Summary

In this lesson, students observe several combustion reactions and will identify some common chemical species involved. They are then led through some hints for writing and balancing chemical equations for combustion reactions.

Grade Level

High School

NGSS Alignment

This lesson will help prepare your students to meet the performance expectations in the following standards:

• HS-PS1-7: Use mathematical representation to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.
• Scientific and Engineering Practices:
  • Using Mathematics and Computational Thinking
  • Developing and Using Models

Objectives

By the end of this lesson, students should be able to:

• Identify the common species in combustion reactions.
• Describe different kinds of fuels for combustion reactions.
• Write and balance chemical equations for combustion reactions.

Chemistry Topics

This lesson supports students’ understanding of:

• Combustion
• Balancing equations

Time

Teacher Preparation: 15 minutes, to gather supplies

Lesson: 45 minutes, with additional time needed for potential extensions

Materials (for 1 class)

• 3-4 wood splints (1 for demo, and the rest for potential discussion)
• Clay or play dough, about the size of a golf ball, to support the wood splint
• 1 candle (any type, tealights work well), supported in candle stick or other
• 1 glass cup or jar, to extinguish the candle (must be taller than the candle)
• 1 lab burner with tubing and gas source (natural gas or propane are typical)
• Matches or lighter, for the wood and the candle
• Flint striker, for the lab burner

Safety

• Students and teacher should wear proper safety gear during chemistry demonstrations. Safety goggles and lab apron are required.
• Always use caution around open flames. Keep flames away from flammable substances.
• Always be aware of an open flame. Do not reach over it, tie back hair, and secure loose clothing.
• Open flames can cause burns. Liquid wax is hot and can burn the skin.
• When lighting the match and wooden splint, be cautious with the flame.
• An operational fire extinguisher should be in the classroom.

Teacher Notes

• This lesson is intended to be a first introduction to combustion reactions and their equations.
• Students should be familiar with writing and balancing equations for other types of reactions, such as synthesis, decomposition, single replacement, and double replacement reactions.
• True/False questions on the student handout are based on the facts listed on the Western Fire Chiefs Association webpage Wildfire Facts for Kids.
  • Several of the statements are much more meaningful with a little more information than true/false (see answer key for examples). If time allows, the link above can be provided to students so they can find their own answers before the teacher conducts a review. In different parts of the country, students will be differently knowledgeable about wildfires, so how this section is addressed is left to the discretion of the teacher.
• While reviewing the wildfire statements, use a fire map, such as this map from the Western Fire Chiefs, to show students the number of wildfires occurring in the US and Canada at the time of viewing. Some maps have different filters to explore. This can be used, depending on the teacher’s lesson goals, to spark conversation about topics such as:
  • Conditions that promote or inhibit fires
  • Reasons some fires are set intentionally
  • Importance of forests – in US and around the world
  • The nature of smoke from wildfires
  • Climate change topics like deforestation, forests as carbon sinks, carbon cycle, air pollution
  • How people who live in fire-prone areas protect themselves
• Encourage students to consider trying to explain fire, or wildfires, and to think of specific questions they could ask to find out more information.
• Demonstration – Once the class has discussed the nature of wildfires, tell students that they will spend a few minute observing fires from different sources. Direct them to pay close attention to differences in what the fires look like and in how they start for each of the different sources.
• There are three substances used to demonstrate how different things burn:
  • Wood
    • Wood splint (or a craft stick can be used) – secure the splint in a metal clamp or in a clump of Play-Doh. (Play-Doh is manufactured to be non-flammable. The safest option is to make a ball of it, drop it onto a surface to allow the bottom to flatten, then use a finger to press a depression in the middle of the top. Pour a small amount of water into the depression so the flame will extinguish when it reaches the bottom of the splint. Then, after lighting the splint, poke it into the middle of the depression while students observe the flame.)
    • Light the splint with a match or lighter. A splint will burn down within a few seconds. A craft stick will take a bit longer.
    • If needed, extinguish by dipping the flame in water.
  • Candle
    • Any type of candle is fine. Tealights work well as they do not need a holder and are low to the table and easy to cover with a glass cup or jar.
    • Light the candle with a match or lighter.
    • Extinguish by placing a clear glass cup or jar upside down to cut off the air.
    • This is as simple as lighting the candle, but additional discussion concepts and ideas are included in the AACT resource Observing a Candle.
  • Lab burner
    • Burners may be fed through a natural gas line or using external tanks.
    • Ignite using a spark from a flint striker.
    • Extinguish by cutting off the gas flow.
• Follow-up to demonstrations
  • Students should complete questions 1-4 on their own.
  • Lead a brief discussion to highlight how the flames differed.
  • Provide the chemical formulas for each of the sources and then direct students to answer questions 5-7.
• Review student answers, then reveal that there are two products that are common to all combustion reactions: H₂O and CO₂.
  • If combustion is perfect, then only H₂O and CO₂ are formed, but combustion is rarely perfect. This introductory resource focuses on complete combustion, but an extension to the lesson could have students research products of incomplete combustion, such as carbon monoxide (CO), carbon (soot), smaller pieces of carbohydrate molecules, sulfur dioxide (SO₂) from fuel impurities, and nitrogen oxides from reactions with nitrogen in the air. For more, see Combustion of hydrocarbon fuels and Atmospheric pollutants.
• Guide students through balancing the combustion of methane (on the worksheet). Use this to point out how, in combustion reactions, atoms of an element can be part of several reactants and products, so this can complicate balancing. Show them that only the fuel changes for different combustion reactions when considering complete combustions.
  • The candle combustion reaction can be used for extra guided or independent practice.
  • Three additional skeleton equations are included for practice.
• Guide students through writing and balancing equations for combustion of the more complicated formulas for alcohol and wood. There is a downloadable Tips sheet available in the sidebar that provides pointers for these more complicated formulas, which can be distributed to students if desired.
  • Additional fuels are included for practice writing and balancing combustion equations.
• The challenge questions can be used in several ways, including as small-group discussion topics or as homework to be discussed during the following class session. They could also be used as prompts for an additional teacher demonstration:
  • Using crucible tongs, hold the bottom of a clean evaporating dish in the flame from burning wood for about 20 seconds.
  • Using a different dish, do the same with the blue flame of a Bunsen burner.
  • Show students that the wood fire left black residue, but the blue flame did not.
    • The jumpier, yellow-orange flame is a “cooler” flame and the combustion is much less complete than is possible with hotter flames.
    • The black residue is “soot,” or unburned carbon and small carbon compounds that didn’t completely oxidize into CO₂. This incomplete combustion is typical of most fires involving any substance more complex than simple hydrocarbon fuels.
    • The yellow-orange flame color can be compared to flame tests, as the flame test for carbon and small carbon compounds would be yellow-orange. When heated, but not enough to burn, carbon particles will glow with this characteristic color.
    • “Fire” is the heat and light produced when fuel particles are oxidized, and what we see as fire is often composed of matter that is “on fire”. The wood flame, for example, is composed of the heat and light from many small hydrocarbon products of the incomplete combustion that are within the volume of the flame and glowing as a result of being hot.
• Additional resources to help further teach this topic:
  • Video: 3 Trees, 3 Conditions, 2 Fires – an ACS Reactions video addressing an internet rumor about preventing Christmas trees from burning.
  • Video: The Internal Combustion Engine – an AACT video showing how combustion reactions are used in automobile engines.
  • AACT classroom resources on classification and balancing of chemical reactions:
    • Lesson Plan: Counting Atoms and Balancing Equations
    • Activity: Balancing Equations with Note Cards
    • Activity: Partner Work: Classifying and Balancing Chemical Reactions
    • Animation: Classifying Chemical Reactions
    • Lab: Classifying Reaction Types
  • Article: Exploding Cabin – a ChemMatters article about the methane explosion of a cabin.