Oxidation Number-1

OXIDATION NUMBER

An oxidation number is a number that is assigned to an atom to indicate its state of oxidation or reduction during a chemical reaction. Each atom in a redox reaction is assigned an oxidation number to understand its ability to donate, accept, or share electrons. It shows the total number of electrons that have been removed from or added to an element to get to its present state. For example, in Fe2O3, the oxidation number of Fe is +3, and in FeO, it is +2. A loss of electrons corresponds to an increase in oxidation number. On the other hand, a gain of electrons corresponds to a decrease in oxidation number .

Oxidation Number Rules 

In order to assign oxidation numbers to atoms, we need to follow a set of rules.

1. The oxidation number of an element in its free state is zero.

Example: The oxidation number of Zn, Al, H2, O2, and Cl2 is zero

2. The oxidation number of a monatomic ion is the same as the charge on the ion.

Example: The oxidation number of Na+ is +1, Mg2+ is +2, Al3+ is +3, Cl-1 is -1, and O2- is -2.

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

Example: In Fe2O3, the oxidation number of Fe is +3, and that of O is -2. The sum of all oxidation numbers is: +3 x 2 + (-2) x 3 = 0. The result is expected since Fe2O3 is neutral.

4. The oxidation number of an alkali metal in a compound is +1, and the oxidation number of an alkaline earth metal in a compound is +2.

Example: In NaCl, Na is an alkali metal, and its oxidation number is +1. In MgO, Mg is an alkaline earth metal, and its oxidation number is +2.

5. The oxidation number of oxygen in a compound is usually -2. However, if the oxygen is in a category of compounds called peroxides, its oxidation number is -1. If the oxygen is bonded to fluorine, the number is +1 or +2, depending upon the compound.

Example: The oxidation number of O in H2O is -2, in H2O2 is -1, in OF2 is +2, and in O2F2 is +1

6. The oxidation number of hydrogen in a compound is usually +1. In the case of a binary metal hydride, the oxidation number is -1.

Example: The oxidation number of H is +1 in H2O and -1 in NaH.

7. The oxidation number of fluorine is always -1.

Example: The oxidation number of F in NaF is -1.

8. Chlorine, bromine, and iodine usually have an oxidation number of -1 unless bonded to oxygen or fluorine.

Example: The oxidation number of Cl in NaCl is -1 and in ClO2 is +4, and in FCl is +1.

How to Find Oxidation Number

The oxidation number of an atom in an ion or compound can be determined using the above rules. Let us look at a few examples [1-6].

1. Sulfuric Acid (H2SO4)

The oxidation number of hydrogen (H) and oxygen (O) are +1 and -2, respectively. Sulfuric acid is a neutral compound. Let x be the oxidation number of sulfur (S). Therefore,

(+1) x 2 + x + (-2) x 4 = 0

Or, 2 + x – 8 = 0

Or, x = +6

2. Nitric Acid (HNO3)

The oxidation numbers of hydrogen (H) and oxygen (O) are +1 and -2, respectively. Nitric acid is a neutral compound. Let x be the oxidation number of nitrogen (N). Therefore,

+1 + x + (-2) x 3 = 0

Or, +1 + x – 6 = 0

Or, x = +5

3. Potassium Permanganate (KMnO4)

The oxidation numbers of potassium (K) is +1 and oxygen (O) is -2. KMnO4 is a neutral compound. Let x be the oxidation number of magnesium (Mn). Therefore,

+1 + x + (-2) x 4 = 0

Or, +1 + x – 8 = 0

Or, x = +7

4. Dichromate Ion (Cr2O72-)

Dichromate is a complex ion. The oxidation number of oxygen (O) is -2. The charge of Cr2O72- is -2. Let x be the oxidation number of chromium (Cr).

2x + (-2) x 7 = -2

Or, 2x -14 = -2

Or, x = +6

5. Carbonate (CO32-)

The oxidation number of oxygen (O) is -2 and the charge on CO32- is -2. Let x be the oxidation number of carbon (O). Therefore,

x + (-2) x 3 = -2

Or, x – 6 = -2

Or, x = +4

6.  Phosphite (PO33-)

The oxidation number of oxygen (O) is -2 and the charge of PO33- is -3. Let x be the oxidation number of phosphorous (P). Therefore,

x + (-2) x 3 = -3

Or, x – 6 = -3

Or, x = +3

7. Potassium Perchlorate (KClO4)

The oxidation numbers of potassium (K) and oxygen (O) are +1 and -2, respectively. Let x be the oxidation number of chlorine (Cl). KClO4 is a neutral compound. Therefore,

+1 + x + (-2) x 4 = 0

Or, 1 + x – 8 = 0

Or, x = +7

8. Potassium Nitrate (KNO3)

The oxidation number of potassium (K) and oxygen (O) are +1 and -2, respectively. KNO3 is a neutral compound. Let x be the oxidation number of nitrogen (N). Therefore,

+1 + x + (-2) x 3 = 0

Or, 1 + x – 6 = 0

Or, x = +5

Examples of oxidation numbers in a compounds

Compound

Oxidation number of elements

H2O2

H = +1

O = -1

CaH2

Ca = +2

H = -1

CHCl3

C = +2

H = +1

Cl = -1

Ba(MnO4)2

Ba = +2

Mn = +7

O = -2

K2MnO4

K = +1

Mn = +6

O = -2

H4P2O7

H = +1

P = +5

O = -2

CH2Cl2

C = 0

H = +1

Cl = -1

Types of Redox Reactions

The different types of redox reactions are as follow:

  • Decomposition reactions
  • Combination reactions
  • Displacement reactions
  • Disproportionation reactions

Combination reactions

“The type of redox reaction in which two atoms or molecules are combined to form a single compound is known as a combination reaction.”

Examples-

 3Mg + N2 → Mg3N2

 H2 + Cl→ 2HCl

Decomposition reactions

“The reaction in which one molecule breaks into two or more components is known as a decomposition reaction.”

Decomposition reactions are endothermic reactions. This is because they require heat for the reaction. One of the reactants should be in the elemental form.

Examples

 Ag2O→ 2Ag + ½ O2

ZnCO3 → ZnO + CO2

2Al2O→ 4Al + 3O2

“The type of redox reaction in which one atom or ion displaces another atom or ion in a compound is known as displacement reaction.”

Displacement reactions

Types of displacement reactions

There are two types of displacement reactions.

Metal displacement reaction

“The type of displacement reaction in which one metal present in the compound is displaced by another metal is known as metal displacement reaction.”

Examples

CuSO4 + Zn → Cu + ZnSO4

Cu + 2AgNO3 → 2Ag + Cu(NO3)2

                                                               

 

 

 

All decomposition reactions are not redox reactions. For example,

CaCO3 → CaO + CO2

Non-metal displacement reactions

“The reaction that mainly involves the hydrogen displacement from the water and acid is known non-metal displacement reaction.”

Examples

 Zn + 2HCl → ZnCl2 + H2

2Na + 2H2O → 2NaOH + H2

Depending upon the capability of the reducing metal or non-metal, the following cases arise;

I. Dihydrogen is displaced from cold water by all alkali metals and some alkaline earth metals (Ca,Sr and Ba), which are excellent reducing agents. For example,

II. Less reactive metals such as magnesium and iron react with steam to produce dihydrogen gas. For example,

III. Dihydrogen can be displaced from acids by various metals, including  “In this type of redox reaction, one element is simultaneously oxidized or reduced.”

This is a special type of redox reaction.

Examples

 

ones that do not react with cold water.


IV. Very less reactive metals such as silver (Ag)(Ag) and gold (Au)(Au) which may occur in the native state, do not react even with dilute hydrochloric acid