From Lightning to Lab

Why it matters

Lightning splits the sky, forcing stubborn nitrogen to bond with oxygen. Those oxides wash down in the rain as dilute nitric acid — and once in the soil they become nitrates, the natural fertiliser that plant roots drink up. Every storm quietly feeds the fields.

Predict first

Why does nitrogen react with oxygen only during a lightning strike?

Nitrogen is inert at room temperature due to its triple bond. Lightning provides the extreme heat needed to break this bond.

• Nitrogen is highly reactive and needs a spark to start
• The high temperature provides energy to break the strong N≡N bond — correct
• Oxygen pushes nitrogen into reaction

What you do

Balance the three-step sky-to-acid chain: N₂ + O₂ → 2NO in the lightning bolt, 2NO + O₂ → 2NO₂ as the air cools, then 4NO₂ + 2H₂O + O₂ → 4HNO₃ in the falling rain.

Check yourself

What is the immediate observation when NO reacts with O₂?

2NO + O₂ → 2NO₂. Nitrogen dioxide (NO₂) is a reddish-brown, pungent gas.

Which net equation represents the formation of nitric acid from NO₂ as it dissolves in rainwater in the presence of atmospheric oxygen?

In the atmosphere, NO₂ reacts with water and oxygen to form nitric acid: 4NO₂ + 2H₂O + O₂ → 4HNO₃.

Why is concentrated sulphuric acid used in the lab preparation of nitric acid?

Concentrated H₂SO₄ is non-volatile (high boiling point). It displaces volatile HNO₃ (low boiling point) from solid nitrates.

After a storm, how does the nitrogen 'fixed' by lightning end up helping plants?

Plants cannot use the inert N₂ in the air. Lightning fixes it: N₂ → NO → NO₂ → nitric acid → nitrates in the soil, which roots take up to build proteins. Lightning is a natural nitrogen-fixer.

• A colourless gas remains
• A reddish-brown gas appears — correct
• A white precipitate forms
• 3NO₂ + H₂O → 2HNO₃ + NO
• 4NO₂ + 2H₂O + O₂ → 4HNO₃ — correct
• NO₂ + H₂O → HNO₃
• It is a stronger acid than nitric acid
• It is non-volatile and displaces volatile nitric acid from nitrates — correct
• It acts as a catalyst
• The nitric acid in the rain forms nitrates in the soil, which roots absorb as nutrients — correct
• Plants breathe in N₂ gas directly through their leaves
• The lightning heats the soil and makes seeds sprout
• Acid rain only harms plants and gives no benefit