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Activity

Molecular modelling of combustion

Summary: 
Use molecule models to figure out the chemical reaction of combustion.
Science content (2016 curriculum): 
Chemistry: Atoms, Molecules (3-7)
Chemistry: Chemical Changes (2, 7)
Physics: Energy forms, Conservation of Energy (1, 3, 4, 5)
Lessons activity is in: 
Materials: 
    molecule pieces for combustion of ethanol:
  • ethanol molecule: two black carbons, one red oxygen, six white hydrogens, eight grey bonds
  • three oxygen molecules: each two red oxygens, 2 grey bonds
  • molecule pieces for combustion of hydrocarbon:

  • methane molecule: one black carbon, four white hydrogens, four grey bonds
  • two oxygen molecules: each two red oxygens, 2 grey bonds
Procedure: 

Either build the starting molecules, or ask students to make them: one ethanol molecule and three oxygen molecules.

Tell/remind students that ethanol is a fuel that burns in oxygen, and that when the fuel and oxygen chemically react their atoms come apart and recombine into new molecules.
One of those new molecules is water (H2O), and it makes three of them.
Ask students to pull apart their ethanol and oxygen molecules, and rearrange them to make three water molecules.
Then tell them that the remaining atoms and bonds make the other reaction product. There are two identical molecules of this second product.
Give them time to use up all the remaining atoms and bonds to make two identical molecules. With time, they should arrive at CO2, or carbon dioxide.

So when ethanol (or other fuels) burn in oxygen the reaction products are water and carbon dioxide.
Sometimes carbon (as soot) is also made, when not all the carbon combines with oxygen (called incomplete combustion).

Additional information on the flames that are often present during combustion:
The flame is a mixture of hot gases, primarily CO2, water vapour, oxygen and nitrogen (nitrogen comes from other materials that burn).
Energy in the flame excites the electrons in some of the transient reaction intermediates such as the Methylidyne radical (CH) and Diatomic carbon (C2), which results in the emission of visible light (blue and green for these radicals) as these substances release their excess energy.
https://en.wikipedia.org/wiki/Flame

Grades taught: 
Gr 4
Teacher: 
Ingrid
Teaching site: 
ingridscience afterschool
Activity originally developed and delivered: 

ingridscience afterschool