Chemical reactions are classified into four categories. These are combination, decomposition, single displacement and double displacement reactions.

Combination Reactions

A reaction in which two or more substances combine to form a single substance is called a combination reaction. In a combination reaction, two elements, two compounds, or an element and a compound react to form a single compound. Combination reactions can be represented by the following general form of equation.

A + B → AB

where the reactants A and B are elements or compounds, the product AB is a compound. Such type of reaction is also known as synthesis or composition reaction.

Decomposition reactions

A decomposition reaction is a reaction that involves the breaking down of a single compound into two or more elements or simpler compounds.

A decomposition reaction can be carried out using heat, light, electricity or a catalyst. But most decomposition reactions are carried out when heat is supplied and this heat energy is indicated by a ‘delta’ (Δ) symbol above the arrow.

The general form of equation for a decomposition reaction is:

AB → A + B

where the reactant AB must be a compound and the products A and B could be elements or compounds.

Single displacement reactions

A reaction in which one element displaces another element from its compound is known as single displacement or replacement reaction. Such a reaction is represented by the following two general forms.

A + BC → B + AC

If A is a metal, it will displace B to form AC, provided A is a more active metal than B.

A + BC → BA + C

If A is a non-metal, it will displace C to form BA, provided A is a more active non-metal than C. In general, a more reactive element displaces a less reactive element from a compound.

Examples of single-displacement reactions

(a) Active metals displace hydrogen from acids.

Reactive metals such as potassium, calcium, sodium, and zinc displace hydrogen gas from dilute acids.

For example, zinc is an active metal, and it displaces hydrogen from hydrochloric acid; but copper metal cannot do so.

Zn + 2HCl ZnCl₂ + H₂
Au + HCl No reaction

(b) Reactive metals, such as potassium, calcium, and sodium react vigorously with water to displace hydrogen:

2Na + 2H₂O 2NaOH + H₂
Ca + 2H₂O Ca(OH)₂ + H₂

Double Displacement Reactions

Double displacement reaction is a reaction in which two compounds react together to form two new compounds by exchange of the positive and negative ions of each reactant. Such a reaction is also known as double replacement reaction or metathesis.

AB + CD → AD + CB

Combustion reactions

Combustion reactions must involve O₂ as one reactant. The combustion of hydrogen gas produces water vapor:

2H₂ (g) + O₂ (g) 2H₂O (g)

Notice that this reaction also qualifies as a combination reaction.

Propane (C₃H₈) is a gaseous hydrocarbon that is commonly used as the fuel source in gas grills.

C₃H₈ (g) + 5O₂ (g) → 3CO₂ (g) + 4H₂O (g)

Oxidation-reduction reactions

An oxidation–reduction or redox reaction is a reaction that involves the transfer of electrons between chemical species (the atoms, ions, or molecules involved in the reaction). Detail explanation on oxidation reduction reactions will be given in the next unit.