Chemical Equivalence
Definition and meaning of Chemical Equivalence in chemistry.
Chemical equivalence describes the relationship in which reacting substances combine in proportions corresponding to equal numbers of chemical equivalents, so that one equivalent of any reactant reacts completely with exactly one equivalent of another.
In more detail
An equivalent is the amount of a substance that supplies, accepts, or reacts with one mole of a specified reactive unit, one mole of H+ or OH- ions in acid-base reactions, one mole of electrons in oxidation-reduction reactions, or one mole of charge in precipitation reactions. The equivalent weight (gram equivalent mass) equals the molar mass divided by the substance's n-factor (its valence, ionic charge, or number of electrons transferred per formula unit). Because equivalents always react in a fixed 1:1 ratio regardless of a compound's molecular formula, chemical equivalence underlies normality, a concentration unit still favored in titrimetric analysis for comparing reagents of different reactive capacity.
Key facts
| Equivalent weight formula | Molar mass ÷ n-factor (valence, charge, or electrons transferred) |
|---|---|
| Concentration unit | Normality (N) = equivalents of solute per liter of solution |
| Common uses | Acid-base, redox, and precipitation titrations |
| Field | General Chemistry |
In H2SO4 + 2NaOH → Na2SO4 + 2H2O, each mole of H2SO4 furnishes 2 moles of H+, so it has 2 equivalents per mole (equivalent weight = 98/2 = 49 g/eq). Thus 1 mole of H2SO4 is chemically equivalent to 2 moles of NaOH, and equal numbers of equivalents (1 eq acid to 1 eq base) react completely.
Frequently asked questions
What is one equivalent of a substance?
One equivalent is the quantity of a substance that reacts with, releases, or accepts exactly one mole of a defined reactive unit, such as H+, OH-, or an electron, in a given reaction.
Why is normality still used if molarity is more common today?
Normality directly accounts for a substance's reactive capacity (its n-factor), so equivalents always react 1:1, simplifying calculations for titrations involving acids, bases, or redox reagents with different numbers of reactive protons or electrons.