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 relationship 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 is called excess reagents whereas the
latter are called limiting regents.
Classification is a good method for the organizaiton of information.
It is a good start in any study, because it helps to organize our thought
and to put information in am order.
We can classify chemical reactions according to their common features,
and divide chemical reactions into a few types:
Classification is a useful scientific method, and we divide chemical reactions into some common 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,
HCl + NaOH = NaCl + H2 . . . neutralization
HCl + NH3 = NH4Cl . . . formation of a salt from vapour
CH3COOH + H2O = CH3COO- +
H3O+ . . . ionization
CH3COOH + NaHCO3 = NaCH3COO + CO2
. . . gas formation
A combination reaction refers to one in which two or more substances react
(or combine) to give one new product. Elements combine to form compound,
but compounds may also do the same.
The following are combination reactions:
H2(g) + Cl2(g) = 2 HCl(g)
Fe(s) + S(s) = FeS(s)
NH3(g) + HCl(g) = NH4Cl(s, appear as a white smoke)
2 Na(s) + Cl2(g) = 2 NaCl(s)
2 NO(g) + O2(g) = 2 NO2(g)
P4O10(s) + 6 H2O(l) = 4 H3PO4(aq)
A substance reacts with oxygen or oxygen in the air to give oxides in a
For example the combustion of alcohol, sulfur and magnesium are
C2H5OH(l) + O2(g) = CO2(g) + H2O(g)
S(s) + O2(g) = SO2(g)
2 Mg(s) + O2(g) = 2 MgO(s)
It's interesting to point out that at very high temperature, a combustion
reaction favors the formation of CO rather than CO2, especially
when oxygen supply is limited. The combustion of CO with oxygen
release energy, and at high temperature, abundance of energy in the system
keeps CO from further oxidation.
A compound or substance separate 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:
2 H2O(l) = 2 H2 + O2 . . . electrolysis of water
2 HgO(s) = 2 Hg(l) + O2(g) . . . generation of oxygen from heating HgO
2 KClO3(s) = 2 KCl(s) + 3 O2(g) . . . another way to produce oxygen
CH3COOH = CH4 + CO2
Note however, the following reactions are often called ionization
or dissociation reactions rather than decomposition reaction.
The solvent participates in these reactions.
NaCl(s) + 6 H2O = Na+(H2O)6 + Cl-
NH4Cl(s) = NH4+(aq) + Cl-
HNO2 = H+(aq) + NO2-(aq)
Some decomposition reactions are Disproportion Reactions.
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:
Zn + H2SO4 = ZnSO4 + 2 H2
. . . Zn displaces hydrogen
2 HCl + F2 = 2 HF + Cl2 . . . F displaces Cl
2 Na + H2O = 2 NaOH + H2 . . . Na displaces H
2 Al + Fe2O3 = 2 Fe + Al2O3
. . . a thermite reaction
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
or Daniel cell operations.
Before the realization of electron transfers, reactions with oxygen were
called oxidation reaction, 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
There are many redox reactions, and balancing redox reaction equation
is a basic skill for an educated person. A unit will be devoted to
balance 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
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).
AgNO3 + NaCl = NaNO3 + AgCl(s)
PbNO3 + KI = PbNO3 + PbI(s)
Precipitation reactions take place in
solutions, and ions exchange in these reactions.
Thus, they are often called exchange reactions or metathesis
Chemical reactions can be beneficial and hazardous.
Chemical Reaction Hazards Forum is an Internet site that discusses
some recent event related to chemical reactions.
oscillating reactions, and chain reactions are interesting.
These and millions of reactions are still being investigated now.
Skill Developing Problems
Define and exemplify combination reactions.
Define and exemplify decomposition reactions.
Define and exemplify redox reactions.
Define and exemplify displacement reactions.
This problem also illustrates a strategy for chemical analysis.
This is also a redox reaction (reduction and oxidation reaction).
These reactions are used for the detection of any one of the reactants.
Indicators change color due to acid-base reactions.