Thermal decomposition

1. Thermal decomposition

2. Reaction of breaking down compounds (carbonates, nitrates or hydroxides) by heating them is called thermal decomposition. CaCO3(s) → CaO(s) + CO2(g) Thermal decomposition of metal hydroxides • Most metal hydroxides decompose when heated, forming oxide and water. Zn(OH)2(s) → ZnO(s) + H2O(g) • All Group II hydroxides decompose in similar way. • But, most of Group I alkali metal hydroxides do not decompose, except lithium hydroxide! 2LiOH(s) → Li2O(s) + H2O(g) • Lithium is the least reactive of the alkali metals. • Thermal decomposition depend on the reactivity of the metal.

3. Thermal decomposition of nitrates • Nitrates are decomposing when heated, but there are some differences in how they decompose. • The alkali metal nitrates decompose to form a nitrite and oxygen. 2KNO3(s) → 2KNO2(s) + O2(g) Potassium nitrate → potassium nitrite + oxygen • Nitrates of other metals decompose on heating to form an oxide, nitrogen dioxide and oxygen. 2Mg(NO3)2(s) → 2MgO(s) + 4NO2(g) + O2(g) • Lithium nitrate, a compound of the least reactive alkali metal, decomposes in this way too.

4. Nitrates of very unreactive metals, such as silver (Ag), decompose to form the metal when they are heated: 2AgNO3(s) → 2Ag(s) + 2NO2(s) + O2(g) • The more reactive the metal, the more stable to thermal decomposition its compound is. • This is well demonstrated if we look at the temperatures at which carbonates decompose when heated. Thermal decomposition of carbonates • The table shows temperatures at which the Group II carbonates decompose. • The reactivity down the Group II increases, and as we go down the group it gets more difficult to decompose the carbonates. • The more reactive the metal, the more stable its nitrate, carbonate or hydroxide.