Equivalent Concept in Chemistry

The equivalent concept is a fundamental aspect of stoichiometry in chemistry that deals with the quantitative relationships between reactants and products in a chemical reaction. It is based on the principle that substances react in certain proportionate amounts, called equivalents.

Understanding Equivalents

An equivalent (Eq) is the amount of a substance that reacts with or supplies one mole of hydrogen ions (H+) in an acid-base reaction, one mole of electrons in a redox reaction, or one mole of a monovalent ion in a precipitation reaction. The concept of equivalents allows chemists to calculate the amount of reactants needed or products formed in a chemical reaction.

Equivalent Weight

The equivalent weight of a substance is the mass that supplies or reacts with one mole of electrons or hydrogen ions. It is calculated by dividing the molar mass of the substance by its valence (the number of electrons lost or gained in a redox reaction, or the number of hydrogen ions replaced or supplied in an acid-base reaction).

$$ Equivalent\ Weight = \frac{Molar\ Mass}{Valence} $$

Normality

Normality (N) is a measure of concentration that is based on the equivalent concept. It is defined as the number of equivalents of solute per liter of solution.

$$ Normality (N) = \frac{Number\ of\ Equivalents}{Volume\ of\ Solution\ in\ Liters} $$

Table of Differences and Important Points

Aspect	Molarity (M)	Normality (N)
Definition	Moles of solute per liter of solution	Equivalents of solute per liter of solution
Dependency	Depends on the molecular weight	Depends on equivalent weight and valence
Reaction Type	General use	Acid-base, redox, and precipitation reactions
Calculation	$ M = \frac{moles}{volume\ (L)} $	$ N = \frac{equivalents}{volume\ (L)} $
Use Case	Used for most solutions	Used when equivalents are more relevant

Examples

Example 1: Acid-Base Reaction

Consider the reaction between hydrochloric acid (HCl) and sodium hydroxide (NaOH):

$$ HCl + NaOH \rightarrow NaCl + H_2O $$

In this reaction, 1 mole of HCl reacts with 1 mole of NaOH. Since HCl is a monoprotic acid (it donates one H+ ion), its equivalent weight is equal to its molar mass. Therefore, 1 equivalent of HCl reacts with 1 equivalent of NaOH.

Example 2: Redox Reaction

In a redox reaction, the equivalent weight of a substance is related to the number of electrons transferred. For example, in the reaction where iron (Fe) is oxidized by copper(II) sulfate (CuSO4):

$$ Fe + CuSO_4 \rightarrow FeSO_4 + Cu $$

Iron goes from an oxidation state of 0 to +2, losing 2 electrons. Therefore, the equivalent weight of iron is half its molar mass.

Example 3: Precipitation Reaction

In a precipitation reaction, the equivalent concept can be used to determine the amount of precipitate formed. For instance, when silver nitrate (AgNO3) reacts with sodium chloride (NaCl) to form silver chloride (AgCl) precipitate:

$$ AgNO_3 + NaCl \rightarrow AgCl + NaNO_3 $$

Silver has a valence of 1 in AgNO3, so its equivalent weight is equal to its molar mass. One equivalent of AgNO3 will react with one equivalent of NaCl to form one equivalent of AgCl.

Conclusion

The equivalent concept is a valuable tool in stoichiometry for calculating the amounts of reactants and products in various types of chemical reactions. By understanding the differences between molarity and normality, and how to calculate equivalent weight, chemists can accurately predict and measure the outcomes of chemical processes.