Types of Chemical Reaction

Types of chemical reactions

Chemical reactions can be categorized into the following types:

i. Decomposition reactions

A decomposition reaction occurs when a large molecule (compound) breaks down into simpler molecules or elements. These reactions can be influenced by heat or catalysts.

Types of decomposition reactions:

a) Decomposition by heating (thermal decomposition)

• Thermal decomposition reactions are irreversible.

Example:

CaCO3(s) → CaO(s) + CO2(g)

b) Thermal dissociation

• Thermal dissociation is a type of decomposition reaction that is reversible.

Example:

NH4Cl(s) ⇌ NH3(g) + HCl(g)

c) Decomposition by catalysts

Some decomposition reactions are influenced by the presence of a catalyst.

Example: Decomposition of hydrogen peroxide:

2H2O2(aq) → 2H2O(l) + O2(g)

ii. Synthesis (combination) reactions

In a synthesis reaction, two or more simpler substances combine to form a single compound. This is the opposite of decomposition.

Example: Formation of water:

2H2(g) + O2(g) → 2H2O(l)

iii. Displacement reactions

A displacement reaction occurs when one element replaces another element in a compound.

Example: Reaction of zinc with hydrochloric acid:

Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)

iv. Double decomposition reactions

In a double decomposition reaction, two compounds exchange their radicals (ions) to form new compounds. For this reaction to occur:

• The reactants must be soluble in water.
• One of the products must be a solid (precipitate), gas, or liquid.

Example: Reaction of silver nitrate with sodium chloride:

AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)

v. Neutralization reactions

A neutralization reaction occurs when an acid reacts with a base to produce a salt and water.

Example: Reaction of hydrochloric acid with sodium hydroxide:

HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

Additional notes

• Bases vs. alkalis:
  • A base is any substance that neutralizes an acid.
  • An alkali is a base that dissolves in water to form hydroxide ions ($OH^-$).
  • Not all bases are alkalis, but all alkalis are bases.