What Happens When Zinc Nitrate Is Strongly Heated? — Equation and Observations

Balanced equation: 2Zn(NO₃)₂ → 2ZnO + 4NO₂ + O₂

Word equation: Zinc nitrate → Zinc oxide + Nitrogen dioxide + Oxygen

Observations:

1. Brown fumes are produced — nitrogen dioxide (NO₂) gas.
2. A white/pale yellow solid remains — zinc oxide (ZnO). (ZnO is white when cold, yellow when hot — thermochromic)
3. A gas that relights a glowing splint is produced — oxygen (O₂).
4. The solid mass decreases as gases are evolved.

Type of reaction: Thermal decomposition (endothermic — requires heat to proceed).

1. Zinc Oxide (ZnO): • White powder at room temperature, yellow when hot (thermochromic property) • Amphoteric oxide — reacts with both acids and bases • With acid: ZnO + H₂SO₄ → ZnSO₄ + H₂O • With base: ZnO + 2NaOH → Na₂ZnO₂ + H₂O

2. Nitrogen Dioxide (NO₂): • Brown/reddish-brown coloured gas • Pungent, choking smell • Toxic gas • An acidic oxide — dissolves in water: 3NO₂ + H₂O → 2HNO₃ + NO

3. Oxygen (O₂): • Colourless, odourless gas • Supports combustion — relights a glowing splint • Produced as a byproduct of nitrate decomposition

Metal nitrates decompose on heating based on the position of the metal in the activity series:

1. Very reactive metals (above Mg) — nitrates give metal nitrite + O₂: 2KNO₃ → 2KNO₂ + O₂

2. Moderately reactive metals (Mg to Cu) — nitrates give metal oxide + NO₂ + O₂: 2Zn(NO₃)₂ → 2ZnO + 4NO₂ + O₂ 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂ 2Cu(NO₃)₂ → 2CuO + 4NO₂ + O₂

3. Less reactive metals (below Cu — Ag, Au) — nitrates give the metal itself + NO₂ + O₂: 2AgNO₃ → 2Ag + 2NO₂ + O₂

Zinc falls in the middle category (moderately reactive) — hence gives ZnO + NO₂ + O₂.