Action of Heat of Materials

Some compounds are not stable to heat, and upon heating these compounds can decompose to give a number of products.

When analysing the stability of different materials it is best to keep in mind these 7 rules:

  1. All hydrogen carbonates decompose to give the carbonate, carbon dioxide and water.
  2. All group I carbonate are stable.
  3. All group II carbonates decompose to give the oxide and carbon dioxide.
  4. All group I hydroxides are stable.
  5. All group II hydroxides decompose to give the oxide and water.
  6. All group I nitrates decompose to give the nitrite and oxygen.
  7. All group II nitrates decompose to give the oxide, nitrogen dioxide and oxygen.

Hydroxides

It must be noted that all group 1 hydroxides are stable while all the other metal hydroxides are unstable and would give water and the metal oxide.

Ca(OH)_2 \rightarrow CaO + H_2O

Hydrogen Carbonates

All Hydrogen Carbonates decompose on heating due to the act that these have got an extra water molecule trapped in them. These would give CO2, H2O and a carbonate.

2NaHCO_3 \rightarrow Na_2CO_3 + CO_2 + H_2O

Ca(HCO_3)_2 \rightarrow CaCO_3 + H_2O + CO_2

Carbonates

More over group 2 metals onwards do not even have got stable carbonates, and these would decompose on heating to form an oxide and carbon dioxide.

CaCO_3 \rightarrow CaO + CO_2

Nitrates

Nitrates are the most complex group, since all nitrates decompose, but not all groups decompose in the same way. Group one metals decompose to produce nitrites and oxygen while group 2 metals would decompose to produce the oxide together with oxygen and nitrogen dioxide.

Group 1 Nitrates

2NaNO_3 \rightarrow 2NaNO_2 + O_2

Group 2 Nitrates

2Ca(NO_3)_2 \rightarrow 2CaO + 4NO_2 + O_2

2Mg(NO_3)_2 \rightarrow 2MgO + 4NO_2 + O_2

 

Hydrates

Another important group are the hydrates, which are further discussed in the chapter of water. This group covers the water of crystallization, where the water molecules would be trapped in between bigger and larger molecules. this can be removed by heating the compound above 100oC where water would boil off but most of these compounds can easily revert to the hydrated state by taking in water either from the atmosphere or from solution.