Oxidation – Reduction (Redox) Reactions – Balancing Redox Reactions

Redox Reactions – Balancing Redox Reactions

In many important chemical reactions, electrons are transferred from atom to atom. Such reactions are called either oxidation – reduction reactions or redox reactions. Many important chemical reactions involve oxidation and reduction. In fact, most reactions used for energy production are redox reactions. In living species the oxidation of sugars, fats, and proteins provides the energy necessary for life. The electric current from the batteries that power computers and cell phones is generated by oxidation-reduction reactions.  One of the most familiar redox reactions is corrosion of a metal. Corrosion is the conversion of a metal into a metal compound by a reaction between the metal and some substance in its environment. When a metal corrodes, each metal atom loses electrons and a cation is formed, which can combine with an anion to form an ionic compound.

The bright metallic surface of calcium tarnishes as CaO forms:

 

2Ca (s)  + O2  (g)   →   2 CaO (s)

 

In the above reaction Ca is oxidized to Ca+2 and O2 which is neutral becomes O-2 is reduced.

As a matter of fact the old definition for oxidation and reduction was:

Oxidation is the reaction of an element with oxygen (O2) to form an oxygen-containing compound and reduction is the removal of oxygen from a compound

For example:

 

C  + O2   →   CO2    (oxidation, combined with O)

 

ZnO +  C  →   Zn + CO (reduction, loss of O)

Also,

Oxidation is called the removal of hydrogen from a compound and reduction the reaction with (H2) to form a hydrogen-containing compound

In all the above (redox reactions) where an element reacts with O2 or H2, there is some kind of electron transfer from one atom to another. The fact, that the first redox reactions studied by chemists were reactions with O2, gave the name oxidation to this process.

However, there are other reactions where electron(-s) are transferred –the reaction is a redox - but there is no O2 or H2 in the reactants or in the products (Fig. I.2).

For example,

 

 

 

 

 

 

 

 

A more general definition for oxidation and reduction was proposed to include reactions like one shown in Fig. I.2 where O2 or H2 is not a reactant or product.

Oxidation is called the loss of electrons by an element or compound

Reduction is called the gain of electrons by an element or compound

There are though reactions where there is no complete transfer of electron(-s) from one element to another (as in the case of Na and Cl, Fig. I.2) but a partial transfer of electrons to the more electronegative element. Such a reaction is between Η2 and Br2 (Fig. I.1). The bond formed between H and Br atoms is a polar bond with a partial negative charge on Br atoms and a partial positive charge on H atoms (Fig. I.3)

 

Becauce the reaction of Na(s) and Cl(g) and that of H2(g) and Br2(g) are so much alike, chemists have expanded the definition of oxidation - reduction reactions to include partial as well as a complete transfer of electrons.

Oxidation is defined as the complete or partial loss of electrons by an  element or compound

Reduction is defined as the complete or partial gain of electrons by an element or compound

The Na atom (Fig. I.2) when it reacts with Cl2 is oxidized to Na+ and the Cl atom is reduced. Because the Na atom makes it possible for the Cl atom to be reduced is called a reducing agent.

The Cl atom (Fig. I.2) when it reacts with Na is reduced to Cl-1. and the Na atom is oxidized. Because the Cl atom makes it possible for the Na atom to be oxidized is called an oxidizing agent.

However, by simply reading a chemical reaction does not always tell us whether oxidation or reduction has occurred. Chemists have developed a numerical system to help identify a reaction as redox. The first step in this system is to assign a number called oxidation number to each atom in the reaction. The concept of oxidation numbers (also called oxidation states) was devised as a way of keeping track of electrons gained by the substance being reduced and electrons lost by the substance being oxidized.

The general steps for assigning oxidation numbers to atoms in a molecule or ion are the following:

The oxidation number for each atom in a pure element is zero
The oxidation number of a monoatomic ion is equal to its charge
When H atoms are combined with nonmetals their oxidation number is +1 (i.e. ΗCl, NH3, H2O, CH4…). When H atoms are combined with metals their oxidation number is -1 (i.e. NaH)
The oxidation number of O is usually -2 (i.e. CO, CO2, SO2). The major exception is peroxides where its oxidation number is -1 (i.e. Η2Ο2) and OF2 where its oxidation number is +2
When fluorine atoms are combined with atoms of other elements, their oxidation number is -1

If a compound’s formula contains one element for which an oxidation number cannot be assigned using the above steps, the oxidation number is calculated as follows:

The sum of the oxidation numbers of all atoms in a neutral compound is zero.
The sum of the oxidation numbers for the atoms in a polyatomic ion is equal to the overall charge on the ion

By using the concept of oxidation number a new definition for oxidation and reduction was proposed:

Oxidation is a reaction where an increase in the oxidation number of an element occurs

Reduction is a reaction where a decrease in the oxidation number of an element occurs

 

Example #1

Calculate the oxidation number of the atoms in: α) CO₂   b) SF₆ 

α)

Step 1: Find the most electronegative atom

The most electronegative atom in CO₂ is the O atom

 

Step 2: The oxidation number of the most electronegative atom is equal to the charge of its ion

For O is O^-2 and therefore the oxidation number is -2

 

Step 3: Oxidation numbers that are not known have to be calculated.

The oxidation number of C in CO₂ is unknown and let us suppose that is x

Then since CO₂ is neutral the sum of the oxidation numbers must be equal to 0

 

CO₂

x + 2 * (-2) = x – 4 = 0

x = 4

Therefore the oxidation number of C in CO₂ is equal to +4

 

b)

Step 1: Find the most electronegative atom

The most electronegative atom in SF₆ is the F atom

 

Step 2: The oxidation number of the most electronegative atom is equal to the charge of its ion

For F is F^-1 and therefore the oxidation number is -1

 

Step 3: Oxidation numbers that are not known have to be calculated.

The oxidation number of S in SF₆ is unknown and let us suppose that is x

Then since SF₆ is neutral the sum of the oxidation numbers must be equal to 0

SF₆

x + 6 * (-1) = x – 6 = 0

x = 6

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References

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Key Terms

oxidation, reduction, redox, oxidation number, balancing redox reactions,

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