NO to NO2: The Brown Gas
Oxidation of nitric oxide to nitrogen dioxide
What this lesson covers
Why it matters
A colourless gas from car exhaust meets the air and instantly turns into a choking, reddish-brown haze. This is the birth of smog.
The idea in plain words
Count the atoms: ensure the 2:1 ratio of NO to O2 produces exactly two NO2 molecules, keeping mass conserved.
Predict first
What happens when colourless NO gas reacts with atmospheric oxygen?
Nitrogen monoxide is unstable in air; it oxidizes rapidly to form nitrogen dioxide, a toxic reddish-brown gas.
- It forms a colourless, odourless gas
- It turns into a reddish-brown, pungent gas — correct
- It produces white smoke
- Nothing happens; NO is stable in air
What you do
Balance 2NO + O₂ → 2NO₂ — adjust the coefficients until every atom is accounted for, and watch the brown colour appear as you succeed.
Check yourself
Which equation correctly represents the formation of nitrogen dioxide?
The balanced equation is 2NO + O₂ → 2NO₂. Two molecules of NO react with one molecule of O2 to form two molecules of NO2.
Why is nitrogen dioxide considered harmful in the atmosphere?
NO2 is a reddish-brown, pungent gas that is toxic to humans and a key component of photochemical smog.
What type of reaction is 2NO + O2 → 2NO2?
Two substances (NO and O2) combine to form a single new substance (NO2), making it a combination reaction.
- NO + O2 → NO2
- 2NO + O2 → 2NO2 — correct
- N2 + O2 → 2NO
- 2NO2 → 2NO + O2
- It is a greenhouse gas that traps heat
- It is a pungent, toxic gas that irritates lungs and contributes to smog — correct
- It depletes the ozone layer directly
- It is highly flammable
- Decomposition
- Displacement
- Combination — correct
- Neutralization