1. Show the snippet in class.
2. If necessary, prepare an introductory lecture providing the class with appropriate information helpful in meeting curriculum goals.
3. If this is a review class, ask students to recall other cases of oxidation-reduction reactions of which they are aware. Write their answers on the board to build a list of redox reactions with increasing speed and increasing amounts of heat and light release, from paper becoming yellow to detonations. If the class is an introduction to the topic, the same question can be asked, but the list will have to be built with substantial help and input by the teacher.
4. Start with the familiar processes of very slow oxidation, as in the yellowing of paper and cut apples turning brown. In very slow redox reactions, no heat is released. In the case of paper, it is the component called lignin which is subject to oxidation by the oxygen in the air. Fine white paper that resists yellowing is made by eliminating the lignin through chemical processes. In the case of fruit, like apples for example, oxygen oxidizes compounds from the cells that come into contact with the air when the cells are damaged through cutting or biting.
5. The next level in the speed of oxidation reactions is rust on iron. It may seem similar to oxidation of paper or cut-up fruit, because it is also slow, but heat is released during the rusting process, although it is very little.
6. Energy is released in the form of heat and light in combustion, familiarly known as fire, which is the next step up in oxidation reactions in terms of speed and energy output. Materials of organic origin like wood, paper, cotton or alcohol are especially prone to catch fire, as the oxidation of the carbohydrates they are made of has a low ignition temperature.
7. We humans are made of organic materials, too, but remember that we are made of about two-thirds water, which makes it difficult for a body to catch fire. When people are seen to be in flames, what happens is that they are covered with flammable stuff, for example gasoline, and it is that substance and their clothes which undergo combustion in the form of fire, while skin and body suffer burns of various degrees due to the excess temperature, but without having caught fire. (When bodies are burnt, they have to dry out first before they ignite and can undergo proper combustion.)
However, there are claims about human bodies spontaneously catching fire and burning partially or completely into ashes. This is called spontaneous human combustion and it is inconsistent with current scientific theory in that it is the body that spontaneously undergoes combustion. Most cases investigated were found to have an alternative and scientifically sound explanation, like people smoking in bed and falling asleep with a lit cigarette in their hands and the bed linen (cotton rich) catching fire.
But there are instances of spontaneous combustion (not human) around, when the ignition temperature of a particular material is very low, or becomes so under particular circumstances. Review the examples mentioned in the Helpful Background section, mentioning the following ignition temperatures:
Triethylborane: -20 °C (-4 °F)
White phosphorus: 34 °C (93 °F)
Carbon disulfide: 90 °C (194 °F)
Diethyl ether: 160 °C (320 °F)
Diesel or Jet A-1: 210 °C (410 °F)
Paper: 218°-246°C (424-474°F)
Gasoline (Petrol): 246–280 °C (475–536 °F)
Butane: 405 °C (761 °F)
Magnesium: 473 °C (883 °F)
Hydrogen: 536 °C (997 °F)
Particularly interesting here is the case of Triethylborane, which obviously ignites and burns spontaneously when it gets into contact with the air. This feature makes it interesting for some technological applications, but also dangerous to handle.
The spontaneous combustion of hay is very frequent It can happen with a small stack of grass from mowing your lawn if left untouched for some days, or if the hay was stacked before left to dry, or if there is too much ambient humidity in the air. A hot mass in the center of the stack will start to emit smoke, as a sign that combustion has begun. In large scales it can become dangerous. Show the class News Report of Spontaneous Combustion of Hay.
8. In any oxidation reaction, as in any chemical reaction, there are substances that are formed as products: those written on the right hand side of the equations shown above. In a fire burning organic material, for example, CO2 is released into the atmosphere as we well know due to the problem of climate change and the possibility that increased CO2 concentrations in the atmosphere due to human activity is responsible for a global rise in average temperatures. The CO2 is gradually released as it is formed, and is mixed with the surrounding air being diluted away from the combustion zone. But if the oxidation reaction is too rapid for the products to get away gradually, this happens violently through a shock wave, which, together with the corresponding sound and the heat and light, we know as an explosion.
In an explosion, it is usually this shock wave which causes the most damage. Explosives used to loosen minerals in mines or bombs falling on concrete facilities or on open fields are examples of this. But we also know that they can also trigger a quickly propagating fire when dropped on buildings with wood, textile, and plastic elements. The explosion produces high temperatures that can reach and exceed the ignition temperature of those materials around, which will then begin to undergo combustion and further heat other materials, getting them to burn too, in a destructive chain reaction. Such an explosion is called deflagration, and can be seen in this astonishing real-life example in which the shock wave can be clearly distinguished: Destroyed in Seconds- Chemical Plant Explosion from the Discovery Channel.
Now, let us recall that in nature, temperature is closely linked to pressure and volume (in what is known as the equation of state). So, in order to raise the temperature of a particular material, besides heating it up with an external heat source, there is the possibility of increasing the pressure on it. If the shock wave of an explosion is sufficiently intense to do this, it will not only cause damage through its blast, but also because it drags along an overpressure that will ignite flammable materials as it advances. This is then called a detonation. The segment of the movie Daylight used here shows how the shock wave advances as a flame. The overpressure of the shock wave is sustained over a long time because it all happens inside a tunnel. There the gases cannot expand freely and therefore maintain a sufficiently high pressure at the wavefront of the shock wave throughout the tunnel, until they reach open air, as is seen at the end of the snippet.
Watch this interesting video on the audible difference between a deflagration and a detonation of the same fuel. The difference is that first it is oxidized by air (21% oxygen) and then with pure oxygen. Oxidation occurs much more rapidly. The Audible Difference between Deflagration and Detonation.
9. Save the last 10 minutes of the class period to address fire safety, using the sequence of the person with burning clothes of the snippet as a starting point to introduce recommendations to act in such and other cases that involve fire.