What are redox reactions?

We explain what redox reactions are, the types that exist, their applications, characteristics and examples of redox reactions.

What are redox reactions?

In chemistry , it is known as redox reactions, oxide-reduction reactions or reduction-oxidation reactions, to any chemical reaction in which an electron exchange occurs between the atoms or molecules involved.

That exchange is reflected in the change in oxidation state of the reagents. The reagent that yields electrons undergoes oxidation and the one that receives them reduces them.

The oxidation state is the tendency of an atom of an element to yield or take electrons when it is part of a chemical reaction. It is also called oxidation numbers or valences .

It is expressed in whole numbers around zero (for neutral elements), being able to reach levels of +1, +2 or +6, for example, or on the contrary, -1, -2 or -6. Some atoms also have varying oxidation states depending on the reaction.

Thus, in every redox reaction there are two types of reagents that influence each other:

  • An oxidizing agent , which picks up electrons and lowers their initial oxidation state, thus suffering a reduction. Or what is the same, increases its negative electromagnetic charge to gain electrons.
  • reducing agent , which yields electrons and increases their initial oxidation state, thus suffering oxidation. Or what is the same, increases its positive electromagnetic charge by losing electrons.

Only in some cases certain reagents (ampholytes) can be oxidized and at the same time reduced, in what is known as ampholization. Of the rest, redox reactions are one of the most common chemical reactions in the universe and are part of the fundamental reactions for the continuity of life.

Rules to know the State of Oxidation

The main rules to know the oxidation state ( EO ) of the elements in a Redox reaction are:

  • Simple uncombined elements is 0 
  • The sum of the oxidation states of a neutral molecule is 0
  • Combined hydrogen is +1 except for metal hydrides with -1
  • Oxygen is -2 except for peroxides with -1
  • Combined metals are always positive with the value of the ion charge

Characteristics of redox reactions

Redox reactions are around us daily. The oxidation of metals, the combustion of gas in the kitchen or even the oxidation of glucose to obtain ATP in our body are examples of this.

In most cases, they involve a significant amount of energy released , as well as a permanent change in the chemical nature of the materials involved.

The end result of a redox reaction is usually different from the initial compounds , both in their combinatorial and electromagnetic charge level. This process is often understood from “semi-reactions” or partial reactions, which are a way of segmenting the global redox process in two: the reduction process and the oxidation process.

For example:

  • Reduction semireaction : Cu 2+ + 2e  -> Cu (reduction of copper when receiving two electrons).
  • Semi-reaction of oxidation : Fe -> Fe 2+ + 2e  (oxidation of iron when losing two electrons)

Which makes up the global reaction Fe + Cu 2+ -> Fe 2+ + Cu.

Types of redox reactions

redox reactions oxidation reduction example engine combustion
Redox combustion reactions release energy that can create movement.

There are different types of redox reactions, with different characteristics. The most common types are:

  • Combustion . All forms of combustion, from gasoline in a car engine or gas in our kitchens, are redox reactions that involve a compound (fuel) and oxygen, and that release a great deal of energy , be it heat , light or movement (as in the case of explosions). Logically, oxygen acts as an oxidizing agent, removing electrons from the compound.
  • Oxidation . Oxidation is commonly described as the degradation of certain materials, especially metal , by the action of oxygen on them. It is a worldwide known and everyday phenomenon, especially in coastal towns, where salts from the environment accelerate (catalyze) the reaction. That is why a car, after taking us to the beach, must be cleaned of any trace of salt water.
  • Disproportion . Also known as dismutation reactions, they have a single reagent that reduces and oxidizes at the same time, as their molecules act with each other. A typical case of this is the decomposition of hydrogen peroxide (H 2 O 2 ).
  • Simple displacement . Also called simple substitution reactions, it occurs when two elements exchange their respective places within the same compound. That is, one element replaces another in its exact place in the formula, balancing its respective electromagnetic charges with other atoms as appropriate. This is what happens when a metal displaces hydrogen in an acid and salts form, as happens when the batteries of an apparatus break down.

Examples of redox reactions

Examples of redox reactions are very abundant. We will try to give an example of each of the types described above:

  • The combustion of octane , a hydrocarbon component of gasoline, takes place in the engine of our cars. This occurs as oxygen and octane react, oxidizing and reducing respectively, releasing energy used to generate work in the engine, and byproducting carbon dioxide and water vapor in the process. All according to the following formula:

2C 8 H 18 + 25O 2 -> 16CO 2 + 18H 2 O + E (energy)

  • The decomposition of hydrogen peroxide , also called hydrogen peroxide, occurs when in the presence of water and oxygen, its own constituent atoms, the hydrogen peroxide loses its oxidation state -1 and ends up becoming simply water and elemental oxygen, recovering an electromagnetic state neutral. This occurs according to the formula:

2H 2 O 2 -> 2H 2 O + O 2

  • Displacement of silver by copper , the classic example verifiable with an aqueous solution of silver nitrate and a piece of copper. When the latter is immersed in the solution, we will immediately notice a color change (going towards blue) and the formation around copper of small metal crystals (precipitate). This will be indicative that you have been replacing the silver in the solution, according to the following formula:

Cu + 2AgNO 3 -> Cu (NO 3 ) 2 + 2Ag

Examples of Adjustment of Redox Reactions:

  • Zn + AgN O 3 → Zn (N O 3 ) 2 + Ag
    • Oxidation states :
      • Reduction: Ag changes from oxidation state + 1 to 0
      • Oxidation: Zn goes from oxidation state 0 to +2
    • Semi-feedback :
      • Zn → Zn +2 + 2e 
      • 2 Ag + + 2 e  → 2Ag
    • Global reaction :
      • Zn + 2 Ag + + 2 e  → Zn +2 + 2Ag + 2e 
    • Adjusted reaction :
      • Zn + 2 Ag N O 3  →  Zn (N O 3 ) 2 + 2Ag

Balancing a redox reaction

HNO 3 + Sn + H 2 0 -> H 2 Sn0 3 + NO

Step 1. Put the oxidation numbers (All the hydrogens that are accompanied and at the beginning are worth 1. All the oxygen accompanied and at the end are worth 2. The elements that are alone are worth 0).

+1 N +5 O -2 + Sn 0 + H 2 + 1 0 -2 -> H +1 Sn +4 0 -2 + N +2 O -2

Step 2. We are going to review which elements were reduced and which ones increased

+5 -> N +2   (3) Reduction

Sn 0 -> Sn +4   (4) Oxidation

Step 3. We cross the numbers

4 N -> 4 N

3 Sn-> 3 Sn

Step 4. Put the numbers in the equation to balance it.

4 HNO 3 + 3 Sn + H 2 0 -> 3 H 2 Sn0 3 + 4 NO

Step 5 . We list the elements to see the amount of elements that exist on each side of the formula, preferably hydrogen and oxygen last.

4 N 4

3 Sn 3

6 H 6

13 or 13

Answer = 4 HNO 3 + 3 Sn + 1 H 2 0 -> 3 H 2 Sn0 3 + 4 NO

  1. Industrial applications

redox reactions reduction oxidation power generator
In power plants, redox reactions manage to move large motors.

The industrial applications of redox reactions are endless. For example, combustion reactions are ideal for producing work , that is, movement or electricity: this is what power plants that burn coal do to obtain heat and evaporate water, for example. It also occurs within the engine of fossil fuel vehicles .

On the other hand, redox substitution and displacement reactions are useful for obtaining certain elements in a state of purity that is not often seen in nature.

For example, silver is highly reactive. Although it is rare to find it pure in the mineral subsoil, a high degree of purity can be obtained through a redox reaction. The same happens when obtaining salts and other compounds.

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