5.4 Some Types of Chemical Reactions

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.

A combination (composition) 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 + 3O2 → 2Fe2O3

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

Fe2O3 + 3SO3 → Fe2(SO4)3

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

Example 3

Which equations are combination reactions?

  1. Co(s) + Cl2(g) → CoCl2(s)
  2. CO(g) + Cl2(g) → COCl2(g)
  3. N2H4(ℓ) + O2(g) → N2(g) + 2H2O(ℓ)

Skill-Building Exercise

Which equations are combination reactions?

  1. P4(s) + 6Cl2(g) → 4PCl3(g)

  2. SO3(ℓ) + H2O(ℓ) → H2SO4(ℓ)

  3. NaOH(s) + HCl(g) → NaCl(s) + H2O(ℓ)

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

2NaHCO3(s) → Na2CO3(s) + CO2(g) + H2O(ℓ)

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

2KClO3(s) → 2KCl(s) + 3O2(g)

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

Note

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.

A combustion reaction occurs when a substance combines with molecular oxygen to make oxygen-containing compounds of other elements in the reaction. One example is the burning of acetylene (C2H2) in torches:

2C2H2 + 5O2 → 4CO2 + 2H2O

Oxygen (in its elemental form) is a crucial reactant in combustion reactions, and it is also present in the products.

Note

Energy in the form of heat is usually given off as a product in a combustion reaction as well.

Example 4

Identify each type of reaction.

  1. 2K(s) + S(s) + 2O2(g) → K2SO4(s)
  2. (NH4)2Cr2O7(s) → N2(g) + Cr2O3(s) + 4H2O(ℓ)
  3. CH4(g) + 2O2(g) → CO2(g) + 2H2O(ℓ)

Skill-Building Exercise

Identify each type of reaction.

  1. C2H5OH + 3O2 → 2CO2 + 3H2O

  2. 2Ca(s) + O2(g) → 2CaO(s)

  3. CaCO3(s) → CaO(s) + CO2(g)

Concept Review Exercises

  1. What is the difference between a combination reaction and a combustion reaction?
  2. Give the distinguishing characteristic(s) of a decomposition reaction
  3. How do we recognize a combustion reaction?

Key Takeaway

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

Exercises

  1. Identify each type of reaction.

    1. C6H5CH3 + 9O2 → 7CO2 + 4H2O
    2. 2NaHCO3 → Na2CO3 + H2O + CO2
    3. C + 2H2 → CH4

  2. Identify each type of reaction.

    1. P4O10 + 6H2O → 4H3PO4
    2. FeO + SO3 → FeSO4
    3. CaCO3(s) → CO2(g) + CaO(s)

  3. Identify each type of reaction.

    1. 2NH4NO3(s) → 2N2(g) + 4H2O(g) + O2(g)
    2. Hg(ℓ) + ½O2 (g) → HgO(s)
    3. CH2CH2(g) + Br2(ℓ) → CH2BrCH2Br

  4. Identify each type of reaction.

    1. Ti(s) + O2(g) → TiO2(s)
    2. H2SO3(aq) → H2O(ℓ) + SO2(g)
    3. 3O2(g) → 2O3(g)