Equivalents
The equivalent of a substance is the mass which supplies or consumes one mole of another substance in a reaction. Equivalents can be used to simplify balancing chemical equations for many reactions. When using equivalents it is often not necessary to write balanced equations to find the amount of substances participating in the reaction. As a result, although to determine the equivalent weight of a substance you must specify the reactants and the products involved in a reaction, the equation does not have to be balanced. Because one equivalent always reacts with one equivalent, the use of equivalents can be of great help when trying to work with reactions where the balanced equation is not known.
In acid-base reactions the equivalent weight is the gram molecular weight of acid that yields an Avogadro number (6.022 x 1023) of hydrogen ions (or, depending on the acid-base scheme, the mass required to form one mole of hydronium, H3O+, ions). For acids such as hydrochloric acid (HCl) that contribute one H+ proton to an acid-base reaction, the equivalent weight is the gram molecular weight of the acid. According to this definition, 35.5g of HCl can contribute 6.022 x 1023 hydrogen protons to an acid-base reaction.
For diprotic acids—those acids such as sulfuric acid (H2SO4) that contribute two H+ protons to an acid-base reaction--the equivalent weight is one-half the gram molecular weight of the acid. For Triprotic acids--those acids such as phosphoric acid (H3PO4) that contribute three H+ protons to an acid-base reaction--the equivalent weight is one-third the acid's gram molecular weight.
The equivalent weight of a base is the gram molecular weight of base that contributes an Avogadro number of hydroxyl ions (OH-). Bases such as sodium hydroxide (NaOH) that contribute one hydroxyl ion to an acid-base reaction, have an equivalent weight equal to their gram molecular weight.
Bases such as calcium hydroxide, Ca(OH)2, that contribute two hydroxyl ions to an acid-base reaction, have an equivalent weight one-half their gram molecular weight. Bases such as aluminum hydroxide, Al(OH)3, that contribute three hydroxyl ions to an acid-base reaction have an equivalent weight equal to one-third their gram molecular weight.
With regard to oxidation-reduction reactions, one equivalent of a reducing substance is the mass of the substance that gives up or releases an Avogadro number (6.022 x 1023) of electrons in a reaction. Correspondingly, one equivalent of an oxidizing substance is the amount of the substance that collects an Avogadro number or one mole of electrons. Balancing oxidation-reduction reactions is thus made easier because one equivalent of an oxidizing agent reacts with one equivalent of reducing agent.
The equivalent weight of any compound entering into an oxidation-reduction reaction is determined by dividing the weight of one mole by the number of electrons gained or lost per formula unit. The products of the reaction must be known to determine equivalent weights in oxidation reduction reactions because it is necessary to know the changes in oxidation numbers between the products and the reactants. When V2O5 is reduced to VO2, for example,its equivalent weight is 90.9g. When V2O5 is reduced to VO, however, its equivalent weight is 30.3g.
Equivalent weight is also called combining weight because elements, compounds, and ions enter into combinations proportional to their equivalent weights.
Normality is based on the concept of equivalent weight. Normality is defined as the number of equivalents per liter of solution. When one mole of sulfuric acid (H2SO4) is put into solution there are two equivalents present in the solution and therefore the solution is a two normal (2 N) solution.
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