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Chemistry LibreTexts

Features of Chemical Reaction

Key Words

  • acid-base reaction or neutralization reactions
  • combination reaction
  • decomposition reaction
  • combustion reaction
  • disproportion reaction
  • displacement reaction
  • oxidation-reduction reactions (redox reaction)
  • precipitation reaction (exchange or metathesis reactions)

Skills to Develop

  • Define some common features such as acid-base reaction (neutralization), combination, combustion, decomposition, displacement, precipitation (exchange or metathesis), and redox reactions.
  • Describe features of some common reactions.
  • Classify chemical reactions according to their features.


Quantitative relationships among reactants and products are called stoichiometry. Stoichiometry problems are easily solved when amounts of substances are converted from mass (in units of g, kg etc), volume (L), into moles. Amounts in moles depend on how the chemical formula or the chemical reaction equations are written. Chemical reaction equations are the basis for reaction stoichiometry, but when the reactants are not stoichiometric mixtures, some reactants will be in excess whereas others will be in limited supply. The former are called excess reagents whereas the latter are called limiting reagents.

Classification is a good method for the organization of information. It is a good start in any study, because it helps to organize our thought and to put information in order. We can classify chemical reactions according to their common features, and divide chemical reactions into a few types:

Acid and Bases

Many substances are known as acids and bases. In the HandbookMenu, some properties of acids and bases are given in two tables and you may click them to find out their names and formulas. Reactions between acids and bases are called acid-base reactions. Some of them are often called neutralization reactions.

However, the concepts of acids and bases have been generalized and much more discussion can be found in modules or units in Chem123/125. Thus, acid-base reactions are further subdivided into other types. A few examples are given below:

\textrm{neutralization: }\,&\mathrm{HCl + NaOH \rightarrow NaCl + H_2}\\
\textrm{formation of a salt from vapour: }\,&\mathrm{HCl + NH_3 \rightarrow NH_4Cl}\\
\textrm{ionization: }\,&\mathrm{CH_3COOH + H_2O \rightarrow CH_3COO^- + H_3O^+} \\
\textrm{gas formation: }\,&\mathrm{CH_3COOH + NaHCO_3 \rightarrow NaCH_3COO + CO_2}


A combination reaction refers to one in which two or more substances react (or combine) to give one new product. Elements combine to form a compound, but compounds may also do the same. The following are combination reactions:

\ce{H_{2\large{(g)}} + Cl_{2\large{(g)}} &\rightarrow 2 HCl_{\large{(g)}}}\\
\ce{Fe_{\large{(s)}} + S_{\large{(s)}} &\rightarrow FeS_{\large{(s)}}}\\
\ce{NH_{3\large{(g)}} + HCl_{\large{(g)}} &\rightarrow NH_4Cl}_{\large\textrm{(s, appears as a white smoke)}}\\
\ce{2 Na_{\large{(s)}} + Cl_{2\large{(g)}} &\rightarrow 2 NaCl_{\large{(s)}}}\\
\ce{2 NO_{\large{(g)}} + O_{2\large{(g)}} &\rightarrow 2 NO_{2\large{(g)}}}\\
\ce{P_4O_{10\large{(s)}} + 6 H_2O_{\large{(l)}} &\rightarrow 4 H_3PO_{4\large{(aq)}}}


A substance reacts with oxygen or oxygen in the air to give oxides in a combustion reaction. For example, the combustion of alcohol, sulfur and magnesium are given below:

\ce{C_2H_5OH_{\large{(l)}} + O_{2\large{(g)}} &\rightarrow CO_{2\large{(g)}} + H_2O_{\large{(g)}}}\\
\ce{S_{\large{(s)}} + O_{2\large{(g)}} &\rightarrow SO_{2\large{(g)}}}\\
\ce{2 Mg_{\large{(s)}} + O_{2\large{(g)}} &\rightarrow 2 MgO_{\large{(s)}}}

It's interesting to point out that at very high temperature, a combustion reaction favors the formation of \(\ce{CO}\) rather than \(\ce{CO2}\), especially when oxygen supply is limited. The combustion of \(\ce{CO}\) with oxygen releases energy, and at high temperature, abundance of energy in the system keeps \(\ce{CO}\) from further oxidation.


A compound or substance separates into two or more substances which may or may not be different from each other, but the products are usually different from the original compound. Some examples of decomposition reactions are:

\textrm{electrolysis of water: }\,&\mathrm{2 H_2O_{\large{(l)}} \rightarrow 2 H_2 + O_2}\\
\textrm{generation of oxygen from heating HgO: }\,&\mathrm{2 HgO_{\large{(s)}} \rightarrow 2 Hg_{\large{(l)}} + O_{2\large{(g)}}}\\
\textrm{another way to produce oxygen: }\,&\mathrm{2 KClO_{3\large{(s)}} \rightarrow 2 KCl_{\large{(s)}} + 3 O_{2\large{(g)}}}\\
&\mathrm{CH_3COOH \rightarrow CH_4 + CO_2}

Note however, the following reactions are often called ionization or dissociation reactions rather than decomposition reactions. The solvent participates in these reactions.

\ce{NaCl_{\large{(s)}} + 6 H_2O &\rightarrow Na^+(H_2O)_6 + Cl^-}\\
\ce{NH_4Cl_{\large{(s)}} &\rightarrow NH^+_{4\large{(aq)}} + Cl^-}\\
\ce{HNO_2 &\rightarrow H^+_{\large{(aq)}} + NO^-_{2\large{(aq)}}}

Some decomposition reactions are Disproportion Reactions. For example,

\ce{3 Cl_{2\large{(g)}} + 6 OH^-_{\large{(aq)}} &\rightarrow 5 Cl^- + ClO^-_{3\large{(aq)}} + 3 H_2O_{\large{(l)}}}\\
\ce{4 KClO_{3\large{(s)}} &\rightarrow KCl_{\large{(s)}} + 3 KClO_{4\large{(s)}}}


Due to difference in affinity, part of a compound may be replaced by another element or a group of atoms. The following reactions illustrate not only the displacement reactions, but also the relative reactivity of some elements:

\textrm{Zn displaces hydrogen: }\,&\mathrm{Zn + H_2SO_4 \rightarrow ZnSO_4 + 2 H_2} \\
\textrm{F displaces Cl: }\,&\mathrm{ 2 HCl + F_2 \rightarrow 2 HF + Cl_2}\\
\textrm{Na displaces H: }\,&\mathrm{2 Na + H_2O \rightarrow 2 NaOH + H_2}\\
\textrm{a thermite reaction: }\,&\mathrm{2 Al + Fe_2O_3 \rightarrow 2 Fe + Al_2O_3}

Oxidation and Reduction

Oxidation and reduction reactions usually involve the transfer of electrons. A gain of electron is called reduction, and a loss of electron is oxidation. Since electrons can neither be created nor destroyed, a gain of electrons by one species must be accompanied by a loss of another species. Thus, oxidation reactions are always accompanied by reductions, and reactions of this type are called redox reactions. These reactions are the basis for battery or Daniell cell operations.

Before the realization of electron transfers, reactions with oxygen were called oxidation reactions, and reactions with hydrogen or with base-forming elements such as metals were called reduction reactions. However, the more modern definition will be used in our discussion of chemistry.

There are many redox reactions, and balancing redox reaction equations is a basic skill for an educated person. A unit will be devoted to balancing redox reaction equations in this course. This type of reactions may belong to types mentioned earlier. Please identify the species that are oxidized and reduced in the following reactions:

\ce{Zn + H_2SO_4 &\rightarrow ZnSO_4 + 2 H_2 \:(zinc\: oxidized)} \\
\ce{2 HCl + F_2 &\rightarrow 2 HF + Cl_2 \:(flourine\: reduced)}\\
\ce{2 Na + H_2O &\rightarrow 2 NaOH + H_2\:(hydrogen\: reduced)} \\
\ce{2 Al + Fe_2O_3 &\rightarrow 2 Fe + Al_2O_3}\\
\ce{2 HgO &\rightarrow 2 Hg + O_2\:(oxygen\: oxidized)}\\
\ce{2 KClO_3 &\rightarrow 2 KCl + 3 O_2\:(chlorine\: reduced)} \\
\ce{C_2H_5OH + O_2 &\rightarrow CO_2 + H_2O}


Solids appear when two solutions are mixed if the products formed are not soluble. The solids are called precipitates, and such reactions are called precipitation reactions. In reaction equations, we use (s) to represent the formation of a solid or precipitate (ppt).

\ce{AgNO_3 + NaCl &\rightarrow NaNO_3 + AgCl_{\large{(s)}}}\\
\ce{PbNO_3 + KI &\rightarrow PbNO_3 + PbI_{\large{(s)}}}\end{align}\)

Precipitation reactions take place in solutions, and ions exchange in these reactions. Thus, they are often called exchange reactions or metathesis reactions.

Chemical reactions can be beneficial and hazardous. Chemical Reaction Hazards Forum is an Internet site that discusses some recent events related to chemical reactions.

Mechanisms of oscillating reactions and chain reactions are interesting. These and millions of reactions are still being investigated now.

Skill Developing Problems

  1. In what class are the following reactions?
  • \(\mathrm{Ca_{\large{(s)}} + O_{2\large{(g)}} \rightarrow CaO_{\large{(s)}}}\)
  • \(\mathrm{2 Fe_{\large{(s)}} + 3 Cl_{2\large{(g)}} \rightarrow 2 FeCl_{3\large{(s)}}}\)
  • \(\mathrm{2 FeO_{\large{(s)}} + O_{2\large{(g)}} \rightarrow Fe_2O_3\: (corrosion)}\)
  • \(\mathrm{As_{\large{(s)}} + H_{2\large{(g)}} \rightarrow AsH_3\:(formation\: of\: arsine)}\)

Hint: combination

Skill: Define and exemplify combination reactions.

  1. In what class are the following reactions?
  • \(\mathrm{CaCO_{3\large{(s)}} \rightarrow CaO + CO_{2\large{(g)}}}\)
  • \(\mathrm{4 KClO_{3\large{(s)}} \rightarrow KCl_{\large{(s)}} + 3 KClO_{4\large{(s)}}}\)
  • \(\mathrm{H_2S_{\large{(g)}} \rightarrow H_2 + S_{\large{(s)}}}\)

Hint: decomposition

Skill: Define and exemplify decomposition reactions.

  1. In what class are the following reactions?
  • \(\mathrm{2 HCl + F_2 \rightarrow 2 HF + Cl_2}\)
  • \(\mathrm{CaCO_{3\large{(s)}} \rightarrow CaO + CO_{2\large{(g)}}}\)
  • \(\mathrm{3 Cl_{2\large{(g)}} + 6 OH^-_{\large{(aq)}} \rightarrow 5 Cl^- + ClO_{3\large{(aq)}}^- + 3 H_2O_{\large{(l)}}}\)
  • \(\mathrm{4 KClO_{3\large{(s)}} \rightarrow KCl_{\large{(s)}} + 3 KClO_{4\large{(s)}}}\)
  • \(\mathrm{H_2S_{\large{(g)}} \rightarrow H_2 + S_{\large{(s)}}}\)

Hint: oxidation and reduction (redox)

Skill: Define and exemplify redox reactions.

  1. In what class are the following reactions?
  • \(\mathrm{2 K_{\large{(s)}} + 2 H_2O_{\large{(l)}} \rightarrow KOH + H_{2\large{(g)}}}\)
  • \(\mathrm{KCl_{\large{(aq)}} + F_2 \rightarrow KF_{\large{(aq)}} + Cl_{2\large{(g)}}}\)
  • \(\mathrm{2 H_2S_{\large{(g)}} + O_{2\large{(g)}} \rightarrow H_2O_{\large{(g)}} + S_{\large{(s)}}}\)
  • \(\mathrm{Cu_{\large{(s)}} + 2 AgNO_{3\large{(aq)}} \rightarrow 2 Ag + Cu(NO_3)_{2\large{(aq)}}}\)

Hint: displacement

Skill: Define and exemplify displacement reactions.

  1. Classify the following collection of chemical reactions into these categories: acid-base, combination, decomposition, combustion, displacement, redox, and precipitation reactions. A reaction may belong to more than one category.
  1. \(\mathrm{Ca_{\large{(s)}} + O_{2\large{(g)}} \rightarrow CaO_{\large{(s)}}}\)
  2. \(\mathrm{2 Fe_{\large{(s)}} + 3 Cl_{2\large{(g)}} \rightarrow 2 FeCl_{3\large{(s)}}}\)
  3. \(\mathrm{CaO_{\large{(s)}} + CO_{2\large{(g)}} \rightarrow CaCO_{3\large{(s)}}}\)
  4. \(\mathrm{2 AgBr_{\large{(s)}} \rightarrow 2 Ag + Br_{2\large{(g)}}}\)
  5. \(\mathrm{PbO_{\large{(s)}} + C_{\large{(s)}} \rightarrow Pb_{\large{(s)}} + CO_{\large{(g)}}}\)
  6. \(\mathrm{Cu_{\large{(s)}} + 2 AgNO_{3\large{(aq)}} \rightarrow 2 Ag + Cu(NO_3)_{2\large{(aq)}}}\)
  7. \(\mathrm{NH_4Cl_{\large{(s)}} \rightarrow NH_{3\large{(g)}} + HCl_{\large{(g)}}}\)
  8. \(\mathrm{CH_2CH_{2\large{(g)}} + H_{2\large{(g)}} \rightarrow CH_3CH_{3\large{(g)}}}\)
  9. \(\mathrm{CH_2CH_{2\large{(g)}} + 3 O_{2\large{(g)}} \rightarrow 2 CO_{2\large{(g)}} + 2 H_2O_{\large{(l)}}}\)
  10. \(\mathrm{Al_{\large{(s)}} + AgS_{\large{(s)}} \rightarrow Al_2S_3 + Ag_{\large{(s)}}}\)
  11. \(\mathrm{NH_3 + H_2O \rightarrow NH_{4\large{(aq)}}^- + OH^-_{\large{(aq)}}}\)
  12. \(\mathrm{HNO_2 \rightarrow NO_2^- + H^+}\)
  13. \(\mathrm{AgNO_3 + NaOH \rightarrow Ag(OH)_{\large{(s)}} + NaNO_3}\)


acid base: k, l

combination: a, b, c, h

decomposition: d, g

combustion: a, i

displacement: f, j

redox: a, b, c, d, e, f, h, i, j

precipitation: m

Skill: This problem also illustrates a strategy for chemical analysis.

  1. The reaction equation

\(\mathrm{2 Al_{\large{(s)}} + Fe_2O_{3\large{(s)}} \rightarrow 2 Fe_{\large{(liquid)}} + Al_2O_3}\)

is the thermite reaction when a mixture of aluminium and iron oxide is ignited. This reaction is a

  1. combination reaction
  2. decomposition reaction
  3. displacement reaction
  4. combustion reaction
  5. acid-base reaction
  6. precipitation reaction

Hint: displacement

Discussion: This is also a redox reaction (reduction and oxidation reaction).

  1. How would you classify these reactions?
  • \(\mathrm{Ca(OH)_2 + CO_2 \rightarrow CaCO_{3\large{(s)}} + H_2O}\)
  • \(\mathrm{BaCl_2 + H_2SO_4 \rightarrow BaSO_{4\large{(s)}} + 2 HCl}\)
  • \(\mathrm{K_2CrO_4 + Pb(CH_3COO)_2 \rightarrow PbCrO_{4\large{(s,\: yellow)}} + 2 K(CH_3COO)}\)

Hint: precipitation

Discussion: These reactions are used for the detection of any one of the reactants.

  1. How would you classify these reactions?
  • antacid and gastric juice reaction: \(\mathrm{Mg(OH)_2 + HCl \rightarrow MgCl_2 + 2 H_2O}\)
  • lime and lemon juice reaction: \(\mathrm{Ca(OH)_2 + H_3C_6H_5O_7}\)
  • ionization of aspirin: \(\mathrm{HC_9H_7O_4 + H_2O \rightarrow C_9H_7O_4^- + H_3O^+}\)

Hint: acid-base reaction

Discussion: Indicators change color due to acid-base reactions.