Thermal Decomposition Reactions

Stable and Unstable Substances

• Stable in Chemistry means unreactive in the conditions stated.

• Unstable means reactive in the conditions stated.

Thermal Decomposition

• When 2 elements combine to form a compound, the product requires heat energy for the elements to be reformed.

• i.e. the compound is more stable than the elements at room temperature.

• Many compounds undergo thermal decomposition – the compound breaks up into compounds of lower mass, or into its elements.

Thermal Decomposition

• The thermal stability of a metal compound is directly related to the chemical activity of the metal.

– Sodium compounds are the most thermally stable.

– Gold compounds do not exist because they are thermally unstable.

Activity Series of Metals

• By observing the reaction of metals with oxygen, acid and water, it is possible to arrange the metals in an order of activity.

• This is the activity series.

Potassium > Sodium > Lithium > Calcium > Magnesium > Aluminium > Zinc > Iron > Lead > (hydrogen) > Copper > Silver > Gold

Metal Carbonates and Metal Hydrogen Carbonates

• The conditions required for the decomposition of metal carbonates and metal hydrogen carbonates depend on the chemical activity of the metal – Sodium and potassium carbonate are thermally stable and can not be decomposed by heating.

Metal Carbonates and Metal Hydrogen Carbonates

• Highly reactive metals form stable compounds that require high energy to undergo thermal decomposition. – Calcium carbonate – the high reactivity of calcium metal results in a stable compound and thus high energy is needed to decompose calcium carbonate.

Thermal Decomposition of Copper Carbonate

• Copper is low in the activity series so, copper carbonate is thermally decomposed with low energy. The equation is:

CuCO 3(s) CuO (s) CO 2(g)

Thermal Decomposition of Copper Carbonate

• Copper carbonate is a green powder, on gentle heating, it decomposes into copper oxide, a black powder and carbon dioxide gas.

Thermal Decomposition of Iron II Hydroxide Fe(OH) 2(s)

Green solid

FeO (s)

Black solid – changes to rust red on contact with air.

H 2 O (g)

Thermal Decomposition of Copper Hydroxide Cu(OH) 2(s)

Blue-Green solid

CuO (s)

Black solid

H 2 O (g)