Three Demos for Chemistry ClassDon’t Try this at home

James T. Stemmle, PhDHagerstown Community College

AFACCT 2012, Montgomery College – Rockville, session 6.13, 6 Jan 2012jstemmle@comcast.net

1) Ice Cream• Freezing point depression

DTf = m Kf

• Where Kf = 1.86 oC/m,

m = molality, i.e., moles/kg solvent

Which is an example of Colligative properties, namely properties dependent only on the number of solute particles and not on the nature of the solute.

Items needed

• 1 lg ziplock baggie• 1 sm ziplock baggie• Ice c. 1 lb• Salt c. ¼ lb• ~ 1/3 c water

And maybe:• Small dish• Plastic spoons• Thermometer• Watering can/water bottle

• 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

Some pertinent facts and calculations

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.

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.

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

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.)

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.

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.

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?

3) Candles

• What do you see?

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

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

• 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

Factoids

Temperature Profile

The Faraday Lectures

• Royal Academy Christmas Lectures for young People, 1860-1861. (during our Civil war) (Available for free download from www.gutenberg.org/etest/14474)

Excerpts• And now, my boys and girls, I must first tell you of what candles are

made. Some are great curiosities. I have here some bits of timber, branches of trees particularly famous for their burning. And here you see a piece of that very curious substance taken out of some of the bogs in Ireland, called candle-wood.

• A candle, you know, is not now a greasy thing like an ordinary tallow candle, but a clean thing, and you may almost scrape off and pulverise the drops which fall from it without soiling anything.

• There is not a law under which any part of this universe is governed which does not come into play, and is touched upon in these phenomena. There is no better, there is no more open door by which you can enter into the study of natural philosophy, than by considering the physical phenomena of a candle

Greatest Technological Development of the 19th Century

• Per NY Times article, approaching the year 1900.

• Three guesses …

• Why yes we have come a great distance

So now the demonstration

• Waxy smoke• Soot deposit• Round scorch marks• Parlor trick: re-ignighting a candle w/o

touching the wick

Lab Experiment

• We know the heat of combustion of a candle, namely 43.3 ± 0.3 kJ/g

• We know how to determine the amount of heat getting into a container of water, namely, Q = m C ΔT

• So, we can answer a question like: What is the heating efficiency of a candle?

• Or several other questions: – Affect of wire gauze– Affect of container, e.g., glass vs tin can. – Affect of distance from the flame

Steps• Weigh the aluminum can• .Weigh candle• .Measure 50 ml water into can• .Measure initial temp• .Light candle • .When temperature has risen ~ 50o measure final temp. Note: candle cannot be allowed to drip

its liquid candle wax. If this is about to happen, gently extinguish the candle and measure the temperature immediately, even if the ÄT is not 50o.

• .Gently Extinguish candle (do not provoke loss of liquid candle wax. Try suffocating the flame with another small tin can.

• .Allow candle to cool• .Weigh candle• .Calculate the heat that transferred to water Qw = m SH ÄT where m = mass of water @ 1 g/ml,

SH = specific heat = 4.184 J/g oC. ÄT = change in temp = T2 - T1

• .Calculate the total heat given off by the candle: heat = Qc = (mass lost by the candle)(43.8 kJ/g) = m(43.8 kJ/g)

• .Calculate the percent efficiency = Qw/Qc x 100

Data Sheet

1. Mass of aluminum can ______________________ g

2. Initial mass of candle: ______________________ g

3. Volume of water in the can: ____________________ mL

4. Mass of the water in the can/ beaker: ____________________ g

5. Initial temperature of the water _________________ oC

6. Time lapse to reach final temp:_________________________ sec

Data Sheet, cont.

7. Final Temperature of the water: _________ oC

8. Final mass of the candle: ________________ g

9. Mass lost by the candle: (2) - (8) __________ g

10. Increase in temp of the water: (7) - (5) ____ oC

11. Heat that flowed into the water: ___________ J Qw = m SH (T2-T1) = (4) x 4.184 J/goC x (10)

12. Heat given off by the candle: ___________ kJ (9) x 43.8 kJ/g

13. Heating efficiency: ____________________% (11)/(12) x 100

Why these?

Some myths•Chemistry is a forbidding scary course•Chemistry is what happens in a lab•Chemistry is no fun•Chemistry is arcane – consisting of secret knowledge known to only a few•You need a lot of math to understand chemistry

Facts. OK more myths. Better ones

• Chemistry is the context of our lives. • Everything is chemical. OK everything at least

has a chemical dimension• Chemistry is the basis for the way we live• Food, materials, energy, shelter all have their

basis in chemistry. • For a person to be comfortable in his or her

own skin and on this planet, he or she needs to know chemistry

References

• “Chemistry of the Christmas Candle” by Klaus Roth, Chemie in unserer Zeit, 2 Novb 2011, and online at http://www.chemistryviews.org/details/ezine/1369631/Chemistry_of_the_Christmas_Candle__Part_1.html

• “A Candle in the Wind”, by Robert J. Eierman, Journal of Chemical Education, vol 85, No. 4, April 2008