CHAPTER 1
FLAMES AND EXPLOSIONS
Part 1
Student: "Are you going to talk about atoms and molecules?"
Presenter: "That's all we are going to talk about."
Content, Occasions, Location, Audiences, and Sponsors: Selections from Faraday Christmas Lectures by Henry A. Bent in Pittsburgh's Soldiers and Sailors Memorial Auditorium for Honor Students in Chemistry from Western Pennsylvania, Eastern Ohio, and Northern West Virginia, supported by the Society of Analytical Chemists of Pittsburgh, the Spectroscopy Society of Pittsburgh, and the Department of Chemistry, University of Pittsburgh.
Purpose. To spark interest in science in sparkable youths by safe execution of striking demonstration-experiments and their explanations in terms of kinetic-molecular theory.
Setting. A large stage beneath a high ceiling fronted by a colorful display of floating balloons, with half a dozen large rectangular tables supporting and surrounded by an array of demonstration equipment including: large tanks of gases (hydrogen, helium, oxygen, nitrogen, methane, and propane) chained to dollies, large fire extinguishers, fire blankets, explosion shields, protective metal shields for table tops, spare goggles, face masks, ear muffs, propane torches, bottle-rocket launching rods, cannons, large tall glass flasks, hot plates with magnetic stirrers, plastic buckets, a large Dewar of liquid nitrogen, a large chest of dry ice, a large fish bowl of rectangular cross section, a large bell jar, a Plexiglas-sided candle stair case, molecular models, posters, an overhead projector, bottles and cans of chemicals, &c. Also familiar things (used in unfamiliar ways), including: balloons, candles, tennis balls, pop bottles, kitchen pan, kitchen towel, air, and water. Briefly put, THE BIG THREE: Flammable Gases, Liquid Nitrogen, and Dry Ice and equipment to exhibit their behavior.
Hands-On/Eyes-On Anticipatory Events for an Arriving Audience: A Preview of Coming Attractions: Crushing by hand of liquid-nitrogen-cooled flowers; watching balloons containing or attached to flasks containing subliming dry ice expand (and burst); ignition of soap bubbles filled with hydrogen and hydrogen-oxygen mixtures; ignition of hand-held propane-filled soap bubbles; and a slide show of highlights of the life of Michael Faraday.
Dress Rehearsals. Attended by middle school students and their teachers and, one evening, the general public. The same demos work for all audiences. Chemistry is chemistry. Combustion of hydrogen is the same for middle schoolers as for senior scientists. One merely says different things about it, depending on the sophistication of an audience.
Presenters. Staff of a Department of Chemistry's Outreach Program. Usually included: the program's director, a post-doctoral fellow, a visiting professor, a high school teacher on sabbatical leave, several graduate and undergraduate students, and, to handle lights and recording equipment, several volunteers from SACP and SSP.
[Boxed statements] stand for posters or projected images, on a large screen.
Text statements in bold face are descriptions of demonstration-experiments.
Statements in small type are messages for the reader.
P1 Principal Presenter 1
P2 Principal Presenter 2
Host (Current chairman of the SACP/SSP Faraday Lecture Committee). Signals end of Anticipatory Events.. Tells youths standing in line (usually a long line) that ignitions of soap-bubbles filled with flammable gases will resume for interested individuals at the end of the main program. Thanks providers of equipment. Introduces PP1 and PP2, briefly!
P1 Welcome to Faraday Lecture 2000. It follows in the footsteps of Michael Faraday and his famous Christmas Lectures for Juvenile audiences at the Royal Institution about, in his words, The Chemical History of the Candle.
The star of our program is in this tank.
P1 places an arm around a tank of hydrogen.
We can learn a lot about it just by observing the character of its container. It has only curved surfaces, top and sides, and bottom, also, concave inward, so that the tank can be stood upright, if somewhat precariously, hence this chain.
P2 unchains the tank from its dolly and, with P1's help, exhibits its bottom.
P2 The tank is pretty heavy. One might guess that it has thick walls.
P1 Access to the tank's contents is by means of a valve, at its top, protected by this sturdy screw cap.
P1 unscrews the cap, revealing a valve, to which he attaches reducing valves, "to reduce the pressure in steps," and adds that -
Everything about the tank
curved surfaces
large mass
protected valve
dolly of moving it about
chain to its dolly
suggests the same thing.
P1The tank is designed to contain a gas at high pressure and, thus, many molecules. For, as you know, -
For ideal gases
PV = nRT
==>
n = P(V/RT).
Thus, for given V and T,
if P is large, then n,
population of molecules,
is large.
For gases, molecular population is proportional to pressure.
P1 So much we infer from the tank's shape and mass. As Yogi Berra has said, "You can observe a lot just by watching." The tank's gas is hydrogen.
P2 Evidently hydrogen molecules are not be very sticky toward each other.
P1 In fact, at any temperature merely 23 Celsius degrees, or more, above absolute zero hydrogen cannot be liquefied however high the pressure may be.
P2 The tank is chained to its dolly, as we noted, to lessen the chance that it might tip over.
P1 For, should that happen, the tank might snap off its valve if it struck something on the way down.
P2 Created would be a rocket.
P1 It's happened. One time a tank of hydrogen in a chemistry building fell over, snapped off its valve, took off down a long hall, and exited the building through a wall at the far end.
P2 Somewhat like this:
P2 inflates and releases a balloon.
P2 As air in the balloon exited one way, the balloon — by Newton's Law of Action and Reaction — moved in the opposite direction.
P1 It's an example of a general rule: Gases tend to expand from regions where their pressure is high (e.g., inside the balloon, somewhat) into regions where the gas pressure is lower (in an auditorium's space outside the balloon).
P2 Here's another example.
P2 inflates a balloon from the tank of hydrogen. Young children are fascinated by that event. They know that balloons don't self-inflate. Yet the person standing near it wasn't blowing into it.
P1 Hydrogen's principal physical property is that it has the lowest density of any known substance, suggested by the fact that a hydrogen-filled balloon floats in air.
P2 ties a string to the hydrogen-filled balloon and releases it, holding on to the string.
P1 Here's how we might picture in our minds' eyes what we just saw with our optical eyes.
P2 Floating is a matter of density.
P2 has a young volunteer pick up a lead brick and hand it to the presenter.
This heavy brick floats in mercury but not, unlike our hydrogen-filled balloon, in air. Floating in mercury would be easy to show — but safe removal afterwards of small droplets of toxic mercury adhering to the brick, not so much so.
P2 drops the lead brick onto an orange crate resting on the auditorium's floor off the front of the stage, smashing it to bits.
P1 As the brick fell, it displaced air upwards, from where the brick ended up to where the brick started its descent.
P1 How is a fallen lead brick like the levitated balloon? Both events increased the disorder in the universe. (One can always say that.) The floor and the brick are a bit warmer — and, accordingly, internally, atomically more disordered — after the heat-producing arrested fall than before it.
P2 Kelvin's statement generalized applies to all events.
P1 Entropy is a numerical measure of atomic and molecular disorder.
P2 For all natural events -
ΔSUniverse > O
P1 An event's entropy-production is a measure of its impact on the environment.
P2 Hence -
THE ENTROPY ETHIC
Live leanly.
Do not create entropy unnecessarily.
Conserve transformable forms of energy.
P1 The "Entropy Ethic" addresses most issues regarding man(un)kind's impact on the environment.
P2 Our nation needs a national energy policy. Our globe needs a personal Entropy Ethic.
P1 Accordingly, -
Haste makes waste
(as it's said)
as thermal pollution
(heat)
&
entropy
P2 Walk or ride a bicycle if you can.
Eschew powerful devices, such as 300 horse power internal combustion engines mounted on mobile chasses.
Remember -
Power
=
Rate of change
of
Transformable Energy
to
heart, usually.
P1 Here's the sort of thing that happens inside internal combustion engines.
P1 torches the hydrogen-filled balloon with a propane torch taped to a pole.
Balloons and soap bubbles (recommended by Mendeleev) are ideal reaction vessels for explosive reactions whose reactants are gases.
P1 We just saw is a sure fire-demo. Nature is Lawful. Nature always does her thing.
P2 Illustrated by the falling lead brick and combustion of hydrogen is the difference between a physical change and a chemical change.
The atomic rearrangement in the combustion has so profound an effect on participating matter's properties we change the substances' names. We say that -
"Hydrogen" + "Oxygen" = "Water" + "Heat"
P1 Here's another illustration of the difference between physical and chemical changes.
P1 drops a box of matches and says: "Free and arrested fall. Physics."
P1 strikes a match, and says: "Combustion! Chemistry!
Just joking. There's a lot of physics in combustion.
P2 Combustion illustrates in -
Chemistry
COLLISION THEORY
To react chemically, molecules must collide physically
with bond-breaking violence.
Kitchens
THE COOK'S IMPERATIVE
Mix and heat.
Fire Fighting
THE TRIANGLE OF FIRE
Fuel + Oxidizer + Heat = Fire
P1 For fire, three things must be at the same place at the same time: molecules of fuel, molecules of oxidizer, and activation.
P2 Recall the size of the hydrogen-produced fireball.
P1 It was much larger than the inflated balloon.
P2 The hydrogen had to reach out — or, as we say, diffuse — into the room for its oxygen.
P1 The flame is called, accordingly, a diffusion flame.
P2 The flame of a torch (as usually used) is a pre-mixed flame.
P1 Gaseous propane and air mix down here at these air ports.
P2 closes the torch's air ports with his hand: "A bushy, diffusion flame."
P2 removes his hand from the torch's air ports: "A hotter, premixed flame."
P1 How about premixing hydrogen and oxygen, in a balloon?
P2 Would it be a safe thing to do?
P1 Hydrogen-oxygen mixtures are said to be explosive.
P2 Yet watch this:
P2 attaches with rubber tubing a hollow glass wand to tank hydrogen's pressure gauge, opens the gauge, slightly, and squirts issuing hydrogen into oxygen-containing air and, for good measure, at the floor, at a sheet of paper, at personal clothing, at his or her hand, face, and hair, and, finally, into a beaker of water.
P1 CONCLUSION: Hydrogen doesn't react with the iron of the steel of its tank, or with the copper and zinc of its brass pressure gauge, or with the rubber and glass of this rubber tubing and glass wand, or with the nitrogen and oxygen of air, or with water. Nor is it toxic.
(squirts self in the face again).
P2 Briefly put: Hydrogen at room temperature is inert stuff. Faraday was fascinated by the inertness of cold fuels. "What are they waiting for?" he wondered.
Faraday harbored doubts — some 150 years ago — regarding the reality of atoms and molecules. The kinetic-molecular model of matter and its extraordinary explanatory power was not a functional part of his mental tool kit.
P1 Chemists encode cold hydrogen's inertness with three ideas.
P2 So, it seems that it would be safe to mix cold hydrogen gas with cold oxygen gas. For complete reaction, the equation, -
2 H2 + O2 = 2 H2O
calls for a molecular ratio of 2-to-1. According to -
AVOGADRO'S LAW
Equal volumes of gas at the same T and P contain equal numbers of molecules.
Accordingly, a 2-to-1 molecular ratio corresponds to a volume ratio of 2-to-1.
Avogadro's Law reflects the fact that from a physical point of view gases are very much alike: mostly empty space.
P1 For most gases the ideal gas equation of state, mentioned earlier, is approximately true.
If T and P are the same, then V/n is the same.
P2 We can eyeball the 2-to-1 ratio because, for our purposes, the precise ratio is not important.
Hydrogen, because of its small molecular mass, has the highest random molecular velocities of any gas, the highest flame velocities, and, accordingly, the widest limits of flammability.
P1 RReads from a Handbook of Chemistry and Physics:
Chemical Limits of Flammability
Formula Lower Upper
H2 4% 74%
P2 adds oxygen and hydrogen to a balloon, and pauses.
P1 Safety first.
P2 Prudent practice in the laboratory is imagining, using one's memory of personal experiences and memory of additional knowledge and by use of common sense applied to observations, the worst things that might happen, and preparing for them. We call it -
[ILLUSTRATION OMITTED]
P1 Do we need rain coats or umbrellas, owing to formation of water?
Not in our experience.
But do cover your ears, to protect ear drums.
P2 dons ear muffs and (finally!) torches the balloon filled with a mixture of hydrogen and oxygen.
BOOM!!
P1 That was explosive formation of water, from hydrogen and oxygen.
Yet no need for raincoats.
It was too hot, in the flame's region, for water molecules to stick together and form droplets of liquid water.
Needed in order to show the presence of water molecules, which are invisible in the gaseous state, as water vapor, or superheated steam, is a cold, molecule-condensing surface, for the alleged water molecules to condense on, such as the glass of this bell jar, at room temperature, which is cold, compared to water's boiling point, at which temperature liquid water forms gaseous water, at 1 atm.
P2 sets about filling the bell jar with hydrogen.
P1 We could use a volunteer.
A volunteer places a hand, as requested, up into the bell jar (resting on wooden blocks).
P1 Hot or cold?
"Cold."
P1 At room temperature fast moving hydrogen molecules conduct energy away from warmer bodies, such as a person's hand, faster than do heavier molecules of air; hence the cool feeling, in hydrogen.
P2 removes a small plug in the end of a short piece of glass tubing that passes through a rubber stopper that stoppers an opening at the top of the bell jar and ignites the emerging hydrogen gas.
P1 Hydrogen's flame is nearly colorless. Hydrogen contains no soot-forming carbon, which, as hot soot, in a candle's flame, glows (yellow hot).
P2 Now our flame has heated the soft glass tubing and is vaporizing some of its sodium, which lends a characteristic yellow color to flames.
P1 sprays a sodium chloride solution into the flame of a Fisher burner, then solutions of chlorides of copper and strontium.
P2 Now imagine what's going on with the hydrogen inside our bell jar.
As denser air, attracted gravitationally by the earth, sinks and displaces less dense hydrogen gas upward and out the exit at the top of the bell jar, air begins to enter the bell jar at its bottom and mix with hydrogen above it, forming a hydrogen-air mixture, perhaps explosive.
As the hydrogen-air mixture begins to exit the bell jar, the burning velocity of the flame, no longer a pure diffusion flame, increases.
Eventually the exiting gas's burning velocity exceeds its flow velocity upward, and the flame advances down the glass tube, until, cooled by the tube, it retreats upward.
