Tested Overhead Proiection Series Compiled by HUBERT N. ALYEA Princeton University
8. REACTION RATES
AND
EQUILIBRIA
(See Dem. 48 through 54)
C. Equilibrium
Dem. 391-Sugar
4. EFFECT OF TEMPERATURE (Continued)
Experiment developed by W. H. Slabaugh. Shifting equilibrium by heating and cooling
Materials: PBrS, two culture tubes with necks drawn down for sealing off. Preparation of PBrs: Dissolve 30 g Br: in 100 ml CSz. Add it slowly (over a 5 min period) to 8 g red P in a distilling flask. Place the flask in boiling water (10O0C) reflux, and (CARE-FIRE HAZARD) finally distill off and discard the volatile CS2. The residue, upon standing, is lemon yellow crystals of PBrs. Place a gram of PBrs in each of two culture tubes, and seal off. Procedure: Project H-3 with tank of near boiling air can water (CARE: do not use flame: PBr6 explode). Immerse one tube in the hot water; have the other at room temperature. Then reverse their locations.
+
Observations: Brown Br, forms on heating, recombines with PBrs upon cooling. heat
(yellow) PBr, E=? P B s 0001
+ Brs (reddish-brown)
Note: PBr6 melts below 100°C; above 106'C it decomposes. PBr, melts at -41.5"C. and boils at 173°C. With excess brcmine, PBr8 forms. Dem. 390-Cobah
+ Chloroie + Acid
Experiment developed by Evangeline B. Klug.
Dem. 389--Phosphorous Peniabromide Heaied
To show: PBrs.
D. Redox
Complexes Heaied
Experiment developed by Kenneth V. Jackman. To show: A temperaturesensitive equilibrium involving cobalt salts. Materials: CoClz.6H20; saturated NaC1-aq; isopropyl alcohol; medicine dropper; H-3. Procedure: (a) Project H-3. Warm a few pink crystals of CoCI2.6Hz0until a deep blue melt is formed. Cool. (b) Now add sat. NaC1-aq (5-15 ml) to form a deep pink color. (c) Using a medicine dropper, gently add a 5 ml layer of isopropyl alcohol. (d) Warm the bottom of the tube; then cool. Observations: (a) Blue CoCI2. ( b Pink Go++. (c) Blue alcohol layer on top, colorless interface-layer, pink Co++ bottom layer. (d) Warming turns the pink (lower) layer blue; upon cooling, it turns back to pink again. Reference: BUTLER,S. B., J. C h a . Educ., 37, A739, 1960; but see WHEATON, R., J Chem. Educ., 8, A23, 1961. See also SIDGWICK, N. V., Chemical Element8 and their Compounds, Oxford Press, 1950, page 1383.
To show: agent.
An exothermic reaction of a violent oxidizing
Materials: (DANGER:do not use any powdered chemicak.) Granular KClOa, granulated sugar, conc. H2S04,pipet, H-4. Procedure: Try first on an open table, to learn the violence of this reaction before trusting it to a projector. Gently mix (DANGER, do not grind) 1 ml (no more) granular KCIOI an equal volume of granulated sugar. Place in a coke cap in H-4 and project. Now (CARE; keep face and hands away) cautiously add 2 drops conc. H2SOPto the mixture. Step back.
+
Observations: Violent flare. The mixture forms CIOz a violent oxidizing agent which ignites the sugar. Reactions:
+
KC10. (conc.) H*SOd 3HClOs CIOl organic matter
+
---
+
KHSOI HClOs HCIOl H20 ZCIOz violent reactlon
+
+
9. ATOMIC STRUCTURE. RADIOACTIVITY
0. Nuclear Phenomena: Radioactivity Dem. 392-Electroscope
Measuring Radioactivity
Device developed by Albert A. Surina. To show: activity.
Use of an electroscope in measuring radio-
Materials: Cats-fur, plastic comb or rod, radioactive material. Special Device SD-392, an electroscope, see Figure. The box dimensions are 13 X 4.5 X 9 cms; the ball and plate are of brass; the foil of very thin aluminum. (One can also adapt the Mp box, inserting a onehole stopper bearing the brass and aluminum foils; evacuate this Mp box when using it as an electroscope.) Procedure: (a) Place 1g of radioactive material inside, or close to, the electroscope. Charge it in the usual manner by rubbing cats-fur over the comb, then touching it to the brass ball, then touching it with the finger. Record time in seconds for the foil to fall between two marks on SD-392. (b) Repeat using 2 g of radioactive material. (c) Repeat, moving the radioactive material to different distances from the electroscope. Volume 44, Number 6, June 1967
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