10.3 Types of Chemical Reactions
chemical
equations
follow certain
patterns
The number of possible chemical reactions is very large. However, there is a limited
number of structural patterns that every chemical reaction may fall into. By learning
these patters we will be able to better understand these chemical reactions.
These patterns can be best seen by considering the following four classifications of
chemical reactions:
1. Combination reactions (Synthesis reactions)
2. Decomposition reactions
3.
Single replacement reactions
4. Double replacement reactions (metathesis reactions)
We will now describe the structure of each one of these classes or reactions.
Combination
reactions
(Synthesis
reactions)
A reaction in which two substances combine to form a third substance is called a
combination or synthesis reaction. The general form of a combination reaction is
Synthesis of
compounds
from elements
This symbolic form shows substances A and B combining to form a new substance AB.
The substances A and B can be two compounds, two elements or an element and a
compound. The formation of water from hydrogen and oxygen is a synthesis reaction that
combines two pure elements to form a compound.
2H2(g) + O2(g) → 2H2O (g)
Synthesis of
compounds
from
compounds
Another example of a synthesis reaction combines magnesium oxide (MgO) with water
to make magnesium hydroxide (Mg(OH)2), also known an milk of magnesia. In this
example, two compounds combine to form a third compound.
Synthesis reactions combine different substances therefore the number of products is less
than the number of reactants.
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Section 10.3 Types of Chemical Reactions
Decomposition and single displacement reaction
Decomposition
is the opposite
of synthesis
A decomposition reaction is the opposite of a synthesis reaction. In a decomposition
reaction a single substance breaks apart (decomposes) to form two or more new
substances. The new substances may be elements or compounds.
Decomposition
of limestone
Calcium oxide (CaO), also called lime, has many uses including
water treatment, glass manufacturing, food preservation and
cement manufacturing. Lime is produced from the
decomposition of calcium carbonate (CaCO3) in limestone. The
reaction occurs when limestone is heated to 825°C.
CaCO3(s) → CaO(s) + CO2(g)
Another important reaction is the decomposition of water which happens when electrical
current passes through it.
2H2O(l) → 2H2(g) + O2(g)
Decomposition
usually requires
energy
Most decomposition reactions require the addition of some type of external energy to
proceed. The energy can be in the form of heat or electrical current as in the previous
examples. Energy can also be in the form of light as in the case of the destruction of
ozone (O3) in the upper atmosphere.
O3 + light → O2 + O
Single
displacement
reactions
A single displacement reaction happens when an element reacts with a compound
replacing an element in it. The general form of a displacement reaction is
Examples of
displacement
reactions
The element A replaces the B part of the compound BD. It could also replace the D part
of the compound BD. A single displacement reaction occurs when zinc (Zn) or iron (Fe)
are immersed in a solution of copper sulfate (CuSO4)
Zn(s) + CuSO4(aq) → Cu(s) + ZnSO4(aq)
Fe(s) + CuSO4(aq) → Cu(s) + FeSO4(aq)
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Double replacement reactions (precipitate reactions)
Ions “trade”
partners
Precipitate reactions often involve the exchange of parts of the reactants. The general
form of this type of reaction is
Ions “trade
partners” in
double
displacement
Here we see that parts B and X , which represent the negative ions, of the compounds AB
and DX exchange places giving us the new compounds AX and DB. Basically the
positive and negative ions “trade partners.”
Precipitate
reaction
A good example is the reaction between lead nitrate (Pb(NO3)2) and potassium iodide
(KI). The pictures show a 0.1 M solution of lead nitrate, and a 0.1 M solution of
potassium iodide. Look carefully at their colors. Notice that they are both essentially
clear and colorless. When Pb(NO3)2 is added notice the brilliant yellow color! The
yellow comes from an insoluble precipitate (PbI2) which is a product of the reaction.
The chemical
equation for
the reaction
The chemical reaction shows that the lead ion displaces the potassium ion in KI. The
potassium ion in turn displaces the lead ion in Pb(NO3)2.
Double replacement or precipitate reactions are one very common reaction type that take
place in aqueous solutions. Recall that a precipitate is the formation of an insoluble solid
which forms when certain aqueous solutions are mixed. The precipitate separates from
the solution and can be easily observed.
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Section 10.3 Types of Chemical Reactions
Precipitates and solubility
The nitrate
anion
Before we look why a precipitate forms, consider the polyatomic
nitrate ion (NO3-). The nitrate ion acts as a “unit” so the nitrogen
atom and three oxygen atoms remain covalently bonded together.
The negative charge is the charge of the whole nitrate ion. A helpful
comparison is to treat nitrate ion as if it were a monatomic ion Cl-.
When a
precipitate
forms
A precipitate forms when one of the products of a reaction is insoluble in water. To be
able to predict which ionic compound is the solid precipitate we need refer to a set of
solubility rules. The rules summarize which compounds are soluble and which are not.
TABLE 10.1. Solubility Rules for Common Ionic Compounds in Water
Soluble Compounds
Insoluble Compounds
(except when with group I metal ions and NH4+)
Group I metal ions:
Li+, Na+, K+, Rb+, Cs+
Using the
solubility table
Carbonates,CO32-
Hydroxides, OH-, (ex Ba2+)
Ammonium: NH4+
Chlorides of Cu, Pb, Ag, and Hg
C2H3O2-
Bromides of Cu, Pb, Ag, and Hg
Nitrates, NO3-
Iodides of Cu, Pb, Ag, and Hg
SO42-
Sulfides, S2-,
Chlorides, Bromides, and Iodides
except with Cu, Pb, Ag, and Hg
Phosphates, PO43-
The balanced chemical equation for the reaction between lead nitrate and potassium
iodide is given below.
Note from the table that nitrates are typically soluble. Therefore we expect potassium
nitrate to be soluble, and it is. The table also says that iodides are generally soluble
except for compounds with copper (Cu), lead (Pb), silver (Ag), and mercury (Hg).
Potassium iodide is soluble. Lead iodide is insoluble, and forms a precipitate.
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Precipitate reaction uses and analysis
“Hard” water
Precipitate reactions are important to our environment and drinking water supplies.
Minerals such as Mg2+ and Ca2+ leach into our water supplies and cause a phenomena
known as water “hardness”. You have experienced water hardness if you ever been in the
shower and the soap did not lather well. “Hard” water leaves your skin feeling sort of
sticky and filmy, because soaps and oils do not come off as easily. Iron, Fe2+ and
manganese, Mn2+ also cause some other problems, such as staining of clothing and
increased bacterial growth.
Water
softening
To condition (or soften) the water in areas
where there are lots of dissolved minerals,
water treatment plants add special ions that
precipitate out some of the Mg2+ and Ca2+
ions. Hydroxide, OH- and carbonates, CO32are added to the water supply to precipitate
out large amounts of these contaminating
ions. For example Mg2+ + OH- forms
Mg(OH)2 (s) and this solid settles to the
bottom and is easily removed by filtering.
Gas forming
reactions
Some double replacement reactions involve the production of a gas. For example when
magnesium carbonate, MgCO3, and hydrochloric acid, HCl, are mixed:
MgCO3(s) + 2HCl(aq) → MgCl2(aq) + H2O(l) + CO2(g)
In this reaction, Mg2+ and H+ ions exchange places resulting in magnesium chloride,
MgCl2 and H2O and carbon dioxide, CO2 is evolved.
Predict what happens when we mix a solution of silver nitrate (AgNO3) with a
solution of sodium chloride (NaCl). Follow the steps and write the complete
balanced reaction.
Asked:
What is the reaction that represents the mixing of AgNO3 and NaCl?
Given:
formulas for both reactant solutions
Relationships: Solubility Rules tell us that group I metal ions (here Na+) and
nitrates are soluble, but chlorides of silver are insoluble.
Solve:
AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)
According to the solubility table, sodium nitrate is soluble and
dissociates to Na+(aq) and NO3-(aq).
Silver chloride is insoluble and forms a precipitate.
Discussion:
We know the precipitate is AgCl because it does not have a group I
metal ion or nitrate. All ions are balanced.
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Polymerization reactions
Polymers are
long chains of
repeated units
Many important molecules in both technology and biology are polymers. The word is a
clue to how a polymer is made. The prefix “poly” means “many” and the Greek word
“polymeres” means “of many parts”. A polymer is a molecule made of many identical
parts called monomers. A good example of a naturally occurring polymer is starch.
Starch is a long chain molecule, built from repeated addition of sugar molecules.
Polymerization
Photosynthesis in plants produces glucose. However, glucose is also rapidly used to
release energy, even by plants. To save chemical energy for long term use, plants convert
glucose into starch by polymerization. Polymerization is a repeated addition reaction
that assembles a polymer from smaller molecules. Anyone who likes fresh corn has
tasted the difference between sugar and starch. Fresh picked ripe corn has a high sugar
content. Over a period of a day or two, enzyme reactions in the corn convert the sugar to
starch. Starch does not taste sweet and the polymerization reaction is responsible for the
“old corn” taste.
Dehydration
synthesis
The polymerization reaction that produces starch is called dehydration synthesis. Look
carefully and notice that glucose has the chemical formula C6H12O6 while the monomer
for starch is C6H10O5. The difference is one water molecule. To dehydrate means to
“take away water” and to synthesize means “to put together.” Dehydration synthesis
builds up the starch molecule by removing an oxygen and two hydrogens from each
glucose molecule that gets added to the chain. The bond between adjacent monomers is
through the oxygen.
Starch is the staple carbohydrate of many animal diets,
including humans! Bread, pasta, and all grains are
mostly starches. The body reverses the synthesis
process and digests starch back into glucose.
Starch is a
staple food
polymer - a molecule built up from many repeating units of a smaller molecular
fragment.
polymerization - a reaction that assembles a polymer through repeated additions of
smaller molecular fragments.
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