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V Chemical Reactions
Essential Questions:
1) What is a chemical reaction?
2) How do we represent the chemical reaction in a way that is convenient and easy to
understand?
3) What is a balanced equation? And why a chemical equation has to be balanced?
4) Can we predict a chemical reaction? If we can, then how do we predict a chemical
reaction.
A chemical reaction: is a process in which one set of substances called reactants is
converted to a new set of substances called products. According to atomic theory, a
chemical reaction only rearranges the way that atoms are combined; the atoms
themselves are unchanged.
Some physical evidence to look for that shows a reaction has occurred:
 a color change
 formation of a solid (precipitate) within a clear solution
 evidence of a gas
evolution or absorption of heat
We represent chemical reactions by chemical equation, which is a symbolic way of
representing a chemical reaction in terms of chemical formulas. In a chemical equation,
formulas for the reactants (starting substances in a chemical reaction) are written on the
left sides of the equation and formulas for products are written on the right. The two sides
of equation are joined by an arrow ().
E. g. Consider the reaction of colorless nitrogen monoxide and oxygen gas to form redbrown nitrogen dioxide gas.
1 Substitute chemical formulas for names, to obtain the following expression.
2 Balance numbers of atoms to obtain a chemical equation.
In a balanced equation, for each element present, the total number of atoms of the
element is the same on both sides. A chemical reaction involves simply a recombination
of the atoms; none are destroy and none are created.
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The numbers in front of the chemical formula are coefficients required to balance a
chemical equation and they are called stoichiometric coefficients.
Note: In balancing a chemical equation, the equation can be balanced only by adjusting
the coefficients of formulas, as necessary.
1 Never introduce extraneous atom or molecules into equation
2 Never change formulas for the purpose of balancing an equation
State of Matter
A complete chemical reaction also includes state of matter and the reaction condition.
In a chemical equation, the state of matter or physical form of reactants and products are
presented through symbols in parentheses.
(g) gas
(l) liquid
(s) solid
(aq) aqueous (water)solution
Therefore, the chemical equation of our first example: Consider the reaction of colorless
nitrogen monoxide and oxygen gas to form red-brown nitrogen dioxide gas is
Reaction Conditions
An important aspect of modern chemical research involves working out the condition for
a reaction. The condition of the reaction is written above or below the arrow in an
equation.  means that a high temperature is required.
E.g.
Balancing a Chemical Equation
The requirement that a chemical equation be balanced is a direct consequence of Law of
conservation of mass. How to balance a equation? One of the methods is Balancing
by Inspection: means to adjust stoichiometric coefficients by trial and error until a
balanced condition is found.
Useful strategies for balancing equations
1 If an element occurs in only one compound on each side of the equation, try balancing
this element first.
2 When one of the reactants or products exists as the free element, balance this element
last.
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3 In some reactions, certain groups of atoms (e.g. polyatomic ions) remain unchanged. In
such case balance these groups as a unit
4 It is permissible to use fractional as well as integral numbers as coefficients. At times,
an equation can be balanced most easily by using one or more fractional coefficients and
then, if desired, clearing the fractions by multiplying all coefficients by a common
multiplier.
E.g. NH3 + O2 N2 + H2O
Quick-Think
E.g. Balance the following chemical equations
1) H2 + O2  H2O
2) Na + Cl2  NaCl
3) Hg(NO3)2 + KI  HgI2 + 2KNO3
4) Solid iron(III) oxide react with carbon monoxide to liberate carbon dioxide gas and to
form iron metal.
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Types of reactions
•Combination reaction
•Decomposition reactions
•Displacement reactions
•Metahesis reactions (double-replacement reactions)
Combustion reactions
1) Combination reactions
2) Decomposition Reactions
3)Displacement Reactions
4) Metathesis
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5) Combustion Reactions
Before we start next section, we should add that other ways of classifying chemical
reactions exist. The traditional scheme we have just discussed is useful for simple types
of reactions, but many reactions can also be placed in one of two broad categories: acidbase reactions and oxidation-reduction reactions. Combustion reactions are examples
of oxidation-reduction reactions. Here we will not discuss oxidation-reduction reactions.
You will learn these two broad categories of reactions in some detail in CHE 110 course.
Ions in Aqueous Solution
Most of reactions that we will be looking are reactions that occur in aqueous solution. In
this section we will look closely at three of these kinds of reactions: precipitation,
neutralization reactions, and reaction with gas formation. These reactions can usually be
classified as metathesis reactions. However, they actually involve ions in solution. To,
understand this we must first look at ions in aqueous solution.
Electrolytes
Certain compounds, called electrolytes, form ions when dissolved in water. Pure water is
a poor conductor of electrically, but when sodium chloride, NaCl, is dissolved in it, the
resulting solution becomes electrically conducting. The solution of NaCl is an electrical
conductor and allows electricity to flow through.
Ionic Theory of Solution
The conductivity of aqueous sodium chloride is explained by the ionic theory of solution,
proposed in 1884 by Svante Arrhenius. According to this theory, an electrolyte produces
ions when it dissolves in water. Sodium chloride dissolves in water as Na+ and Cl- ions.
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As seen on the figure left, one electrode is
positively charged by the battery and
-
attracts the negatively charged Cl ions.
The other electrode is negatively charged and
attracts Na+ ions. Thus, the ions move in
the solution. This movement of ions, or
electric charge, is responsible for the electric
current that flows in the solution. Othe the
contratry, the sugar dissolves in the water as
molecular and does not carry any charg.
Thus, the solution is noncoducting solution.
Most soluble ionic substances dissolve in water as ions and are therefore electrolytes.
Some molecular substances dissolve to give ion. Fore example, HCl(g) reacts with water
to give H3O+ and Cl- ions.
Strong electrolyte:
Weak electrolyte
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Table 5.1 Electrolyte Classification of Some Common Substances
Strong Electrolytes
HCl, HBr, HI
HClO4
HNO3
H2SO4
KBr
NaCl
NaOH, KOH
Other soluble
ionic compounds
Weak Electrolytes
CH3COOH
HF
Nonelectrolytes
H2O
CH3OH
C2H5OH
C12H22O11(sucrose)
Most organic compounds
Arrhenius’ Definition of Acids and Bases
When Arrhenius developed his ionic theory of solutions, he also gave the
definition of acids and bases. Although pure water is a very poor electricity conductor, it
does produce a small percentage of ions (about 2x10-7% of the molecules react to gives
ions). The reaction can be written
H2O(l)+ H2O(l)  H3O+(aq) + OH-(aq)
Because of this equilibrium the hydronium inos, H3O+, and hydroxide ions, OH-, assume
special significance in aqueous solution. The hydronium ion, H3O+ , which may be
thought of as H+ bonded to H2O, is often simply written H+(aq) and called the hydrogen
ion.
In modern terms, the Arrhenius definition of
an acid
A base
Strong Acid and Weak Acid
A strong
A weak acid
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Strong Base and Weak Base
A strong base
A weak base
Table 5.2 Common Strong Acids and Bases in aqueous solution
Strong Acids
Strong bases
HClO4
LiOH
H2SO4
NaOH
HI
KOH
HBr
Ca(OH)2
HCl
Sr(OH)2
HNO3
Ba(OH)2
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Quick-Think
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Molecular and Ionic Equations
Earlier we mention that many of reaction that occur in aqueous solution involve ions.
Consider the preparation of precipitated calcium carbonate. One way to prepare this
compound is to react calcium hydroxide with sodium carbonate.
Ca(OH)2(aq) + Na2CO3(aq) CaCO3(s) + 2NaOH(aq)
We called this a molecular equation, an equation in which the substances are written as
if they were molecular substances, even though they may exist as ions in the solution.
Molecular equation is useful because it is explicit about what solutions have been added
and what products are obtained; and to calculate the amount of reactants or products.
However, the molecular equation does not tell us that the reaction actual involves ions in
solution. We know that soluble ionic substances in solution should be represented by
their separate ions. To represent this, the above reaction as an ionic equation, in which
all the ions are explicitly shown:
Ca2+(aq) + 2OH-(aq) + 2Na+(aq) + CO32-(aq)  CaCO3(s) + 2Na+(aq) + 2OH-(aq)
This is an example of ionic equation, which is a chemical equation for a reaction
involving ions in solution in which soluble substances are represented by the formulas of
the predominant species in that solution. The rules for converting molecular equations to
ionic equations follow:
1) Make sure the molecular is balanced
2) Ionic substances indicated in the molecular equation as dissolved in solution, such as
NaCl(aq), are normally written as ions.
3) Ionic substances that are insoluble (do not dissolve) either as reactants or products
(such as precipitate) are represented by formulas of the compounds.
4) Molecular substances that are strong electrolytes, such as strong acids, are written as
ions. Thus, HCl(aq) is written as H3O+(aq) + Cl-(aq) or as H+(aq) + Cl-(aq).
5) Molecular substances that are weak electrolytes or nonelectrolytes are represented by
their molecular formulas.
Note that some ions appear on both side of equation. These we called them spectator
ions; they are ions in an ionic equation that do not take part in the reaction. We can
cancel them from both sides of the equation.
The resulting equation is
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This is the net ionic equation, an ionic equation from which spectator ions have been
canceled. It shows the essential reaction that occurs. In this case, calcium ions and
carbonate ions come together to form solid calcium carbonate.
Aqueous Reactions
Among the most important reactions in aqueous solution are: precipitation reactions, and
acid base reactions (that is, reaction of acids and bases, including neutralization reactions,
and reaction with gas formation). A chemical reaction occurs for a reason that involves
the energies of reactants and products. Without go into detail, think about the “driving
force” that push reactants into product. Here we will learn how to predict when such
reactions will occur.
Way to Drive Metathesis Reactions to Products:
Precipitation, formation of a weak electrolyte, and formation of a gaeouss product.
1) Precipitation
Consider once again the reaction of calcium hydroxide with sodium carbonate.
Ca(OH)2(aq) + Na2CO3(aq) CaCO3(s) + 2NaOH(aq)
We can obtain the products from the reactants side of the molecular equation by
interchanging the anions (or cations). The reaction is therefore a metathesis reaction.
When the aqueous solutions of the reactants are first mixed, we obtain a solution of four
ions.
Ca2+(aq) + 2OH-(aq) + 2Na+(aq) + CO32
Now we have ask whether the reaction will occur or not.
For a reaction to occur, at least two of the ions have to be removed in
someway from the mixture. Otherwise, no reaction occurs; we simply have a mixture
of ions. In this case, calcium ion and carbonate ions come together to form calcium
carbonate, which precipitates from the solution. In this way, calcium ion and carbonate
ions are removed from the mixture, leaving sodium ions and hydroxide ions in the
solution.
Ca2+(aq) + CO32-(aq)
CaCO3(s)
We can say that this metathesis reaction is driven to products by the removal of ions
through precipitation. Precipitation is one way in which ions can be removed from
solution.
2 formation of a weak electrolyte
Another way is through the formation of a molecular substance that is a weak electrolyte.
Consider the reaction of hydrochloric acid and sodium hydroxide. First write the
reactants. Then, assuming a metathesis reaction, we can predict the products by
interchanging the anion
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Hydrogen ions and hydroxide ions react to produce a weak electrolyte, so these ions are
effectively removed from the solution as the molecule H2O.
3) formation of a gaeouss product
A third way in which ions can be removed and a metathesis reaction is driven to products
is through the formation of a gaseous product. Consider the reaction of sodium sulfide
with hydrochloric acid.
Sulfide ion and hydrogen ion are removed as the gas hydrogen sulfide. This reaction is
driven by the removal of ions through the formation of a gaseous product.
In next section, we will look at each of these kinds of reactions.
Precipitation Reactions
Precipitation reactions are process in which soluble reactants yield an insoluble solid
product that falls out of solution. Most precipitations take place when certain cations and
anions combined to produce an insoluble ionic solid called a precipitate. Therefore,
precipitation reactions in aqueous solution depend on the fact that one product does not
dissolve readily in water. In order to predict whether a precipitation is likely to occur, it
is necessary to know whether the potential products are soluble or insoluble.
E.g. the reaction of silver nitrate and sodium iodide in an aqueous water solution yields
sodium nitrate in solution and a yellow precipitate of silver iodide. First we write a
balanced molecular equation
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Let us verify the solubility of the compounds. Form table 5.3, we note that nitrates and
halides are soluble except silver halides and few other ions. Now, we can append the
appropriate labels to the compounds in the above equation
Table 5.3
Empirical Rules for the solubilities of Common Ionic Compounds
Soluble compounds
Exceptions
Sodium, potassium and ammonium
compounds
Acetate and nitrates
Halides (chlorides, bromides, and iodides)
Sulfates
Insoluble compounds
Carbonates and phosphates
Lead(II), silver and mercury(I) halides are insoluble
Calcium, strontium(Sr), barium and lead(II)
sulfate are insoluble
Exceptions
Group 1 metals and ammonium compounds
are soluble
Hydroxides
Group 1 metals compounds are soluble and
calcium, Sr2+ and Ba2+ are slightly soluble
Sulfides
group 1 and group 2 metals and
ammonium compounds are soluble
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E.g. Predict whether a reaction will occur in each of the following case. If so, write a net
ionic equation for the reaction. If no reaction occurs, write NR after arrow.
(a) Al2(SO4)3 + NaOH
Neutralization Reactions
When a base is added to an acid solution, the acid is said to be neutralized. In a
neutralization reaction, an acid and a base react to form water and an aqueous solution of
an ionic compound called a salt.
A neutralization reaction:
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In a neutralization involving the weak base such as NH3
In a neutralization involving the weak acid such as acetic acid CH3COOH
Reactions with Gases FormationSome reactions are driven to products by the
formation of a gas.
E.g carbonates react with acids to produce gases product.