Combustion of Gases (Geoffrey Alch)
This demonstration focuses on combustion of natural gas, which is predominantly methane (CH4). It's a natural product from deep in the earth, so it's not absolutely pure methane. It's what comes out of the spigots in all laboratory classrooms (including ours), and it is also used by your stove at home and your home heating.
This lesson does not have a handout, however we will balance the chemical equation for the combustion of methane (reaction with oxygen). Have a small amount of paper available for this; You can easily do this in a small area on your page of notes for this class. The demonstration will show how combustion is not possible unless sufficient oxygen is present.
Title: The title should be descriptive so others will know what it is about.
"Combustion of Gases"
Principle(s) Investigated: List all principles that apply to this activity.
This demonstration shows the principle that all fires need oxygen, and that without enough oxygen to achieve the fuel/oxygen ration specified in the reaction equation, burning is not possible. For methane, this ratio is two parts by volume of oxygen (O2) per one part by volume of methane. In the first part, the interface of flammable gas with oxygen happens at a tiny hole, so in this way the speed of burn is controlled. However, with unrestricted access to oxygen, as shown in the second half of the demonstration, the speed of the fire is limited only by the flammability of the material, and the energy released is limited only by the quantity of fuel. In this case, methane is highly flammable, and explosion happens very suddenly, once the correct mixture with air is reached (9%fuel / 91% air). With one gallon of properly mixed gas/air mixture, the explosive energy is very startling, and not easily forgotten.
This demonstration also utilizes the concept that air is approximately 20% oxygen (O2) and 80% Nitrogen (N2).
Standards : Paste in the appropriate California content standards.
8PC5. Chemical reactions are processes in which atoms are rearranged into different combinations of molecules. As a basis for understanding this concept:
a. Students know reactant atoms and molecules interact to form products with different chemical properties.
b. Students know the idea of atoms explains the conservation of matter: In chemical reactions the number of atoms stays the same no matter how they are arranged, so their total mass stays the same.
c. Students know chemical reactions usually liberate heat or absorb heat.
Materials: Include a list of materials and sources from which they may be obtained.
This demonstration requires three things:
1. An clean, empty one-gallon paint can with a pressed-on lid. It can be purchased in any store which custom-mixes paint. Cleaning out a paint can which has already had paint in it is quite messy, and probably not worth the bother. My new can cost $3.50. Make a 6mm (1/4") hole in the center of the lid, and a 12 mm (1/2") hole on the side near the bottom. This is most easily done with a drill.
2. A length of hose of a diameter which will grip the barbed gas nipple in a lab. A minimum length of 20 cm should do, and the material is not important. It is needed to fill the paint can with natural gas. This can be bought at most hardware stores, or even a medical supply.
3. A lighter is necessary to light the fire, preferably a long one (suitable for lighting a barbecue or fireplace). This can be bought in a supermarket, hardware or general merchandise store, including the "99 cents only" stores.
Procedure: Give a detailed explanation of the procedure and include diagrams if possible.
Safety of students must be maintained. Do not place the lit paint can too close to students, and remind them that because there is fire, everyone's safety requires they stay in their seats --- as long as they stay in their seats they will stay safe.
Start with a one gallon clean, empty paint can with a press-fitted lid and a 6 mm (1/4") hole drilled in the center of the lid and a 12mm (1/2") hole in the side, near the bottom. Use a hose to fill the can with natural gas from a spigot in the lab. Light the gas at the hole in the center with a long lighter. The "candle" will burn with a big orange flame at first. Explain that the flame will gradually become very tiny, and that you are turning off some of the room lights so that they will be able to still see the flame as it shrinks. If the can is placed on a surface no higher than the students' eye level, they should still be able to see the flame at its tiniest, without having to stand up.
Once the fire is lit, write the time on the board, and explain that you must exactly time the burning. Explain that if it goes out without your noticing it, they should point it out so that you can either relight it, or if it won't relight then you will stop the timing. Explain that one gallon of gas converts to 4.1 liters, and that the burn time and the capacity will be useful later in a calculation.
Next is a good opportunity to review the balancing of the reaction equation: CH4 + 2O2 = CO2 + 2H2O.
Begin without coefficients, and use students' help to calculate them.
Also review that the students understand that atmospheric air is approximately 20% oxygen (O2), and 80% nitrogen (N2), with less than 1% of other gases. Explain that this is important because the nitrogen slows down the burning.
At this point, the flame is probably smaller than at first. Draw attention to this, and explain that it is smaller because air is entering the can through the hole in the side at the bottom, and therefore the gas escaping the hole in the top has a lower concentration of methane, and therefore a smaller flame. Also point out that the flame has transitioned from orange to blue, indicating that the methane is burning more completely now, because it is partially carbureted with oxygen inside the can now, and therefore needs very little air outside the can in order to complete the reaction.
About 5-10 minutes after lighting, the great surprise of the demonstration should happen:
BOOM!!!
The can will explode loudly, the lid probably striking the ceiling, with a brief but very bright flash of orange light.
Immediately pass the can to a student or two, and ask them what they feel. It will be warm. Reassure the students that you planned fr this explosion to happen. Apologize for surprising them, but explain that you think it will help them remember the lesson.
Ask the students to describe the situation immediately before the explosion, and how the explosion happened. They should remember the tiny flame sitting on top of the tiny hole. If no one offers an explanation, suggest that the flame entered the hole. Explain that for complete combustion of methane to take place, there must be a 2-to-1 ratio of oxygen to methane (from the balanced equation), and a 4-to-1 ratio of nitrogen to oxygen (the composition of air). Combining these ratios gives the following proportions:
1/11 (9%) methane
2/11 (18%) oxygen
8/11 (73%) nitrogen
Suggest that the flame entered the hole exactly when the gas mixture inside the can contained enough oxygen to burn with no external supplementation, i.e. approximately in those proportions. The concentration of methane started near 100%, and dropped steadily while the "candle" burned. Once the methane concentration approached 9%, that's when the flame entered the hole, and BOOM!!
The discussion can continue by explaining the "Applications to Everyday Life" below.
Student prior knowledge: What prior concepts do students need to understand this activity?
1. Balancing the chemical equation for combustion of methane is reviewed while the paint can "candle" is burning. This is intended as a review, however, and, will not generally be enough time to be comprehended by a student who has never balanced a chemical equation.
Explanation: Give a thorough explanation of the experiment or demonstration. Your explanation should be written to give your fellow teachers a solid understanding and include greater detail than what you might provide for your secondary students. Make certain to include equations whenever pertinent.
The explanation is integrated with the "Procedure" above.
Questions & Answers: Give three thought-provoking questions and provide detailed answers.
1. Q: Why didn't the explosion happen when the fire was first lit?
A: The flame didn't enter the hole because there was no oxygen inside the can to complete the reaction.
2. Q: Why does the flame get smaller while it burns?
A: Because as the reaction continues the fuel and air are more mixed, and therefore very little outside oxygen is needed to complete the reaction.
3. Q:Why does the flame transition from orange to blue?
A: The temperature of the blue flame is hotter because the combustion is more complete
Applications to Everyday Life: Explain (don't just list) three instances where this principle can be used to explain other phenomenon.
1. First, this lesson illustrates the principle that thoroughly mixing the components of any reaction greatly speeds up that reaction. Suppose you were making bread. If you just sprinkle the yeast on top of the dough, will it rise as high as if you knead it? Maybe it might if you have a week to wait...
2. This lesson illustrates principles that can be extended to any kind of fire. It shows that all combustion requires oxygen, and conversely that depriving a fire of oxygen will slow down or put out the fire. Some fire extinguishers work by depriving fires of oxygen. If your house is on fire, close your interior doors, and don't open any that are shut. Open doors can allow huge volumes of air to flow into the room where the fire is burning. Leaving doors shut will slow the fire down by restricting the flow of air.
3. This lesson has implications for any kind of explosion, for example, gas fires that frequently start immediately following earthquakes. Such a house fire is meticulously demonstrated in the MythBusters link below. "Don't anyone light a match" All fires need three things: Fuel, oxygen and spark (initiation energy). When two of the three are present, you better not supply the third ingredient. This lesson also demonstrates why gas-earthquake-automatic shutoff valves are lifesavers in earthquakes.
Photographs: Include a photograph of you or students performing the experiment/demonstration, and a close-up, easy to interpret photograph of the activity --these can be included later.
See photograph above.
Videos: Include links to videos posted on the web that relate to your activity. These can be videos you have made or ones others have made.
The MythBusters TV show recreates an apartment explosion as shown in the film "The Bourne Supremacy", to test if the annihilation represented in the movie due to a gas line leak would have happened that way in real life. They fill an apartment to 9% concentration of methane and detonate it.. Their verdict: "yes" (although slightly exaggerated in the film).
http://www.youtube.com/watch?v=0QV1zR9kIM0
Here a professor an the University of Utah Department of Chemistry does a vey similar demo.
http://www.youtube.com/watch?v=MhD5pZ5sZb4
Here's a similar demo done in a home with a coffee can.
http://www.youtube.com/watch?v=reiBJIn37QA
And yet another: