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# 6.3.stemmle

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### 6.3.stemmle

1. 1. Three Demos for Chemistry Class Don’t Try this at home James T. Stemmle, PhD Hagerstown Community College AFACCT 2012, Montgomery College – Rockville, session 6.13, 6 Jan 2012 jstemmle@comcast.net
2. 2. 1) Ice Cream• Freezing point depression Tf = m Kf• Where Kf = 1.86 oC/m, m = molality, i.e., moles/kg solventWhich is an example of Colligative properties, namely properties dependent only on the number of solute particles and not on the nature of the solute.
3. 3. Items needed• 1 lg ziplock baggie• 1 sm ziplock baggie• Ice c. 1 lb• Salt c. ¼ lb• ~ 1/3 c waterAnd maybe:• Small dish• Plastic spoons• Thermometer• Watering can/water bottle
4. 4. Some pertinent facts and calculations• Serving temp of Ice cream: -16oC• Corresponding molality: m = 16oC/1.86oC/m = 8.602m• Molarity for NaCl ions: 4.3 m• MW of NaCl: 58.45 g/mol (Na: 23. Cl: 35.45)• Grams of NaCl: 4.3 moles x 58.45 g/m = 251 g NaCl/kg water. So, about one fourth the mass of ice/water.• Solubility in cold water: 35.7g/100g = 357 g/Kg H20• 357/58.45 g/mol = 6.108 moles/Kg, i.e., 6.108 m• Molarity of ions: 12.208 molal• Max FP depression = 12.2 m x 1.86oC/m = 22.7oC• Density of NaCl = 2.165 g/ml
5. 5. So, rule of thumb:• By mass, 4 parts ice/water, 1 part salt, and since density of salt is 2.156 g/ml, …• By volume: 8 parts ice/water, 1 part salt.
6. 6. Why Ice Cream?• It could be used to illustrate colligative properties with calculations and all.• It’s fun. Students get something useful. The product is actually delicious.• It illustrates that chemistry is the context of our lives.• It is safe and can be done with common things found around the house. Acknowledgement: ICE and Ann Caldwell.
7. 7. 2) Espresso• Every morning I think hey this would make a great demo• And I actually use it• It shows the chemistry hidden in ordinary things. Hey, open your eyes. Pay attention.• In particular, it illustrates what goes on in boiling (evaporation) and condensation
8. 8. Some stray facts• Alton Brown (The food Channel Nerd: “Good Eats” show) says none of the coffee makers on the market use hot enough water.• Espresso does not have an x. First syllable pronounced Ess.• My Mr. Coffee espresso machine was acquired for \$3 at a thrift store• This particular machine is the favorite source of steam for at least one established luthier. I , myself have used it to repair a guitar. (Steam softens glue. Is used to remove the neck.)
9. 9. The Espresso Machine• It’s a boiler with a steam outlet and a boiling water outlet.• The boiling water is forced out of the boiler by steam pressure.
10. 10. What do we observe?• First bubbles, hissing sound• Some water ejected• Then louder sound. The peculiar espresso sound mimicked on TV, but no bubbles.• Finally hot bubbles.
11. 11. Why Espresso Demo?• Fits with distillation experiments. Also illustrates phenomena occurring in Hydrogen and Oxygen experiments. Namely contamination of first bottle with initial head gases.• Could be used to illustrate the scientific method. To test powers of observation. In query based teaching. How do we explain that bubbles come first, then no bubbles, then bubbles again. What is going on here?
12. 12. 3) Candles• What do you see?
13. 13. The Chemical Equation• 2 CnH2n+2 + (3n/2 + ½) O2 → 2n CO2 +(2n+1) H2O• For example: n = 21 – C21H44 + 32 O2 → 21 CO2 + 22 H2O
14. 14. Only the yellow part of the flame absorbs light and casts a shadow• primary soot particles have roughly the composition (C3H)n, and each is actually made up of several thousand carbon atoms. The empirical formula suggests cyclic, highly unsaturated, polycyclic aromatic structural elements. The primary soot particles grow through agglomeration, dehydration, and coagulation to as much as a few million carbon atoms [9]. The resulting large soot particles begin in Zone IV at temperatures above 1200 °C to glow.• yellow candlelight is a direct result not of a chemical reaction, but rather of solid soot particles thermally transforming energy into light
15. 15. Factoids• All three phases are present in the wax fuel: solid, liquid gas• The flame produces no visible smoke.• A disturbed flame produces sooty, black smoke made of solid carbon.• When extinguished, a candle produces white smoke that smells like wax
16. 16. Temperature Profile