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.