This is "Unit 7", section 7.2 from the book General Chemistry (v. 1.0).

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7.2 Classifying Chemical Reactions

Learning Objectives

  1. To identify fundamental types of chemical reactions.
  2. To predict the types of reactions substances will undergo.

The chemical reactions we have described are only a tiny sampling of the infinite number of chemical reactions possible. How do chemists cope with this overwhelming diversity? How do they predict which compounds will react with one another and what products will be formed? The key to success is to find useful ways to categorize reactions. Familiarity with a few basic types of reactions will help you to predict the products that form when certain kinds of compounds or elements come in contact.

Most chemical reactions can be classified into one or more of three basic types: acid–base neutralization reactionsA reaction of the general form acid + base → salt., precipitation reactionsA chemical reaction that has the general form AB + C → AC + B or AB + CD → AD + CB., and oxidation–reduction reactionsA chemical reaction that exhibits a change in the oxidation states of one or more elements in the reactants that has the general form oxidant + reductant → reduced oxidant + oxidized reductant.. The general forms of these three kinds of reactions are summarized in Table 7.2(1) , along with examples of each. It is important to note, however, that many reactions can be assigned to more than one classification, as you will see in our discussion. The classification scheme is only for convenience; the same reaction can be classified in different ways, depending on which of its characteristics is most important. In this section our purpose is to distinguish the three types of chemical change. In future sections we will dive into the specific reaction in more detail. Precipitation reactions that involve the the formation of an insoluble salt such as barium sulfate when solutions of two soluble salts are mixed together—will be discussed in the next section of this chapter. Acid-base neutralization reactions and oxidation–reduction reactions are discussed in chapter 8.

Table 7.2(1) Classifications of Chemical Reactions

Name of Reaction General Form Example(s)
oxidation–reduction (redox) oxidant + reductant → reduced oxidant + oxidized reductant C7H16(l) + 11O2(g) → 7CO2(g) + 8H2O(g)
acid–base acid + base → salt NH3(aq) + HNO3(aq) → NH4+(aq) + NO3−(aq)
precipitation
AB + CD → AD + CB BaCl2(aq) + Na2SO4(aq) → BaSO4(s) + 2NaCl(aq)

Oxidation–Reduction Reactions

The term oxidationThe loss of one or more electrons in a chemical reaction. The substance that loses electrons is said to be oxidized. was first used to describe reactions in which metals react with oxygen in air to produce metal oxides. When iron is exposed to air in the presence of water, for example, the iron turns to rust—an iron oxide. When exposed to air, aluminum metal develops a continuous, coherent, transparent layer of aluminum oxide on its surface. In both cases, the metal acquires a positive charge by transferring electrons to the neutral oxygen atoms of an oxygen molecule. As a result, the oxygen atoms acquire a negative charge and form oxide ions (O2−). Because the metals have lost electrons to oxygen, they have been oxidized; oxidation is therefore the loss of electrons. Conversely, because the oxygen atoms have gained electrons, they have been reduced, so reduction is the gain of electrons. For every oxidation, there must be an associated reduction. It is this transfer of electrons that distinguishes an oxidation-reduction reaction. Oftentimes, you can spot an oxidation-reduction reaction when an elemental form is a reactant or product.

4Fe + 3O2 → 2Fe2O3

Acid-Base Neutralization Reactions

In chapter 5, section 5.2, we learned the names of a number of substances that are acids and a number of substances that are bases. For example, we learned the name of HNO3 is nitric acid. We also learned that NaOH is sodium hydroxide and it is a base. When an acid and a base are listed on the left-hand side of the reaction arrow, this reaction can be classified as an acid-base neutralization reaction.

HNO3 + NaOH → NaNO3 + HOH

As one progresses through a sequence of courses in the discipline of chemistry, one finds the definitions of acid and base can be greatly expanded.

Precipitation Reactions

When aqueous solutions of some salts are mixed, there is the possiblity that the cation of one salt combines with the anion of the other salt to form a substance that does not dissolve in water. This substance that does not dissolve is called the precipitate, or the insoluble substance. In the case of the formation of a precipitate, we call the reaction a precipitation reaction. In the next section we will talk about how we know if a precipitate can form. For now, just be able to distinguish a precipitation reaction by recognizing precipitation requires two aqueous solutions combining to form a solid substance.

3Ca(NO3)2(aq) + 2Na3PO4(aq) → Ca3(PO4)2(s) + 6NaNO3(aq)

Note the Pattern

Practically every chemical change can be classified as oxidation-reduction, acid-base or precipitation.

Example 7.2-1

Classify each reaction as an oxidation–reduction reaction, an acid–base reaction, or a precipitation reaction.

  1. MnSO4(aq) + (NH4)2S(aq) → MnS(s) + (NH4)2SO4(aq)
  2. AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)
  3. CaCO3(s) + 2HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l)

    Video: Powdered calcium carbonate is reacted with a solution of hydrochloric acid in a test tube. Video Credit: Part of NCSSM CORE collection: http://www.dlt.ncssm. Please attribute this work as being created by the North Carolina School of Science and Mathematics. This work is licensed under Creative Commons CC-BY https://creativecommons.org/licenses/by/3.0/ via YouTube

  4. 2Ca(s) + O2(g) → 2CaO(s)
  5. Pb(s) + PbO2(s) + 2H2SO4(aq) → 2PbSO4(s) + 2H2O(l)

Given: balanced chemical equation

Asked for: classification of chemical reaction

Strategy:

Look for acids and bases, elemental forms and precipitates, then classify the reaction.

Solution:

  1. Two aqueous solutions, one of manganese(II) sulfate and the other of ammonium sulfide, are reacting to form a solid substance, manganese(II) sulfide. Therefore, this reaction is a precipitation reaction.

  2. Two aqueous solutions, one of silver nitrate and the other of sodium chloride, are reacting to form a solid substance, silver chloride. Therefore, this reaction is a precipitation reaction.

    AgCl(s) precipitates when solutions of AgNO3(aq) and NaCl(aq) are mixed. NaNO3 (aq) is in solution as Na+ and NO3− ions. Image Credit: By Luisbrudna (Own work) [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons

  3. Like all carbonates, calcium carbonate is a base. Hydrochloric acid is not surprisingly an acid. Therefore, this reaction is an acid-base neutralization reaction.

    Bubbles of carbon dioxide form on limestone (calcium carbonate) when tested with a hydrochloric acid solution. Image Credit: By Alessandro e Damiano (Own work) [CC BY 4.0 (http://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons

  4. Two elements, calcium and oxygen, are combining to make a compound. This is an oxidation-reduction reaction.

  5. This reaction occurs in a conventional car battery every time the engine is started. An acid (H2SO4) is present and transfers protons to a metal oxide base, PbO2 to form water during the reaction. The reaction can therefore be described as an acid–base reaction. Note that elemental lead is also a reactant. The lead forms a compound, lead(II ) sulfate. Therefore this can be classified as an oxidation-reduction reaction. So is this reaction acid-base neutralization or oxidation-reduction. Answer: Yes!

Schematic drawing of a 12-volt car battery. The locations of the reactants (lead metal in a spongy form with large surface area) and PbO2 are shown. The product (PbSO4) forms as a white solid between the plates.

Exercise

Classify each reaction as an oxidation–reduction reaction, an acid–base reaction, or a precipitation reaction.

  1. 2Al(s) + 2NaOH(aq) + 6H2O(l) → 3H2(g) + 2NaAl(OH)4(aq)
  2. TiCl4(l) + 2Mg(l) → Ti(s) + 2MgCl2(l)
  3. MgCl2(aq) + Na2CO3(aq) → MgCO3(s) + 2NaCl(aq)
  4. CO(g) + Cl2(g) → Cl2CO(l)
  5. H2SO4(l) + 2NH3(g) → (NH4)2SO4(s)

Answer:

  1. Redox reaction. This is the reaction that occurs when Drano is used to clear a clogged drain.
  2. Redox reaction.
  3. Precipitation reaction. This reaction is responsible for the scale that develops in coffee makers in areas that have hard water.
  4. Redox reaction.
  5. Acid–base reaction.

Summary

Chemical reactions may be classified as an acid–base neutralization reaction, a precipitation reaction, , and an oxidation–reduction (or redox) reaction. Acid-base reactions require both an acid and a base as reactants. Precipitation reactions form insoluble substances. Redox reactions often involve elemental forms.

Numerical Problems

  1. Classify each chemical reaction according to the types listed in Table 7.2(1).

    1. 12FeCl2(s) + 3O2(g) → 8FeCl3(s) + 2Fe2O3(s)
    2. CaCl2(aq) + K2SO4(aq) → CaSO4(s) + 2KCl(aq)
    3. HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
    4. Br2(l) + C2H4(g) → BrCH2CH2Br(l)

  2. Classify each chemical reaction according to the types listed in Table 7.2(1).

    1. 4FeO(s) + O2(g) → 2Fe2O3(s)
    2. Ca3(PO4)2(s) + 3H2SO4(aq) → 3CaSO4(s) + 2H3PO4(aq)
    3. HNO3(aq) + KOH(aq) → KNO3(aq) + H2O(l)
    4. ethane(g) + oxygen(g) → carbon dioxide(g) + water(g)

  3. Balance this chemical equation:

    NaHCO3(aq) + H2SO4(aq) → Na2SO4(aq) + CO2(g) + H2O(l)

    What type of reaction is this? Justify your answer.

Answers

    1. redox reaction
    2. precipitation
    3. acid–base
    4. redox reaction

  3. This is an acid-base neutralization reaction. Bicarbonate ion is acting as a base by picking up hydrogen ions from the acid, sulfuric acid. The reaction is driven forward by the decomposition of carbonic acid to water and carbon dioxide gas.