Chemical Reactions in Air
Learning Objectives
Students will be able to:
- Write and balance simple combustion reactions
- Explain how burning fuels creates air pollutants
- Describe how ground-level ozone forms from precursor chemicals
- Connect chemical reactions to indoor and outdoor air quality
Combustion: Burning Fuels
Combustion is a chemical reaction between a fuel and oxygen that releases energy (heat and light).
Complete vs. Incomplete Combustion
Complete Combustion
Plenty of oxygen available
Products: Carbon dioxide and water (cleaner, but CO2 still a greenhouse gas)
Incomplete Combustion
Not enough oxygen
Products: Carbon monoxide (poisonous!) and soot particles (PM)
Where Combustion Happens Indoors
| Source | Fuel | Products Released |
|---|---|---|
| Gas stove | Natural gas (CH4) | CO2, CO, NO2, H2O |
| Gas furnace/heater | Natural gas or propane | CO2, CO (if malfunctioning), NO2 |
| Candles | Wax (hydrocarbons) | CO2, CO, PM2.5, soot |
| Fireplace | Wood | CO2, CO, PM2.5, VOCs |
| Incense | Plant materials | PM2.5, VOCs, CO |
Nitrogen Oxides (NOx)
High-temperature combustion causes nitrogen and oxygen in the air to react:
Sources of NOx
- Car engines
- Gas stoves
- Power plants
- Any high-temp burning
Why it matters
- Respiratory irritant
- Creates ozone (smog)
- Contributes to acid rain
- Triggers asthma
Ozone Formation: The Smog Recipe
Good Ozone vs. Bad Ozone
Stratosphere (good):
Ozone layer protects us from UV radiation - 10-30 miles up
Ground level (bad):
Ozone is a lung irritant - this is smog!
The Ozone Recipe
Ground-level ozone forms when three ingredients combine:
From vehicles, industry
From gasoline, solvents
This is why smog is worst on hot, sunny days! The chemical reaction needs sunlight energy to proceed.
Activity: Balance These Equations
Balance the following combustion reactions:
-
Methane (natural gas):
___CH4 + ___O2 → ___CO2 + ___H2OShow answer
1CH4 + 2O2 → 1CO2 + 2H2O
-
Propane (grill fuel):
___C3H8 + ___O2 → ___CO2 + ___H2OShow answer
1C3H8 + 5O2 → 3CO2 + 4H2O
-
Ethanol (alcohol):
___C2H5OH + ___O2 → ___CO2 + ___H2OShow answer
1C2H5OH + 3O2 → 2CO2 + 3H2O
Real-World Connections
Electric vs. Gas Stoves
Gas stoves produce NO2 and CO from combustion. Studies show homes with gas stoves have 50-400% higher NO2 levels. Electric stoves produce no combustion pollutants.
Ozone Action Days
On hot, sunny days, cities may issue ozone alerts asking people to limit driving and refueling. These activities release NOx and VOCs that form ozone.
Catalytic Converters
Cars use catalytic converters to reduce pollutants. They convert CO to CO2 and break down NOx before exhaust leaves the tailpipe.
Ventilation While Cooking
Running a range hood while cooking with gas removes combustion products before they spread. Opening windows helps too!
Key Takeaway
Combustion reactions power our lives but also create air pollution. When fuels burn, they produce CO2 (greenhouse gas), and potentially CO (toxic), NOx (smog precursor), and PM (respiratory hazard). Understanding these reactions helps us make choices that protect air quality—like using exhaust fans, maintaining appliances, and supporting clean energy.