Classifying Chemical Reactions and
Predicting Their Products
This classification scheme organizes chemical reactions into nine categories.
These are combination, decomposition, single replacement, double replacement,
acid-base neutralization, combustion, oxidation-reduction, complex ions and
anhydrides. The approach taken here is (1) learn a process for recognizing the
type of reaction by looking at the reactants, and (2) develop a strategy for
predicting the products for that type of reaction. The processes for recognition of
the reactants, and the strategies for predicting the products are expressed in
boldface type. Even though students are not required to balance equations on
the AP exam, equations are balanced in this presentation.
I. Combination - a reaction in which two or more substances react to produce
one product.
2H2(g) + O2(g)  2H2O(l)
2SO2(g) + O2(g)  2SO3(g)
NH3(g) + HCl(g)  NH4Cl(s)
Recognition: Memorize the reactants for the five classic combination reactions.
Strategy: Memorize the products for the five combination reactions.
A. Combination of two elements produces a binary compound containing those
two elements. Predict the product by assuming the elements are in their most
stable oxidation states.
Problem: Predict the formula of the product and write a balanced equation for the
reaction of lithium with nitrogen.
Li+, N3-
Li3N
6Li(s) + N2(g)  2Li3N(s)
B. Metal oxides react with carbon dioxide to form metal carbonates.
MgO(s) + CO2(g)  MgCO3(s)
C. Metal oxides react with sulfur dioxide to form metal sulfites.
CaO(s) + SO2(g)  CaSO3(s)
D Metal oxides react with water to form metal hydroxides.
MgO(s) + H2O(l)  Mg(OH)2(s)
E. Nonmetal oxides react with water to form oxyacids.
N2O5(g) + H2O(l)  2HNO3(aq)
II. Decomposition - a reaction in which one substance is broken down into
two or more simpler substances.
2HgO(s)  2Hg(l) + O2(g)
2NaH(s)  2Na(s) + H2(g)
Recognition: There is just one reactant.
Strategy: Memorize the products for the five types of decomposition reactions.
A. Decomposition of a binary compound produces the elements that make up the
compound.
2Ag2O(s)  4Ag(s) + O2(g)
B. Decomposition of a carbonate forms the metal oxide and carbon dioxide.
CaCO3(s)  CaO(s) + CO2(g)
C. Decomposition of a sulfite forms the metal oxide and sulfur dioxide.
BaSO3(s)  BaO(s) + SO2(g)
D. Decomposition of a hydroxide produces the metal oxide and water.
Ca(OH)2(s)  CaO(s) + H2O(l)
E. Decomposition of a metal chlorate produces the metal chloride and oxygen.
2KClO3(s)  2KCl(s) +3O2(g)
F. Decomposition of an oxyacid yields a nonmetal oxide (called an acid
anhydride) and water.
H2CO3(aq)  H2O(l) + CO2(g)
III. Single replacement (or displacement) - a reaction in which an atom
or an ion in a compound is replaced by an atom or an ion of another element.
A. A single replacement reaction can be recognized as the reaction of an
element with a salt, or the reaction of a metal element with an acid.
B. If the element is a metal or hydrogen, it will displace the metal ion in the
compound. If the element is a nonmetal, it will displace the nonmetal ion in the
compound.
Activity Series
Higher Element displaces Lower element
Li>K>Ba>Ca>Na>Mg>Al>Zn>Cr>Fe>Cd>Co>Ni>Sn>Pb>H>Cu>Hg>Ag>Pt>Au
B. Hydrogen displacement - Any metal above hydrogen in the activity series will
displace hydrogen from an acidic solution.
Cd(s) + 2HCl(aq) CdCl2(aq) + H2(g)
C. Metal displacement - Any metal will displace a metal that is below it in the
activity series from its ionic form in solution.
Zn(s) + CuSO4(aq)  ZnSO4(aq) + Cu(s)
D. Halogen displacement - The halogen activity series is F2>Cl2>Br2>I2. Any
halogen will displace any anion of a less active halogen from solution.
Cl2(g) + 2KBr(aq)  2KCl(aq) + Br2(l)
Problem: Write a chemical equation for the reaction of magnesium with a solution
containing nickel (II) chloride.
Mg(s) + NiCl2(aq)  MgCl2(aq) + Ni(s)
IV. Double replacement - a reaction between two salts in which the cation
of the first salt combines with the anion of the second salt and the cation of the
second salt combines with the anion of the first salt. These reactions are
ordinarily precipitation reactions and the products can be predicted by knowing
the solubility rules.
Solubility Rules
A. The nitrates, chlorates, and acetates of all metals are soluble in water. Silver
acetate is sparingly soluble.
B. All sodium, potassium, and ammonium salts are soluble in water.
C. The chlorides, bromides, and iodides of all metals except lead, silver, and
mercury (I) are soluble in water. Mercuric iodide, HgI2, is insoluble in water, while
PbCl2, PbBr2, and PbI2 are soluble in hot water. The water- insoluble chlorides,
bromides, and iodides are also insoluble in dilute acids.
D. The sulfates of all metals except lead, mercury (I), barium, and calcium are
soluble in water. Silver sulfate is slightly soluble. The water- insoluble sulfates
are also insoluble in dilute acids.
E. The carbonates, phosphates, borates, sulfites, chromates, and arsenates of all
metals except sodium, potassium, and ammonium are insoluble in water but
soluble in dilute acids. Also, MgCrO4 is soluble in water; MgSO4 is slightly soluble
in water.
F. The sulfides of all metals except barium, calcium, magnesium, sodium,
potassium, and ammonium are insoluble in water; BaS, CaS, and MgS are
sparingly soluble.
G. The hydroxides of sodium, potassium, and ammonium are very soluble in
water. The hydroxides of calcium and barium are moderately soluble. The oxides
and hydroxides of all other metals are insoluble.
AgNO3(aq) + NaCl(aq)  AgCl(s) + NaNO3(aq)
FeBr2(aq) + K2CrO4(aq)  FeCrO4(s) + 2KBr(aq)
Problem: Write an equation for the reaction that occurs when solutions of
ammonium chloride, NH4Cl, and calcium nitrate, Ca(NO3)2, are mixed.
Both CaCl2 and NH4NO3 are soluble. No reaction takes place.
Problem: Write an equation for the reaction that occurs when solutions of barium
chloride, BaCl2, and aluminum sulfate, Al2(SO4)3, are mixed.
3BaCl2(aq) + Al2(SO4)3(aq)  3BaSO4(s) + 2AlCl3(aq)
V. Acid-base neutralization - a reaction between an acid and a base
A. Arrhenius
Recognition: An acid reacts with a base
Strategy: A salt and water are formed.
HCl(aq) + NaOH(aq)  NaCl(aq) + H2O(l)
Many chemical reactions go to completion because of the formation of a weak
electrolyte. Water is a weak electrolyte that is very stable, and provides the
driving force for these reactions.
B. Brønsted-Lowry (proton transfer)
Recognition: The acid is a species with a proton to lose, and the base can
accept a proton.
Strategy: Transfer the proton to the base.
HC2H3O2(aq) + NH3(aq)  C2H3O2-(aq) + NH4+(aq)
C. Lewis
Recognition: An electron pair donor reacts with an electron pair acceptor.
Strategy: Combine the acid and base to form a coordinate covalent bond.
BCl3(g) + NH3(g)  Cl3BNH3(g)
VI. Combustion - reaction of a substance with oxygen.
A. Reaction of a metal with oxygen
2Mg(s) + O2(g)  2MgO(s)
B. Reaction of a nonmetal with oxygen
C(graphite) + O2(g)  CO2(g)
C. Reaction of a metal with oxygen produces a metal oxide, and reaction of a
non-metal with oxygen produces a nonmetal oxide. These reactions are also
combination reactions.
D. Combustion of a compound containing carbon and hydrogen (a hydrocarbon),
or carbon, hydrogen and oxygen
CH4(g) + 2O2(g)  CO2(g) + 2H2O(l)
E. Combustion of a compound containing carbon and hydrogen, or carbon,
hydrogen and oxygen produces carbon dioxide and water.
Recognition: Look for any substance plus oxygen as the reactants.
Strategy: Write the oxide as the product, or if it is combustion of a hydrocarbon,
the products are carbon dioxide and water.
Problem: Predict the formula of the product and write a balanced equation for the
reaction between aluminum and oxygen.
Al3+, O2-
Al2O3
4Al(s) + 3O2(g)  2Al2O3(s)
Problem: Write a balanced equation for the chemical reaction involving
combustion of glucose, C6H12O6.
C6H12O6(s) + 6O2(g)  6CO2(g) + 6H2O(l)
VII. Oxidation - reduction (redox) reactions
A. An oxidation-reduction reaction is a chemical reaction involving a transfer of
electrons from one species to another.
1. The species containing the element that increases in oxidation state (loses
electrons) is oxidized.
2. The species containing the element that decreases in oxidation state (gains
electrons) is reduced.
B. In order to recognize the reactants and predict the products of redox reactions,
learn some of the most important oxidizing agents and the species that they
form, and learn some of the most important reducing agents and the species that
they form.
Important Oxidizing Agents
MnO4- (acid solution)
MnO4- (basic solution)
MnO2 (acid solution)
Cr2O72- (acid solution)
CrO42- (basic solution)
HNO3, concentrated
HNO3 dilute
H2SO4, hot concentrated
Metallic ions
Free halogens
HClO4
Na2O2
H2O2
Formed in Reaction
Mn2+
MnO2
Mn2+
Cr3+
Cr3+
NO2
NO
SO2
Metallous ions
Halide ions
ClOHO2
Important Reducing Agents
Halide ions
Free Metals
Sulfite ions
Free halogens (dilute basic solution)
Free halogens (concentrated basic
solution)
C2O42-
Formed in Reaction
Halogens
Metal ions
SO42Hypohalite ions
Halate ions
CO2
Problem: Write the net ionic equation for the reaction that occurs when an acidic
solution of potassium permanganate is added to a solution of iron (II) sulfate.
The products will be Mn2+ and Fe3+.
5Fe2+(aq) + MnO4-(aq) + 8H+(aq)  5Fe3+(aq) + Mn2+(aq) + 4H2O(l)
VIII. Complex ions
A. Complex ions are weak electrolytes and tend to be very stable.
B. The complex ions are ordinarily formed from transition metal ions (or atoms),
Al3+ or Be2+ combined with several ligands.
C. Ligands are anions or molecules that have a lone pair of electrons capable of
being donated to form a coordinate covalent bond to the metal.
D. The number of ligands bonded to the metal cation is called the coordination
number.
1. The aqua complexes (those with water as ligands) of the transition metal ions
tend to have a coordination number of 6.
2. Otherwise, metal ions with a 2+ charge tend to have a coordination number of
4, and metal ions with a 3+ charge tend to have a coordination number of 6.
E. Look for reactions in which a transition metal salt, aluminum salt or beryllium
salt is reacted with one of the following ligands:
I-, Br-, Cl- ,F-, OH-, H2O, C2O42-, NH3, SCN-, CNF. Predict the product by combining a metal ion with a 2+ charge with four
ligands and by combining a metal ion with a 3+ charge with six ligands.
Problem: Write the net ionic equation for the reaction in which a concentrated (15
M) ammonia solution is added to a solution of nickel (II) chloride.
Ni2+(aq ) + 4NH3(aq )  [Ni(NH3)4]2+(aq)
IX. Anhydrides
A. An anhydride is a compound derived from another compound by removal of
water.
B. An acid anhydride is a compound which when added to water produces an
acid.
1. If the acid anhydride is a nonmetal oxide it will form an oxyacid in which the
oxidation state of the nonmetal is the same in the oxyacid as it is in the
anhydride.
2. This type of reaction is recognized as the reaction of a nonmetal oxide with
water. The product is an oxyacid in which the nonmetal has the same oxidation
state as it does in the nonmetal oxide.
3. SO2 is the acid anhydride of sulfurous acid, H2SO3.
SO2(g) + H2O(l)  H2SO3(aq)
C. A basic anhydride is a compound which when added to water produces a
base.
1. Metal oxides are typically basic anhydrides.
2. This type of reaction is recognized as the reaction of a metal oxide with water.
The product is a base.
3. Na2O is the basic anhydride of sodium hydroxide, NaOH.
Na2O(s) + H2O(l)  2NaOH(aq)
D. When nonmetal oxides react with water, the product will be an oxyacid. When
metal oxides react with water, the product will be a base.
Problem: Write the chemical equation for the reaction that occurs when excess
water is added to calcium oxide.
CaO(s) + H2O(l)  Ca(OH)2(aq)