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Reduction Reaction

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Reduction is the reverse of oxidation - the addition of hydrogen or electrons or the removal of oxygen are all reduction reactions. The modern definition for oxidation is that oxidation takes place when a substance loses electrons.

Anytime oxidation takes place and a substance loses one or more electrons another substance must gain the electron. When a substance gains one or more electrons, the process is known as reduction.

Reactions that involve the transfer of one or more electrons always involve both oxidation and reduction. These reactions are known as oxidation-reduction or redox reactions.

Reduction Reaction Definition

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Reduction is the part of an oxidation-reduction reaction in which an atom gains an electron. The name also implies that in any reaction in which oxidation occurs, reduction must also take place. One cannot take place without the other.

Reduction reaction is defined as "the gain of electrons by any chemical species or a reaction that removes oxygen from a compound is called reduction."

Reduction potential definition of a chemical system is a measure of the tendency for reduction reactions to occur. Reduction definition chemistry explains reduction as the gain of electrons.

Reduction Reaction Example

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All the reduction reaction are written in the opposite direction and the net oxidation reduction reaction is the sum of the appropriate reduction and oxidation half-reaction.

Some of the examples of reduction reactions are given below.
  1. Ag+ + e- $\rightarrow$ Ag(s)
  2. Zn2+ + 2e- $\rightarrow$ Zn(s)
  3. Cu2+ + 2e- $\rightarrow$ Cu(s)
  4. PbO(s) + H2(g) $\rightarrow$ Pb(s) + H2O(l)
  5. Cl2(aq) + 2e- $\rightarrow$ 2Cl-(aq)

Reduction Half Reaction

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Half reactions are equations that have electrons as reactants or products. One half reaction represents a reduction process and the other half reaction represents an oxidation process. In a reduction half reaction, electrons are shown on the reactant side. In an oxidation half reaction the electrons are shown on the product side.

A half reaction is a balanced chemical equation that describes either the oxidation or reduction but not both. Thus a half reaction describes half of the redox reaction.

The reduction half reaction chart is given below.

Reduction Half Reaction Chart

We can write the reduction and the oxidation reactions separately, as half reactions; the sum of the half-reactions equals the net oxidation-reduction reaction.

Reduction half reactions examples include
  1. Co3+ + e- $\rightarrow$ Co2+
  2. (IO3)- + 6H+ + 5e- $\rightarrow$ 1/2 I2(s) + 3H2O
  3. Cl2(g) + 2e- $\rightarrow$ 2Cl-
  4. Ag+ + e- $\rightarrow$ Ag(s)

In the direction of an oxidation reaction is reversed, it becomes a reduction reaction; that is if Al3+ accepts 3 electrons, it is reduced to Al(s). All of the reduction reactions are oxidation reactions if they are written in the opposite direction.

Oxygen Reduction Reaction

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Oxygen is the most abundant element in the Earth's crust. The oxygen reduction reaction (ORR) is also the most important reaction in life processes such as biological respiration, and in energy converting systems such as fuel cells. Oxygen reduction reaction in aqueous solutions occurs mainly by two pathways.
  1. The direct 4-electron reduction pathway from O2 to H2O and the 2-electron reduction pathway from O2 to hydrogen peroxide (H2O2).
  2. In non-aqueous aprotic solvent and in alkaline solutions, the 1-electron reduction pathway from O2 to super oxide (O2) can also occur.

Oxidation reduction potential (O/R potential)

It has been known for decades that microorganisms display varying degrees of sensitivity to the oxidation-reduction potential of their growth medium. The O/R potential of a substance may be defined generally as the ease with which the substrate loses or gains electrons. When an element or compound loses electrons, the substrate is oxidized, whereas a substrate the gains electrons becomes reduced.

Carbon dioxide reduction

Carbon dioxide is an extremely stable molecule. Carbon dioxide requires the input of energy or a high energy reductant to drive the reaction. Two possible energy sources which can be used in this capacity are electrochemical potential and light.

Carbon dioxide reduction is about the possible future role of such a technique as a means for converting CO2 into value added chemicals.

Sulfate Reduction Reaction

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Sulfate reduction to sulfide is favored at a pH of 7 and Ea of -200 mV. After available dissolved oxygen nitrate and iron (III) are consumed, available sulfate can e used as an electron acceptor. Sulfate reducing microorganisms are sensitive to temperature, inorganic nutrient, pH and oxidation potential.

Small imbalances in environmental conditions can severely limit the rate of BTEX degradation via sulfate reduction.