Chemical Reaction
What is a Chemical Reaction?
A chemical reaction is a process in which the identity of at least one substance changes. A chemical equation represents
the total chemical change that occurs in a chemical reaction using
symbols and chemical formulas for the substances involved. Reactants are the substances that are changed and products are the substances that are produced in a chemical reaction.
The general format for writing a chemical equation is
reactant1 + reactant2 + … → product1 + product2 + …
With the exception of nuclear reactions, the Law of Conservation of Mass–matter
is neither created nor destroyed during a chemical reaction– is obeyed
in “ordinary” chemical reactions. For this reason a chemical equation
must be balanced–the number of atoms of each element must be
the same on the reactants side of the reaction arrow as on the products
side. Details on balancing chemical equations are found in the units on
Stoichiometry and Redox Reactions.
The general format for writing a chemical equation can be written in a short-hand version as
a A + b B + … → c C + d D + …
where the lower case letters are the stoichiometric coefficients needed to balance a specific equation.
The units on Stoichiometry, Redox Reactions, and Acid-Base Chemistry contain additional background reading, example problems, and information on the topics covered in this unit.
Chemists
classify chemical reactions in various ways. Often a major
classification is based on whether or not the reaction involves
oxidation-reduction. A reaction may be classified as redox in which oxidation and reduction occur or nonredox in which there is no oxidation and reduction occurring.
A
redox reaction can be recognized by observing whether or not the
oxidation numbers of any of the elements change during the reaction.
Example Problem: Classify the reactions as either redox or nonredox.
(1) 4 Fe(s) + 3 O2(g) → 2 Fe2O3(s)
(2) NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
(3) Cl2(g) + H2O(l) → HCl(aq) + HClO(aq)
Answer:
In equation (1), the iron changes oxidation numbers from 0 to +3 and
oxygen changes from 0 to -2. Equation (1) represents a redox reaction.
In equation (2), there is no change in oxidation numbers for the
elements involved: sodium is +1, oxygen is -2, hydrogen is +1, and
chlorine is -1 on both the reactants and products sides. Equation (2)
represents a nonredox reaction. In equation (3), the chlorine changes
from 0 to -1 in HCl and to +1 in HClO. There is no change in the
oxidation numbers of hydrogen (+1 in H2O, HCl, and HClO) and oxygen (-2 in H2O and HClO). Because chlorine is oxidized and reduced, equation (3) represents a redox reaction.
The
reaction described in equation (3) is interesting in that an element in
one oxidation state undergoes both oxidation and reduction. Such a
redox process is known as a disproportionation reaction. The
element undergoing disproportionation must have at least three
different oxidation states–the initial one in the reactant and one
higher plus one lower in the products.
Most simple redox reactions may be classified as combination, decomposition, or single displacement reactions. In a combinations reaction two reactants react to give a single product. The general format of the chemical equation is
a A + b B + … → c C
A
special case of a combination reaction in which the reactants are only
elements in their naturally occurring forms and physical states at the
temperature and pressure of the reaction is known as a formation reaction.
Example Problem: Identify which reactions are redox combination reactions.
(4) 6 Li2O(s) + P4O10(g) → 4 Li3PO4(s)
(5) CaO(s) + H2O(l) → Ca(OH)2(s)
(6) S(s) + 3 F2(g) → SF6(g)
(7) ZnS(s) + 2 O2(g) → ZnSO4(s)
(8) SO2(g) + Cl2(g) → SO2Cl2(g)
Answer:
All of the reaction are classified as combination reactions because
they involved two or more reactants producing a single product.
However, redox is occurring only in equations (6), (7), and (8). In
equation (6), S is oxidized from 0 to +6 and F is reduced from 0 to -1;
in equation (7), S is oxidized from -2 to +6 and O is reduced from 0 to
-2; and in equation (8), S is oxidized from +4 to +6 and Cl is reduced
from 0 to -1.
In a decomposition reaction a single reactant breaks down to give two or more substances. The general format of the chemical equation is
a A → b B + c C + …
If the decomposition reaction involves oxidation-reduction, the reaction is often called an internal redox reaction because the oxidized and reduced elements originate in the same compound.
Example Problem: Identify which reactions are redox decomposition reactions.
(9) CuSO4⋅5H2O(s) → CuSO4(s) + 5 H2O(g)
(10) SnCl4⋅6H2O(s) → SnO2(s) + 4 HCl(g) + 4 H2O(g)
(11) NH4NO2(s) → N2(g) + 2 H2O(g)
(12) (NH4)2Cr2O7(s) → N2(g) + Cr2O3(s) + 4 H2O(g)
Answer:
All of the reactions are classified as decomposition reactions because
they involve a single reactant producing two or more substances.
However, redox is occurring only in equations (11) and (12). In
equation (11), the N in NH4+ is oxidized from -3 to 0 and the N in NO2- is reduced from +3 to 0. In equation (12), the N is oxidized from -3 to 0 and the Cr is reduced from +6 to +3.
In a single displacement reaction
the atoms or ions of one reactant replace the atoms or ions in another
reactant. Single displacement reactions are also known as displacement, single replacement, and replacement reactions. The general format of the chemical equation is
a A + b BC → c AC + d B
Whether or not a redox single displacement reaction occurs will depend on the relative reducing strengths of A and B.
Example Problem: Identify which reactions are redox single displacement reactions.
(13) 2 Al(s) + Fe2O3(s) → 2 Fe(s) + Al2O3(s)
(14) 2 NaI(aq) + Br2(aq) → 2 NaBr(aq) + I2(aq)
(15) Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)
Answer:
All three reactions are redox. Both equations (13) and (14) fit the
general format of the single displacement reaction by assigning A as
Al, B as Fe, and C as O in equation (13) and A as Br, B as I, and C as
Na in equation (14). To classify equation (15) is a little more
difficult. The reaction has been represented by a net ionic equation in
which the anion has been omitted. If an anion X is added to generate
the overall equation, Zn(s) + CuX(aq) → ZnX(aq) + Cu(s), then assigning
A as Zn, B as Cu, and C as X shows that this is also a redox single
displacement reaction.
In addition to the single redox reactions described above, a redox reaction may be classified as a simple redox electron transfer reaction
in which the oxidation numbers of ionic reactants are changed by the
direct transfer of electrons from one ion to the other–typically in
aqueous solutions. For example
2 Fe3+(aq) + Sn2+(aq) → 2 Fe2+(aq) + Sn4+(aq)
Many
redox reactions do not fit into the classifications described above.
For example, redox reactions involving oxygen-containing reactants in
aqueous acidic or basic solutions such as
3 Cu(s) + 8 HNO3(aq, dil) → 3 Cu(NO3)2(aq) + 2 NO(g) + 4 H2O(l)
Cu(s) + 4 HNO3(aq, conc) → Cu(NO3)2(aq) + 2 NO2(g) + 2 H2O(l)
or the combustion of oxygen with more than one element in a reactant
2 CH3OH(g) + 3 O2(g) → 2 CO2(g) + 4 H2O(l)
These types of reactions are classified as complex redox reactions.
There
are several classifications of nonredox reactions–including
combination, decomposition, single displacement, and double
displacement reactions.
The general format of the chemical equation for a nonredox combination reaction is the same as for a redox combination reaction
a A + b B + … → c C
However,
all reactants and the product must be compounds and no changes in
oxidation numbers of the elements occur. Usually these reactions
involve reactants that are acidic and basic anhydrides.
Example Problem: Identify which reactions are nonredox combination reactions.
(16) 2 Na(s) + Cl2(g) → 2 NaCl(s)
(17) SO3(g) + CaO(s) → CaSO4(s)
(18) SO2(g) + H2O(l) → H2SO3(aq)
Answer: All three equations are combination reactions, but only equations (17) and (18) are nonredox.
The general format of the chemical equation for a nonredox decomposition reaction is the same as for a redox decomposition reaction
a A → b B + c C + …
However,
the reactant and all products must be compounds and no changes in
oxidation numbers occur. Quite often one of the products formed will be
a gas.
Example Problem: Identify which reactions are nonredox decomposition reactions.
(19) NH4HCO3(s) → NH3(g) + CO2(g) + H2O(g)
(20) NH4NO2(s) → N2(g) + 2 H2O(g)
(21) CaCO3(s) → CaO(s) + CO2(g)
Answer: All three equations are decomposition reactions, but only equations (19) and (21) are nonredox.
The general format of the chemical equation for a nonredox single displacement reaction is the same as for a redox single displacement reaction
a A + b BC → c AC + d B
However,
there are no changes in the oxidation numbers of the elements during
the reaction. Common nonredox single displacement reactions include
ligand substitution in complexes and formation of more stable
oxygen-containing compounds from less stable oxygen-containing
compounds.
Example Problem: Identify which reactions are nonredox single displacement reactions.
(22) [PtCl4]2-(aq) + 2 NH3(aq) → [Pt(NH3)Cl2](s) + 2 Cl-(aq)
(23) Na2CO3(s) + SiO2(s) → Na2SiO3(l) + CO2(g)
(24) 2 AgNO3(aq) + Cu(s) → Cu(NO3)2(aq) + 2 Ag(s)
Answer: All three equations are single displacement reactions, but only equations (22) and (23) are nonredox.
Finally, the last classification of nonredox reactions is that of nonredox double displacement reactions. The general format of the chemical equation is
a AC + b BD → c AD + d BC
with no oxidation or reduction of A, B, C, or D occurring. These reactions are also known as double replacement, “partner” exchange, and metathesis reactions.
Usually one or more of the products will be a gas, a precipitate, a
weak electrolyte, or water. An important example of a nonredox double
displacement reaction is the reaction of an acid with a base under
aqueous conditions.
Example Problem: Identify the nonredox double displacement reactions.
(25) CaCO3(s) + 2 HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)
(26) HCl(aq) + KOH(aq) → KCl(aq) + H2O(l)
(27) AgNO3(aq) + KCl(aq) → AgCl(s) + KNO3(aq)
Answer: All three reactions are nonredox double displacement reactions.
Most
experienced chemists can classify a given chemical reaction rather
easily and quickly by “inspection” of the formulas of the reactants and
products in the chemical equation. The first decision most chemists
make is to determine whether or not the reaction involves redox. Based
on this decision, the answers to a few more specific questions will
readily lead to the reaction classification. These specific questions
are based on the general formats of the chemical equations for the
different classifications of the reactions described above.
To begin learning what these questions are, you might consider using the online analysis program that is available at http://www.xxx.yyy to classify the reactions given in the Example Problem.
The basis of this analysis program is outlined by the flow charts given
in Figures (1) and (2). Please do not memorize this flow chart–it is
simply a tool to help you learn what to look for and what questions
should be asked as you classify a given reaction.
Figure 1. Flow chart of questions to classify nonredox reactions. (Used by permission ...)
Figure 2. Flow chart of questions to classify redox reactions. (Used by permission...)
Questions:
- Why are chemical reactions that occur in the thermodynamic sometimes does not happen without the help of an enzyme?
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