5.4: Some Types of Chemical Reactions

Skills to Develop

• To classify a given chemical reaction into a variety of types.

Although there are untold millions of possible chemical reactions, most can be classified into a small number of general reaction types. Classifying reactions has two purposes: it helps us to recognize similarities among them, and it enables us to predict the products of certain reactions. A particular reaction may fall into more than one of the categories that we will define in this book.

Combination Reactions

A combination (a.k.a. composition or synthesis) reaction is a chemical reaction that makes a single substance from two or more reactants. There may be more than one molecule of product in the balanced chemical equation, but there is only one substance produced. For example, the equation

\[4Fe + 3O_2 \rightarrow 2Fe_2O_3 \label{Eq1}\]

is a combination reaction that produces Fe₂O₃ from its constituent elements—Fe and O₂. Combination reactions do not have to combine elements, however. The chemical equation

\[Fe_2O_3 + 3SO_3 \rightarrow Fe_2(SO_4)_3 \label{Eq2}\]

shows a combination reaction in which Fe₂O₃ combines with three molecules of SO₃ to make Fe₂(SO₄)₃.

Decomposition Reactions

A decomposition reaction is the reverse of a combination reaction. In a decomposition reaction, a single substance is converted into two or more products. There may be more than one molecule of the reactant, but there is only one substance initially. For example, the equation

\[2NaHCO_{3(s)} \rightarrow Na_2CO_{3(s)} + CO_{2(g)} + H_2O_{(ℓ)} \label{Eq3}\]

is a decomposition reaction that occurs when NaHCO₃ is exposed to heat. Another example is the decomposition of KClO₃:

\[2KClO_{3(s)} \rightarrow 2KCl_{(s)} + 3O_{2(g)} \label{Eq4}\]

This reaction was once commonly used to generate small amounts of oxygen in the chemistry lab.

The decomposition reaction of NaHCO₃ is the reaction that occurs when baking soda is poured on a small kitchen fire. The intent is that the H₂O and CO₂ produced by the decomposition will smother the flames.

Single Replacement Reactions

When an element reacts with a compound to form a new element and a new compound, it is called a single replacement, or sometimes single displacement. Essentially, one element displaces another in the compound. For example, the equation

\[3Al + 3H_2SO_4 \rightarrow 3H_2 + Al_2(SO_4)_3\label{Eq5}\]

is a single replacement in which the element aluminum replaces hydrogen in sulfuric acid, H₂SO₄, to form elemental hydrogen and the new compound, Al₂(SO₄)₃.

Double Replacement Reactions

In a double replacement (a.k.a. double displacement or metathesis) reaction two compounds typically react with each other to produce two different compounds. This type of reaction is often an exchange of positive and negative components of the compounds. Two examples are precipitation and acid-base reactions.

In this double replacement

\[NaCl + AgNO_3 \rightarrow NaNO_3 + AgCl_{(s)}\label{Eq6}\]

the Na⁺ from NaCl and Ag⁺ from AgNO₃ swap partners, or replace each other. At the same time, the Cl^- in NaCl and NO₃^- in AgNO₃ switch and replace each other. An insoluble solid that forms from solution is called a precipitate (AgNO₃ here), hence this is also called a precipitation reaction.

Sulfuric acid, H₂SO₄, and magnesium hydroxide base, Mg(OH)₂, can also participate in a double replacement:

\[H_2SO_4 + Mg(OH)_2 \rightarrow 2H_2O + MgSO_4\label{Eq7}\]

Here the sulfate and hydroxide anions, SO₄^2- and OH^-, trade places while two hydrogen cations, 2 H⁺, replace the magnesium cation, Mg²⁺. We will learn in more detail later how acids (which donate H⁺) can react with the bases (with OH^-) to make water (H₂O) and a salt (MgSO₄ is the formula of Epsom salt, often used in soothing baths). 

Example \(\PageIndex{1}\)

Identify each type of reaction.

1. 2Al(s) + 6HCl(aq) → 2AlCl₃(aq) + 3H₂(g) 
2. (NH₄)₂Cr₂O₇(s) → N₂(g) + Cr₂O₃(s) + 4H₂O(ℓ)
3. 2K(s) + S(s) + 2O₂(g) → K₂SO₄(s)

SOLUTION

1. The element aluminum reacts with the compound hydrochloric acid to form the compound aluminum chloride and the element hydrogen. (Aluminum replaces hydrogen.) This is a single replacement reaction.
2. A single substance reacts to make several products, so we have a decomposition reaction.
3. Multiple reactants are combining to make a single product, so this reaction is a combination reaction.

Summary

There are several recognizable types of chemical reactions: combination, decomposition, and combustion reactions are examples.

Concept Review Exercises

1. How are combination and decomposition reactions similar?
2. How are single replacement and double replacement reactions different?

Answers

1. They are the reverse of each other. In combination, multiple substances make one product (in general: A + B → C). In a decomposition, one substance makes multiple substances (in general: C → B + A).
2. Single replacement reactions must have elements on both sides of the equation, one that becomes a compound and one that is formed from a compound (in general: A + BC → AB + C). Double replacement reactions require more compounds so that two displacements can occur (in general: AB + CD → AD + CB). Other answers are possible.

Exercises

1. Identify each type of reaction.

   1. HCl + NaOH → H2O + NaCl
   2. 2Na + CuBr₂ → 2NaBr + Cu
   3. 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂
   4. C + 2H₂ → CH₄
   5. 2NH₄NO₃(s) → 2N₂(g) + 4H₂O(g) + O₂(g)

Answers

1. 1. double replacement
   2. single replacement
   3. decomposition
   4. combination
   5. decomposition