Chemical reactions can be divided into several classes each having similar characteristics.

These
different types of reactions will be discussed in greater detail throughout the book. You will find
that almost every reaction you see can fall into one of these categories, so make sure that you
understand them.

Contents
[hide]

 1 Synthesis Reactions
 2 Decomposition Reactions
 3 Single Replacement Reactions
 4 Double Replacement Reactions
o 4.1 Precipitation
o 4.2 Acid-Base Neutralization
 5 Combustion
 6 Organic Reactions
 7 Redox

Synthesis Reactions
Helpful Hint!

Synthesis reactions always yield one product.

Reversing a synthesis reaction will give you a
decomposition reaction.

The general form of a synthesis reaction is A + B → AB. Synthesis reactions "put things
together".

This is the most well-known example of a synthesis

reaction—the formation of water via the fusion of
hydrogen gas and oxygen gas.

Another example of a synthesis reaction is the formation of

sodium chloride (table salt).

Because of the very high reactivities of sodium metal and chlorine gas, this reaction releases a
tremendous amount of heat and light energy. Recall that atoms release energy as they become
stable, and consider the octet rule when determining why this reaction is so favorable.
[edit] Decomposition Reactions
These are the opposite of synthesis reactions, with the format AB → A + B. Decomposition
reactions "take things apart". Just as synthesis reactions can only form one product,
decomposition reactions can only start with one reactant. Compounds that are unstable will
decompose quickly without outside assistance.

One example is the electrolysis of water (passing water

through electrical current) to form hydrogen gas and
oxygen gas.

Hydrogen peroxide slowly decomposes into water and

oxygen because it is somewhat unstable. The process is
sped up by the energy from light, so hydrogen
peroxide is stored in dark containers to slow down the
decomposition.

Carbonic acid is the carbonation that is dissolved in

soda. It decomposes into carbon dioxide and water,
which is why an opened drink will lose its fizz.
Decomposition, aside from happening spontaneously in unstable
compounds, occurs under three conditions: thermal, electrolytic, and
catalytic. Thermal decomposition occurs when a substance is heated.
Electrolytic decomposition, as shown above, is the result of an electric
current. Catalytic decomposition happens because a catalyst breaks
apart a substance.

[edit] Single Replacement Reactions

Single replacement reactions, also called single displacement, swap one component with another,
in the format AB + C → AC + B.

Adding hydrochloric acid to zinc will cause a gas to bubble out:

[edit] Double Replacement Reactions

In these reactions, two compounds swap components, in the format AB + CD → AD + CB.

[edit] Precipitation
 Wikipedia has related
information at
Precipitation (chemistry)

A precipitation reaction occurs when an ionic substance comes out of solution and forms an
insoluble (or slightly soluble) solid. The solid which comes out of solution is called a
precipitate. This can occur when two soluble salts (ionic compounds) are mixed and form an
insoluble one—the precipitate.

An example
is lead
nitrate
mixed with
potassium
iodide,
which forms
a bright
yellow
precipitate
of lead
iodide.

Note that the

lead iodide
is formed as
a solid. The
previous
equation is
written in
molecular
form, which
is not the
best way of
describing
the reaction.
Each of the
elements
really exist
in solution
as individual
ions, not
bonded to
each other
(as in
 potassium
iodide
crystals). If
we write the
above as an
ionic
equation,
we get a
much better
idea of what
is actually
happening.

Notice the
like terms
on both
sides of the
equation.
These are
called
spectator
ions because
they do not
participate
in the
reaction.
The can be
ignored, and
the net ionic
equation is
written.

In the solution, there exists both lead and iodide ions. Because lead iodide is insoluble, they
spontaneously crystallise and form the precipitate.

[edit] Acid-Base Neutralization

In simple terms, an acid is a substance which can lose a H+ ion (i.e. a proton) and a base is a
substance which can accept a proton. When equal amounts of an acid and base react, they
neutralize each other, forming species which aren't as acidic or basic.

For example, when hydrochloric acid

and sodium hydroxide react, they form
water and sodium chloride (table salt).
 Again, we get a clearer picture of what's
happening if we write a net ionic
equation.

Acid base reactions often happen in aqueous solution, but they can also occur in the gaseous
state. Acids and bases will be discussed in much greater detail in the acids and bases section.

[edit] Combustion

The combustion of methane (releasing heat and light)

Combustion, better known as burning, is the combination of a substance with oxygen. The
products are carbon dioxide, water, and possible other waste products. Combustion reactions
release large amounts of heat. C3H8, better known as propane, undergoes combustion. The
balanced equation is:

Combustion is similar to a decomposition reaction, except that oxygen and heat are required for
it to occur. If there is not enough oxygen, the reaction may not occur. Sometimes, with limited
oxygen, the reaction will occur, but it produces carbon monoxide (CO) or even soot. In that case,
it is called incomplete combustion. If the substances being burned contain atoms other than
carbon, hydrogen, and oxygen, then waste products will also form. Coal is burned for heating
and energy purposes, and it contains sulfur. As a result, sulfur dioxide is released, which is a
pollutant. Coal with lower sulfur content is more desirable, but more expensive, because it will
release less of the sulfur-based pollutants.

[edit] Organic Reactions

This is carboxylic acid. All functional groups end with an "R"—a placeholder for the rest of the
molecule.

Organic reactions occur between organic molecules (molecules containing carbon and
hydrogen). Since there is a virtually unlimited number of organic molecules, the scope of organic
reactions is very large. However, many of the characteristics of organic molecules are
determined by functional groups—small groups of atoms that react in predictable ways.

Another key concept in organic reactions is Lewis basicity. Parts of organic molecules can be
electrophillic (electron-loving) or nucleophillic (nucleus, or positive loving). Nucleophillic
regions have an excess of electrons—they act as Lewis bases—whereas electrophillic areas are
electron deficient and act as Lewis acids. The nucleophillic and electrophillic regions attract and
react with each other (needless to say, this has inspired many terrible organic chemistry jokes).

Organic reactions are beyond the scope of this book, and are covered in more detail in Organic
Chemistry. However, most organic substances can undergo replacement reactions and
combustion reactions, as you have already learned.

[edit] Redox

The formation of hydrogen fluoride causes fluoride to oxidize and hydrogen to reduce.

Redox is an abbreviation of reduction/oxidation reactions. This is exactly what happens in a

redox reaction, one species is reduced and another is oxidized. Reduction involves a gain of
electrons and oxidation involves a loss, so a redox reaction is one in which electrons are
transferred between species. Reactions where something is "burnt" (burning means being
oxidised) are examples of redox reactions, however, oxidation reactions also occur in solution,
which is very useful and forms the basis of electrochemistry.

Redox reactions are often written as two half-reactions showing the reduction and oxidation
processes separately. These half-reactions are balanced (by multiplying each by a coefficient)
and added together to form the full equation. When magnesium is burnt in oxygen, it loses
electrons (it is oxidised). Conversely, the oxygen gains electrons from the magnesium (it is
reduced).

Redox reactions will be discussed in greater detail in the redox section