, place it in the larger bottle cap in H-4 and project. Now pour a couple of dropperfuls of conc. H2SOaupon the sugar; stir.

Observations: The dramatic growt,h of crystals in the latter cell is observed. I t is especially colorful when viewed through the Polaroid sheets. NaCl docs not, "seed" for crystal growt,h; a crystal of hypo dropped into this first cell will induce crystal growth there too.

Dem. 576-Conc.

MSOI

Observations: The sugar swells and chars; viofent bubbling as steam is given off. Reactim: See previous demonstrat,ion. Dem. 577-Conc.

HSOa

+ Sugar (3)

T o show: The formation of carbon from the action of conc. H2S04on sat. sugar-aq. Materials: Plastic box about 1 1 / X ~ 1 1 /X ~ &in. high; plastic vial 2-in. high X 3/4-in. diam; conc. HtSOa; 10 g granulated sugar dissolved in 10 ml hot water, then cooled. Procedure: Set the vial containing 5 ml sugar-aq inside the plastic box and project. Suddenly (CARE: keep face away) dump 5 ml conc. HISOninto the sugar-aq. Observations: The solution suddenly turns black, froths out of the vial into the plastic box. Equations: as in Dem. 575. Dem. 578-Canc.

MSOI

+ Plastic

Experiment developed by Crosby Lr. Rogers.

Dem. 580-Hear

of rolufion of Na2S103.5Hz0

T o show: Negative heat of solution upon dissolving photographer's fixer, RazS2Oa.5HzO (hypo). Materials:

AT", stirring rod, NazS2O8.5Hz0.

Procedure: Project 12' with the tank I/, full of water. Note the position of the red liquid in the h l o thermometer. Now suddcnly dump in 5 g of Na2S203.5H10 and stir. Observations: The temperature of the solution drops about five degrees, due to negative heat of solution of the hypo. Dem. 5 8 1-Photographic

Fixer

T o sh,ow: How in photography unchanged AgCl (AgBr, AgI) on t.he developed film is dissolved off by hypo fixer (Na2SzOl),forming a complex. ( a ) Ag' CIAgC1 4 (whit,e). (h) AgCl 2Na&Oa NaC1 Nna [Ag(SzO&].

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T o show: Charring action of conc. HzS04on carbonaceous subst,ances.

Materials: AgNOs-aq, HCI-dil., Na2S20a-aq, (Can repeat, substit,uting NaRr-aq or NaI-aq for HCI). Cell C-2.

Materials: conc. HaSOa, 100 ml porcelain casserole, hot plate; 7 X '/,-in. strips of cellophane and other clear plastics; H-3.

Procedure: Project C-2 ' / z filled wit,h wat,er. To this add a few drops each of AgNOa and HCI. St,ir. KOW to one cell add 5 ml of Ag2S20a-aqaud stir.

Procedure: ( a ) I n H-3 projcct culture tubcs full of conc. H2S04(CARE in handling). Lower strips of cellophane and of plastic into them. (b) Heat 50 ml conc. I-12S04to about SOT, fill the culture tubes with this hot, liquid, and project in H-3. Immerse the strips in it.

Observations: The precipitat,~ of AgCl, which first forms, dissolves in the hypo.

Observations: The strips are charred; in hot acid in a few minutes, in cold acid overnight. Note: T r y different clear plastics:eelloph~ne, polyethylene, polystyrene, acrylates, etc. wi1.h hot and with cold conc. HzSOI.

2. THIO-SULFATES Dem. 579-Na3S203 Supersafuration

T o show: Growt,h of hypo crystals from a supersaklrated solution. Materials: Na2S203. 5H?O cryst,aIs, oval cukurc tubes, burner, cotton, NnCl crystals. Two sheets of Polaroid if available. Procedure: Heat two culture tubcs half-full of Ra21 ml water until they have S203.5Hz0crystals melted. Place them undisturbed in C-2 to cool,

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Journal of Chemical Education

Note: As a diversion, slyly slip a silver q~larterinto the AgCl t o which you are to later add hypo. Whon the AgCl dissolves, the silver qnarter remains projecled.

Dem. 582-Titration

of Iodine with Thiorulfate

l3xpcriiizent d~velopctlby Crosby T T . Rogers. T o show: An iodornetric titrat,ion of thiosulfate in the presence of starch. Afaterials: Starch-at{, 0.1N Na&Os-aq, 0.1N Iodineaq (dissolve 13..5 g iodine 24 g I