4.4: Patterns of chemical reactions
General types of chemical reactions
There are several ways to classify chemical reactions. The general types of chemical reactions fall in the categories of combination, decomposition, replacement, and combustion reactions, as illustrated in Fig. 4.4.1.
Combination reactions
A compound is synthesized or formed from two or more substances, e.g.:
\[\ce{C + O2 -> CO2}\nonumber\]
\[\ce{2H2 + O2 -> 2H2O}\nonumber\]
\[\ce{2Mg + O2 -> 2MgO}\nonumber\]
\[\ce{2Na + Cl2 -> 2NaCl}\nonumber\]
\[\ce{CaO + CO2 -> CaCO3}\nonumber\]
Fig. 4.4.2 shows an example of a hydrogen with oxygen combination reaction that is being developed for use as a fuel in the future.
Decomposition reactions
The decomposition reactions are the reverse of the combination reaction, i.e., one compound splits apart into two or more substances, usually by heating, e.g.:
\[\ce{H2CO3 -> H2O + CO2}\nonumber\]
\[\ce{CaCO3 ->[\Delta] CaO + CO2}\nonumber\]
\[\ce{2KClO3 ->[\Delta] 2KCl + 3O2}\nonumber\]
\[\ce{2H2O (l) ->[Electrolyisis] 2H2(g) + O2(g)}\nonumber\]
Fig. 4.4.3 illustrates the last reaction, i.e., decomposition of water through electrolysis.
Replacement or substitution reactions
There are two sub-classes of this category of reactions, i.e., single replacement and double replacement reaction.
Single replacement reactions involve one substance replacing a part of another, e.g.:
\[\ce{Zn(s) + CuCl2(aq) -> ZnCl2(aq) + Cu(s)}\nonumber\]
\[\ce{2Al(s) + 6HCl(aq) -> AlCl3(aq) + 3H2(g)}\nonumber\]
Fig. 4.4.4 shows an example of a single replacement reaction of magnesium resulting in hydrogen gas formation.
Double replacement reactions or metathesis involve the mutual exchange of partners between two substances, e.g. the following precipitation reactions:
\[\ce{NaCl(aq) + AgNO3(aq) -> AgCl(s)(v) + NaNO3(aq)}\nonumber\]
\[\ce{Na2CO3(aq) + CaCl2(aq) -> CaCO3(s)(v) + 2NaCl(aq)}\nonumber\]
Combustion reactions
Combustion is a reaction of a substance with oxygen, often with the formation of flame and release of much heat, e.g.:
\[\ce{C8H16 + 12O2 -> 8CO2 + 8H2O + Heat}\nonumber\]
\[\ce{C + O2 -> CO2 + Heat}\nonumber\]
\[\ce{2H2 + O2 -> 2H2O + Heat}\nonumber\]
\[\ce{2Mg + O2 -> 2MgO + Heat}\nonumber\]
Fig. 4.4.5 shows the above reaction, i.e., combustion of Mg in air.
Usually, combustion is considered as the reaction of a substance containing carbon and hydrogen with oxygen resulting in carbon dioxide, water, flame, and heat, e.g., burning methane on a kitchen stove:
\[\ce{CH4 + 3O2 -> CO2 + 2H2O + Heat}\nonumber\]
Classification of chemical reaction
The chemical reactions are generally classified based on what exchanges during the reaction. These include;
- the transfer of electrons in oxidation-reduction reactions,
- transfer of protons in acid-base reactions, and
- a part of reactants mutually exchanges in precipitation reactions, as described below.
Oxidation-reduction reactions
The oxidation-reduction or redox reaction involves the exchange of electrons. For example, reactions between a metal and nonmetal involve the transfer of electrons from the metal to the nonmetal forming an ionic bond, as shown in Fig. 4.4.6.
Acid-base reactions
The acid-base reactions involve the transfer of protons from an acid to a base, as shown in Fig. 4.4.7.
Precipitation reactions
These are double displacement reactions in water that results in the precipitation of one of the products, as shown in Fig. 4.4.8.
The precipitation reactions and the acid-base reactions are described in the later chapters. The oxidation-reduction reactions are discussed in the following section.