Download The five main types of redox reactions are combination

The five main types of redox reactions are combination, decomposition,
displacement, combustion, and disproportionation.
LEARNING OBJECTIVES [ edit ]
Explain the processes involved in a redox reaction and describe what happens to their various
components.
Recognize the different types of redox reactions from their chemical equations.
KEY POINTS [ edit ]
In combination reactions, two elements are combined. A + B => AB
.
In decomposition reactions, a compound is broken down into its constituent parts. AB => A + B
.
In displacement reactions, one or more atoms is swapped out for another. AB + C => A + CB
.
In combustion reactions, a compound reacts with oxygen to produce carbon dioxide, water,
and heat.
In disproportionation reactions, a molecule may be simultaneously reduced and oxidized; these
types of reactions are rare.
TERMS [ edit ]
redox
a shorthand term for "reduction­oxidation," two methods ofelectron transfer that always occur
together
combustion
a process in which a fuel combines with oxygen, usually at high temperature, releasing CO2, H2O,
and heat
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Redox reactions are all around us. In fact, much of our technology, from fire to
laptop batteries, is largely based on redox reactions. Redox (reduction­oxidation) reactions
are those in which the oxidation states of the reactants change. This occurs because in such
reactions, electrons are always transferred between species. Redox reactions take place
through either a simple process, such as the burning of carbon in oxygen to yield carbon
dioxide (CO2), or a more complex process such as the oxidation of glucose (C6H12O6) in the
human body through a series of electron transfer processes.
The term "redox" comes from two concepts involved with electron transfer: reduction and
oxidation. These processes can be explained in simple terms:
Oxidation is the loss of electrons or an increase in oxidation state by a molecule, atom,
or ion.
Reduction is the gain of electrons or a decrease in oxidation state by a molecule, atom, or
ion.
A simple mnemonic for remembering these processes is "OIL RIG"—Oxidation Is Losing
(electrons), Reduction Is Gaining (electrons).
Redox reactions are matched sets: if one species is oxidized in a reaction, another must be
reduced. Keep this in mind as we look at the five main types of redox reactions: combination,
decomposition, displacement, combustion, and disproportion.
Combination
Combination reactions "combine" elements to form a chemical compound. As usual,
oxidation and reduction occur together.
General equation: A + B → AB
Sample 1. equation: 2 H2 + O2 → 2 H2O
The sum of oxidation states in the reactants is equal to that in the products: 0 + 0 → (2)(+1)
+ (­2)
In this equation, both H2 and O2 are free elements and their oxidation states are 0. The
product is H the oxidation state is ­2 for oxygen and +1 for hydrogen.
Decomposition
Decomposition reactions are the reverse of combination reactions, meaning they are the
breakdown of a chemical compound into its component elements.
General equation: AB → A + B
Sample 2. equation: 2 H2O → 2 H2 + O2
Calculation: (2)(+1) + (­2) = 0 → 0 + 0
In this equation, the water is "decomposed" into hydrogen and oxygen, both of which are
neutral. Similar to the previous example, H2O has a total oxidation state of 0, with each H
taking on a +1 state and the O a ­2; thus, decomposition oxidizes oxygen from ­2 to 0 and
reduces hydrogen from +1 to 0.
Displacement
Displacement reactions, also known as replacement reactions, involve compounds and the
"replacing" of elements. They occur as single and double replacement reactions.
General equation (single displacement): A + BC → AB + CA
A single replacement reaction "replaces" an element in the reactants with another element in
the products.
Sample 3. equation: Cl2 + 2 NaBr → 2 NaCl + Br2
Calculation: (0) + ((+1) + (­1) = 0) ­> ((+1) + (­1) = (0) + 0
In this equation, Cl is reduced and replaces Br, while Br is oxidized.
General equation (double displacement): AB + CD → AD + CB
A double replacement reaction is similar to a double replacement reaction, but involves
"replacing" two elements in the reactants with two in the products.
Sample 4. equation: Fe2O3 + 6 HCl → 2 FeCl3 + 3 H
In this equation, Fe and H as well as O and Cl trade places.
Combustion
Combustion reactions always involve oxygen and an organic fuel. In the following image, we
see methane combusting to release energy.
Redox reaction of methane
This is a basic example of a combustion reaction.
General equation: CxHy + O2 → CO2 + H2O
Disproportionation
In some redox reactions, substances can be both oxidized and reduced. These are known as
disproportionation reactions. One real­life example of such a process is the reaction of
hydrogen peroxide, H2O2, when it is poured over a wound. At first, this might look like a
simple decomposition reaction, because hydrogen peroxide breaks down to produce oxygen
and water:
2 H2O2(aq) → 2 H2O(l) + O2(g)
The key to this reaction lies in the oxidation states of oxygen, however. Notice that oxygen is
present in the reactant andboth products. In H2O2, oxygen has an oxidation state of ­1. In
H2O, its oxidation state is ­2, and it has been reduced. In O2 however, its oxidation state is 0,
and it has been oxidized. Oxygen has been both oxidized and reduced in the reaction, making
this a disproportionation reaction. The general form for this reaction is as follows: however,
its oxidation state is 0, and it has been oxidized. Oxygen has been both oxidized and reduced
in the reaction, making this a disproportionation reaction. The general form for this reaction
is as follows:
2A → A' + A"