Problem #23: Types of Chemical Reactions [Solved]

In previous problems, we have seen what a chemical reaction is and the conditions and factors that affect a chemical reaction. There are many kinds of chemical reactions that can be classified in a few categories or types of chemical reactions.

Question: Name the main categories or types of chemical reactions you know. Give a brief description or explanation for each category/type and for each category/type,  illustrate by an example.

Solution:

Chemical reactions can be classified in many categories; hereafter are one of the most common classifications: Combination, Decomposition, Simple Substitution, Double Substitution, and Combustion.

• Combination Reaction, also called Synthesis: A + B → C
  This is when two or more simple substances combine to form one new substance (more complex).

Ex: 2H₂(g) + O₂(g) → 2H₂O(g)

• Decomposition Reaction: AB → C + D
  When one substance decomposes into two or more simpler substances.

Ex: CaCO₃(s) + heating → CaO(s) + CO₂(g)

• Simple Substitution or Simple Replacement reaction: C + AB → CB + A
  One element replaces another element in a compound.

Ex: 2Na (s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)

Here sodium ion, Na⁺, replaces hydrogen ion, H⁺, in the molecule of water.
This example is also called an Oxidation-Reduction reaction, where one chemical species is oxidized or loses an electron (Na → Na⁺ + e), and at the same time, another chemical species is reduced or gains an electron (2H⁺ + 2e → H₂)

• Double substitution or double replacement reaction: AB + CD → AC + BD
  Involves two elements exchanging places in a reaction.

Ex: HCl(aq) + NaOH(aq) → NaCl(aq) + HOH/H₂O(l)
NaCl(aq) + AgNO₃(aq) → NaNO₃(aq) + AgCl(s)

In the first example, hydrogen and sodium ions exchange places.
This example is also called an acid-base neutralization reaction.

In the second example, sodium and silver ions exchange places. Because there is formation of a precipitate, that kind of reaction is also called precipitation reaction.

• Combustion Reaction: A + O₂(g) → Oxide + Flame
  Reactions with Oxygen accompanied by the production of heat and flame are called combustion reactions. The main product in the combustion reaction is the oxide(s), sometimes plus water if the compound A contains hydrogen atoms in its molecule.

Ex: C(s) + O₂(g) → CO₂(g) + Flame
C₂H₄(g) + 3O₂(g) → 2CO₂(g) + 2H₂O(g) + Flame

But, as already said, some of the above reactions are given other categorization names:

• Acid-Base Neutralization Reaction: reaction between an acid and a base; it is a double substitution, except that it involves specifically an acid and a base.

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

• Addition Reaction in Organic Chemistry: is a combination/synthesis reaction:

H₂C=CH₂ + H₂ → H₃C-CH₃

• Oxidation-Reduction Reaction (Redox): Redox reactions constitute a very important category of chemical reactions, although they are sometimes described, in Organic Chemistry and Inorganic Chemistry, in two ways that may appear different (but without being really different).

The general definition of a Redox Reaction is: a Redox Reaction is a reaction where there is exchange of electron(s) between reagents; in other words, one reagent has its oxidation state increasing, i.e. it is oxidized and is called reducing agent, whereas the other reagent has its oxidation state decreasing, i.e. it is reduced and is called oxidizing agent. This type of reaction can be found in one or another previous categories above.

Ex:
Zn⁰(s) + Cu²⁺Cl₂(aq) → Zn²⁺Cl₂(aq) + Cu⁰(s) (simple replacement)
2H₂⁰(g) + O₂⁰(g) → 2H²⁺O^2-(g) (combination/synthesis, combustion)
C^4-H+₄(g) + 2O⁰₂(g) → C⁴⁺O^2-₂(g) + 2H⁺₂O^2-(g) (combustion)

N.B: The above categories of chemical reactions are the categories found in Chemistry in general; but in Organic Chemistry, there are other sub-categories specific to that branch of Chemistry.