Equivalent Mass of the Substances in Detail

The equivalent mass of a substance is defined as the number of parts by which any element integrates with:

• 1 part of the mass of hydrogen
• 8 parts by mass of oxygen
• 35.5 parts by mass of chlorine or gram equivalent of any other element

Gram Equivalent Mass means the equivalent mass of a substance expressed in grams.  

Equivalent mass will be different in different chemical reactions and under different experimental conditions, which means it is not constant.

Now, let’s discuss the equivalent mass of substances that undergoes oxidation and reduction.

When there is a loss of electrons during a reaction by a molecule, atom or ion, this process is known as Oxidation. 

An oxidising agent, oxidizer or oxidant is a chemical that gets oxidised with other substances; it can increase the oxidation state and make the substance lose its electrons. In a chemical reaction, atoms of oxidising agents remove at least one electron from another atom, and it passes on at least one electronegative atom to a chemical species. 

It is also called an electron acceptor, which means it accepts the electrons given to it by other compounds.

A few examples of oxidising agents: 

1. Oxygen (O2)
2. Nitric acid (HNO3)
3. Hydrogen peroxide (H2O2) 
4. Potassium nitrate (KNO3)
5. Sulfuric acid (H2SO4)
6. Lead dioxide (PbO2)

Example:

Hydrofluoric acid is made of hydrogen (H) and fluorine (F).

H2 + F2 —-> 2HF

Here, hydrogen is oxidised, and fluorine is reduced. If written in two half-reactions, it would look like:

H2 —> 2H+ + 2e-

F2 + 2e- —> 2F-

There is no oxygen in this reaction.

According to the classic definition of oxidation, it happens when oxygen is added to a compound because oxygen (O2) was the first known oxidising agent. 

As per the standard definition, electron loss and an increase in oxidation rate, the addition of oxygen was necessary. The definition has been widened now, which means the modern definition of oxidation also includes the addition of other types of chemical reaction.

Some examples of the classic definition:

• 2Fe + O —> Fe2O3

Iron (Fe) combined with Oxygen (O) to form Iron Oxide.

Here, the iron has been oxidised. The oxidised iron is called Rust.

• C + O2 —> CO2

Carbon (C) is combined with Oxygen (O) to form Carbon dioxide. 

• 2Mg + O2 —> 2MgO

Magnesium metal and oxygen are combined to form Magnesium oxide. 

Equivalent mass of oxidising agent =

The molecular mass of oxidising agent/ Number of electrons involved in the reaction of the one-mole oxidising agent

Let’s understand this with an example: 

2KMnO4 + 3H2SO4 —> K2SO4 + 2MnSO4 + 3H2O + 5[O]

Here, the oxidising agent is KMnO4, which is Potassium Permanganate.

Change that will occur: 

MnO4- —> Mn2+

Change in oxidation state: 

1 mole KMnO4 —> (Mn7+ —> Mn2+)

+7–(+2)=+5

Number of electrons = +5

Atomic Mass: 

Potassium (K) = 39

Manganese (Mn) = 55

Oxygen (O) = 16

The Molecular Mass of KMnO4 is 158.

Equivalent Mass of KMnO4 = Molecular mass of KMnO4 / 5

= 158/ 5 

=31.6

Reduction is the gain of electrons or the oxidation state of a molecule, an atom, or ion decreases. It is the opposite of Oxidation.

A reducing agent or reductant is a chemical that loses electrons and oxidised during a chemical reaction. It is oxidised because in a redox reaction it loses electrons. In an oxidation-reduction (Redox) reaction, it gives electrons to the other reactant thereby reducing the other reactant. The process of reduction is only completed when the reducing agent passes on electrons to other substances in the reaction.

It is also known as an electron donor, which means it gives electrons to other compounds.

A few examples of reducing agents are:

1. Sodium (Na)
2. Iron (Fe)
3. Zinc (Zn)
4. Lithium (Li)
5. Hydrochloric acid (HCI)
6. Hydrogen Sulphide (H2S)

A classic definition of oxidation and reduction defines reaction as per protons or hydrogen. In oxidation, there is a loss of hydrogen, and in reduction, there is a gain of hydrogen.

The modern and more accurate meaning of oxidation involves electrons and oxidation numbers.

An example of reduction:

• CuO + Mg —> Cu + MgO

Copper oxide and magnesium are combined to give copper and magnesium oxide.

• H2C=CH2 + H2 —> H3C — CH3

Here, since there is an addition of hydrogen, it is a reduction reaction

• 2HgO —> 2Hg + O2

Removal of oxygen signifies that this is a reduction reaction. 

Equivalent mass of reducing agent =

The molecular mass of reducing agent/ Number of electrons involved in the reaction of the mole reducing agent

Let’s take an example:

Equivalent mass of reducing agent = 

Molecular mass/ Change in oxidation number

= Molecular mass/ Number of electrons lost

Fe2+ is oxidised Fe3+, where one electron is lost.

Equivalent mass of Fe2+ = molecular mass/ 1

= 55.8/1

=55.8 

Redox Reactions

A redox reaction is a chemical reaction where there is a transfer of electrons between two reactants. A reaction where both oxidation and reduction happens is called a Redox (reduction-oxidation) reaction.

Cellular respiration, combustion, photosynthesis, corrosion and the formation of sodium chloride are a few examples of Redox reactions.

Examples: 

• Mg + F2 —> MgF2

The oxidation state of Mg increased from 0 —> +2

It is an oxidation reaction.

F (Fluorine) is reduced from 0 —> -2

It is a reduction reaction.

There are four types of redox reactions:

1. Displacement
2. Decomposition
3. Disproportionation
4. Combination

Conclusion:

To summarise, it can be said that the equivalent mass of a substance helps us know chemical elements physically and chemically as well.