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The Fire Triangle

Part 2 - Oxygen

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The Fire Triangle

Conceptual Model
Fire has three components

Heat
Fuel
Oxygen

Heat

Fuel

Oxygen

As you recall, the fire triangle is a conceptual model that can be used to understand the factors needed for fire to begin and to continue. Can anyone explain what a conceptual model is? In science, a conceptual model is a simplified representation of a concept or of concepts related to a phenomenon. Often depicted as diagrams, conceptual models show processes or describe the relationships between concepts or factors essential to a phenomenon. Check your diagram and make any necessary corrections as we quickly review the fire triangle. The three required factors in the fire triangle conceptual model are heat, Click fuel, Click and oxygen. Click When the quantities of these three factors are not restricted, Click fire will continue. Remember that firefighters use this model to understand how to extinguish fires. Since we recently conducted an activity investigating heat and fuel, we’ll only review oxygen. Click

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The Fire Triangle - Oxygen

Atmosphere’s Composition

N₂ ~ 78.084%
O₂ ~ 20.940%
Argon ~ 0.934%
Other ~ 0.042%

CO₂ ~ 0.0407%
CO₂ ~ 407 ppm

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Chemical & Physical Changes

A physical change occurs when a specific molecule/substance/material undergoes a change or changes form without the molecule/substance/material having its unique chemical identity changing. Phase changes are a great example.
For a transformation to be a chemical change, the original unique chemical identity of a material/substance must change to a different chemical identity as a result of a chemical reaction. Examples include burning, rusting, cooking, digestion, and photosynthesis.

Complete “Table 1” on your worksheet. After a minute allow students to compare their work with that of their colleagues. Now compare your responses with those of your colleagues. Make any necessary changes or additions. Allow students more time to interact. Monitor students to determine when they have completed their responses. Would anyone like to share their definition for physical change? Can anyone give me an example of a physical change? When water freezes, the relationship between H₂O molecules changes, but the configuration of atoms in the form of H₂O does not change. Click What about a chemical change? Examples? For example, during thermolysis, a chemical change takes place as H₂O molecules are split into H₂ (hydrogen gas) and O₂ (oxygen gas). Click Now write the balanced equations for photosynthesis and cellular respiration in “Table 2.” Give students some time to work on their own. Then allow students to compare with their neighbors. Let’s check your chemical equations. Click

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Balanced Chemical Equations

Photosynthesis

6CO₂ + 6H₂O + Sunlight → C₆H₁₂O₂ + 6O₂

Cellular Respiration

C₆H₁₂O₂ + 6O₂ → 6CO₂ + 6H₂O + Energy

+ HEAT + HEAT & LIGHT

Click Can anyone tell me the reactants and products for photosynthesis? If students don’t include sunlight, ask them if photosynthesis can take place in the dark. Check your balanced chemical equation for photosynthesis and make any necessary changes. Click Give students time to correct their equations. Click What are the reactants and products for cellular respiration? Students will probably include energy in their responses since you brought “sunlight” to their attention earlier if it was necessary. Check and correct your equations for cellular respiration. Click Allow students to change to their equations. Does anyone remember how to modify the cellular respiration equation so it will represent combustion? Students need to include heat with the reactants so all three components of the fire triangle are present on the reactant side of the equation. Students should also state that the energy produced will be in the form of heat and light. Click If vegetative biomass is being burned, glucose is probably in the form of which organic polymer? Cellulose. Take a moment to answer the two prompts beneath “Table 2.” Click

NOTE: If you choose to have students begin the laboratory portion of the activity without knowing that CO₂ will be produced from mixing the clear liquid (vinegar) and white powder (baking soda) in the plastic cup then move on to Slide 6. However, if you prefer that students are familiar with the chemical reactions producing CO₂, then skip to Slides 7 & 8 and then return to Slide 6. Slides 7 & 8 could be used while discussing worksheet responses when students have finished.

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Activity

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Safety Checklist for Fire Activities

Wear appropriate Personal Protective Equipment [PPE].

100% cotton clothing; safety goggles; closed-toed shoes

Tie back loose sleeves and long hair.
Make sure a charged fire extinguisher is close. Know how to use it.
Make sure each station has a spray bottle that is filled with water.
Always be aware of the fire’s location. Never lean over the fire!
Extinguish burned materials with water before putting them in the trash. Fire is not out if there is any smoke or heat coming from the fuels.
If a fire starts on you … Stop! Drop! and Roll!

An instructor must be present for fire to be used.

(Adapted from the USFS “FireWorks Safety Poster”)

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“Plastic Cup” Chemical Reactions

“White powder”

Baking soda
Sodium bicarbonate (NaHCO₃)

“Clear liquid”

Vinegar
Acetic acid (CH₃COOH)

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“Plastic Cup” Chemical Reactions

Reaction #1

CH₃COOH + NaHCO₃ → H₂CO₃ + NaCH₃COO

Reaction #2

H₂CO₃ → CO₂ + H₂O

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Atomic Mass of an Element*

Noble Gases

Helium (He)
Neon (Ne)
Argon (Ar)
Krypton (Kr)
Xenon (Xe)
Radon (Rn)
Organesson (Og)

Slides 9-14 are an “Extension” that could be used to review atomic mass and how to calculate molecular mass. It might be a good idea to go through this extension after students have completed the activity. The extension concludes with why CO₂ accumulated in the plastic cups when vinegar and baking soda were combined. The accumulation of CO₂ displaced air containing O₂. The absence of O₂ kept students from relighting the candle in the plastic cup.

*EXTENSION SLIDE: Let’s take a moment to learn or review how to determine the atomic mass of an element. The atomic mass of an element is important not only to figure out what the mass of an atom of a particular element is but also as a necessary step when calculating molecular mass. We will begin by focusing on the noble gases. Can anyone name a few of the noble gases? Helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), radon (Rn), and oganesson (Og). Click Can anyone explain why the noble gases are considered to be unusual? The noble gases are usually unreactive because all noble gases have an outer electron shell that is full. They are typically encountered as monatomic gases. Refer to a periodic table. What is the atomic mass of helium? 4.003 “atomic mass units” or “grams per mole.” Click The atomic mass of Helium is 4.003 atomic mass units or 4.003 grams per mole. Since we will only be making comparisons between elements or between molecules, it is not important which units are used. However, the magnitude (value) of the mass is extremely important! Does anyone remember what the “2” is? The atomic number which is the number of protons in the nucleus of that element. Thus, helium has 2 protons in its nucleus. What is the atomic mass of neon? Click … of argon? Click … of krypton? Click Which of these gases makes up about 1% of Earth’s atmosphere? Argon Click You might want to make a note of argon’s atomic mass. Click

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Calculating Molecular Mass*

Example: N₂

Molecule	Element	# of Atoms (atoms)		Atomic Mass (amu/atom)		Product (amu)
N₂			x		=

Molecule	Element	# of Atoms (atoms)		Atomic Mass (amu/atom)		Product (amu)
N₂	Nitrogen		x		=

Molecule	Element	# of Atoms (atoms)		Atomic Mass (amu/atom)		Product (amu)
N₂	Nitrogen	2	x		=

Molecule	Element	# of Atoms (atoms)		Atomic Mass (amu/atom)		Product (amu)
N₂	Nitrogen	2	x	14.007	=

Molecule	Element	# of Atoms (atoms)		Atomic Mass (amu/atom)		Product (amu)
N₂	Nitrogen	2	x	14.007	=	28.014

Molecule	Element	# of Atoms (atoms)		Atomic Mass (amu/atom)		Product (amu)
N₂	Nitrogen	2	x	14.007	=	28.014
						28.014

*EXTENSION SLIDE: What we covered in the previous slide will help us calculate molecular mass, the mass of a molecule. Can anyone define molecule for the class? Composed of two or more bonded atoms, a molecule is the smallest identifiable group that a pure substance can be divided into and still maintain that substances’ chemical properties. We will calculate the molecular masses of the prominent gas molecules in Earth’s atmosphere. Be sure you take notes and show your work on “page 4” of your worksheet. You can either round atomic mass values to simplify calculations or you can use a calculator. Nitrogen (N₂) will be our example. What element is represented by “N?” Nitrogen Click Now let’s make our calculation of N₂’s molecular mass. Be sure to follow along and to show your work. Which molecule are we working with? Click Which elements are present in N₂? Click How many atoms of each element are present? Click What is the atomic mass of each element present? Click For each element, find the product of the number of atoms multiplied by the atomic mass. Click Finally, sum all of the “products” together to get the molecular mass. Click This calculation was fairly simple since only one element was involved. Click

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Calculating Molecular Mass*

Example: O₂

Molecule	Element	# of Atoms (atoms)		Atomic Mass (amu/atom)		Product (amu)
O₂	Oxygen	2	x	15.999	=	31.998
						31.998

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Calculating Molecular Mass*

Example: CO₂

Molecule	Element	# of Atoms (atoms)		Atomic Mass (amu/atom)		Product (amu)
CO₂			x		=
			x		=

Molecule	Element	# of Atoms (atoms)		Atomic Mass (amu/atom)		Product (amu)
CO₂	Carbon		x		=
	Oxygen		x		=

Molecule	Element	# of Atoms (atoms)		Atomic Mass (amu/atom)		Product (amu)
CO₂	Carbon	1	x		=
	Oxygen	2	x		=

Molecule	Element	# of Atoms (atoms)		Atomic Mass (amu/atom)		Product (amu)
CO₂	Carbon	1	x	12.011	=
	Oxygen	2	x	15.999	=

Molecule	Element	# of Atoms (atoms)		Atomic Mass (amu/atom)		Product (amu)
CO₂	Carbon	1	x	12.011	=	12.011
	Oxygen	2	x	15.999	=	31.998

Molecule	Element	# of Atoms (atoms)		Atomic Mass (amu/atom)		Product (amu)
CO₂	Carbon	1	x	12.011	=	12.011
	Oxygen	2	x	15.999	=	31.998
						44.009

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Calculating Molecular Mass*

Example: H₂O

Molecule	Element	# of Atoms (atoms)		Atomic Mass (amu/atom)		Product (amu)
H₂O	Hydrogen	2	x	1.008	=	2.016
	Oxygen	1	x	15.999	=	15.999
						18.015

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Importance of Molecular Mass

*EXTENSION SLIDE: This is the abstract from a study investigating why CO₂ concentrations can be greater within cave systems. Click Here’s a table from the published journal article resulting from the study. Take a look at the molar/molecular masses (M_mol) for the listed gases. What do you notice? Give students time to draw some conclusions such as: 1) the values for nitrogen, oxygen, and water vapor are the same as what was calculated in class; 2) the value for carbon dioxide is different (we’re correct, this article is wrong); and air has a mass slightly greater than nitrogen. The value for CO₂ is different than the one we calculated! Who’s correct? We are! What mistake did the author make? A value of “6 amu” was used for carbon in the calculation instead of 12.011 amu. Click What is the molecular mass of methane (CH₄)? Around 16 amu. The author confused carbon’s atomic number with its atomic mass!