Subject: Science/Chemistry: Oxidation-Reduction Reactions
Subject: Science/chemistry: oxidation-reduction reactions.
Ages: 12+
Length: Snippet: 3 minutes
Lesson: One class period
Learner Outcomes/Objectives: Students will learn about real life occurrences of oxidation-reduction reactions. They will learn how these redox reactions are behind many known phenomena, from paper becoming yellow to apples becoming brown to fire to the most destructive of explosions. They will review how oxidation-reduction reactions involve electron transfer between atoms. Students will learn the origin of the words "oxidation" and quot;reduction". Finally, students will receive practical advice on fire safety.
Rationale: A spectacular sequence from the movie emDaylight>/em provides an effective starting point to introduce or review oxidation-reduction reactions in Chemistry and to help students to relate theory learned in class to real life, including fire safety recommendations.
Description of the snippet:
A car being chased by the police drives into a tunnel from New York to New Jersey. As it speeds inside the tunnel swerving between the other cars, it finally crashes and ends up flying into one of various trucks loaded with flammable materials. The snippet shows how the subsequent explosion sets off a shock wave of flames trough the entire tunnel.
Helpful Background:
An explosion like the one shown in em>Daylight>/em and in so many other movies is nothing else than an oxidation-reduction reaction which happens very rapidly. Slower oxidation-reduction reactions are all around us, like the rusting of some metals or a bitten apple getting brown if left in the air. For an example, see <a href=”http://scienceray.com/chemistry/the-oxidation-of-an-apple/”>***</a> The basis of oxidation-reduction (in short: "redox") reactions is the transfer of electrons between atoms. The atoms losing electrons are being oxidized while those receiving electrons are being reduced. See more on rusting and its chemical reactions in <a href=”http://www.chemicalformula.org/rust”>What is Rust?</a> from chemicalforumulas.org.
The name oxidation comes from the fact that forming a bond with oxygen is equivalent to losing electrons to the oxygen atoms, because oxygen is very electronegative. This means that oxygen exerts a strong attraction over the electrons of other atom and basically attaches them to itself, thus gaining negative charges (it becomes a negative ion). This process leaves the other atom positively charged (as a positive ion). The charge difference is then the basis for an electrostatic attraction between both ions is known as ionic bonding.
Recall that in the other type of bonding between atoms, covalent bonding, both atoms involved have a similar tendency to attract electrons (they both have electronegativity) and end up sharing electrons. This is a much stronger bond than the ionic bonding of redox reactions. See figure 1.
So, the term oxidation can refer to the bonding with oxygen or the loss of electrons. These are two different ways to describe the same process. There is a third meaning of the term oxidation, which is the loss of whole hydrogen atoms by a molecule. This is related to both previous meanings in that oxidizing agents remove hydrogen atoms from (generally organic) compounds when they are available.
See a summary of what “oxidation” can mean in a href=http://www.chemguide.co.uk/inorganic/redox/definitions.html>***</a>.
Scientists have devised a way to unify all three meanings in one, defined as “the increase in oxidation number”. This also explains the origin of the word reduction, as it then means “reducing the oxidation number”. For a detailed explanation of oxidation numbers see <a href=”http://www.ausetute.com.au/oxistate.html”>***</a .
More about fire and its chemical reactions can be found in <a href=”http://science.howstuffworks.com/environmental/earth/geophysics/fire1.htm”>***</a>.
Let us now review the oxidation reactions in terms of their speed and release of heat and light. Old paper becoming yellow, or potatoes and apples becoming brown, do not release either. In the rusting of iron, however, it is known that heat is released very slowly. It is not noticeable, but it can be measured. See more in <a href=”http://www.scientificamerican.com/article.cfm?id=experts-why-cut-apples-turn-brown”>Why do Apple Slices Turn Brown After Being Cut?</a> and <a href=”http://science.howstuffworks.com/question463.htm”>Why do Newspapers Turn Yellow Over Time?</a> from howstuffworks.org).
The next step up in oxidation is combustion. A steady release of heat and light is produced. A frequent example is organic compounds made of carbon and hydrogen that are oxidized by gaseous oxygen. This is the case of fossil fuels in their different forms used to power engines, or provide heat in fires or kitchens. Combustion can happen to gases, liquids and solids. A particular case is the combustion of metals, which release light of different colors and is used in fireworks. See a href=”http://chemistry.about.com/od/fireworkspyrotechnics/a/fireworkcolors.htm”>***</a>for a more detailed explanation on the chemistry of firework colors.
Combustion needs the fuel to reach a certain temperature in order to start. When this temperature is very low for a particular material, it can happen that it ignites spontaneously. See <a href=”http://www.omafra.gov.on.ca/english/livestock/dairy/facts/hayfires.htm”>*** , <a href=”http://www.make-biodiesel.org/Biodiesel-Safety/spontaneous-combus-tion.html”>***</a> , and <a href=”http://www.tis-gdv.de/tis_e/ware/nuesse/pistazie/pistazie.htm#selbsterhitzung”>***</a> (for the surpising case of Pistachio nuts). A detailed explanation of why moisture is responsible for the heating up of a stack of hay can be found in a href=”http://www.psla.umd.edu/extension/publications/haycombustionp1c.pdf”>***</a>.
What happens in the clip from <emDaylight</em is more violent still: what we know as an explosion is a very rapid, practically instantaneous oxidation, with intense heat and light release, plus an increased pressure due to rapid release of gases (products of the redox reaction). This makes a loud noise and creates a shock wave. If this shock wave is subsonic what really drives the propagation of the fire in the explosive blast is the rapid heat transfer to the materials above the temperature they need to ignite. This is called a deflagration. When the shock wave is supersonic the high overpressures it propagates heats materials above their autoignition temperature (kindling point). This is called detonation and is the most violent oxidation mechanism. See more on explosions, detonations and deflagrations in <a href=”http://www.interfire.org/res_file/def_det.asp”>***</a> or <a href=”http://www.exponent.com/explosions/”>***</a>
emAdvice on how to deal with a person in flames:</em As seen in the final sequence of the segment, a person who is in flames can be helped by making them roll on the floor and by wrapping them in a blanket or anything that can stop oxygen from feeding the flame. Usually people in this situation are in shock or panic, and they need to be helped, or even forced to let themselves be helped in this way. In some places there are special fire blankets placed beside fire extinguishers. To learn about fire safety, you can visit a href=”www.firesafety.gov”>Tips on Fire Safety</a> from FEMA.
Using the snippet in class:
Preparation
1. Be familiar with the location of the segment which begins at minute 14:00 and ends at minute 17:00.
2. Review the Helpful Background Section including the links cited to determine what information to present to the class.
Step by Step
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 eliminating the lignin in 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 that a body catches 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)
DieselorJet 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 and the hay was stacked before left to dry, or there is too much ambient humidity in the air. If the hay is moved, 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 a href=”http://www.youtube.com/watch?v=FVqyXxrqDXo”>News Report of Spontaneous Combustion of Hay</a>.
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 know well due to the problem of climate change and the possibility that increased CO2 concentrations in the atmosphere due to human activity is responsible of 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: <a href=”http://www.youtube.com/watch?v=_KuGizBjDXo “>Destroyed in Seconds- Chemical Plant Explosion<a> from the Discovery Channel.
Now, let us recall that in nature, temperature is closely linked to pressure and volume (in what is known 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 em>Daylight>/em> 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 wave front 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. a href=”http://www.youtube.com/watch?v=D-dLjHJuFWQ”> The Audible Difference between Deflagration and Detonation</a>.
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.