Early chemists defined equivalent weight as the weight of one substance reacting with another to form a third. Hydrogen was used as the standard, but easily purified metals were later used. Molar masses replaced equivalent weights, but they are still used in acid-base and reduction-oxidation reactions, mining, and polymer chemistry.
Early chemists defined an equivalent weight as the weight of one substance that would react with a second to form a third. As these chemists studied matter, they realized that reactions always occur in set proportions. Many of their reagents appeared to contribute the same level of activity, regardless of the reaction involved.
Tables of equivalent weights, based on hydrogen reactions, were assembled in the late 18th century. Hydrogen was used as the standard, being the least massive element; however, it does not react readily with many elements. Easily purified and accessible metals readily form oxides and have often been used as an experimental basis for determining equivalent values.
The mass gain of the metal was attributed to the oxygen content of the metal oxide. This weight was measured, divided by eight, and reported as the equivalent weight grams of hydrogen for that metal. The weight was divided by eight because oxygen reacts with hydrogen at a weight ratio of eight to one to form water. Oxygen was viewed as the chemical opposite of hydrogen. Modern chemists would agree that oxygen participates in oxidizing reactions and hydrogen in reducing reactions.
This procedure worked well as long as the reaction wasn’t too complex. Many metals have different oxides, as they can yield stable compounds in more than one valence configuration or oxidation state. As chemists learned more about the nature of the reactions they were conducting, the periodic chart replaced the previous tables.
Calculations performed using a table of equivalent weights were followed by the use of molar masses. The molar refers to the number of atoms available to react. The extent of the reaction is based on this number, not the mass of the reactants. One mole of atoms has 6.023 x 10 23 atoms.
The use of the hydrogen standard illustrates the difference. It is known that water contains two hydrogen atoms to one oxygen atom. Since oxygen has a molar mass of 16 grams per mole, while the molar mass of hydrogen is 1 gram per mole, the mass ratio is eight to one, oxygen and hydrogen. The mole ratio is two to one, hydrogen and oxygen, reflecting the actual composition.
Some fields of chemistry have continued to use equivalent weight in limited contexts. In acid-base chemistry, an equivalent weight is the mass of a chemical species that reacts with 1 mole of hydronium ions (H3O+) or 1 mole of hydroxide ions (OH-). In reduction-oxidation reactions, an equivalent weight is the mass of a substance that accepts or donates one mole of electrons.
In the mining industry, equivalent weights have been used to describe the concentration of ore in a sample. Silver, for example, will precipitate as silver chloride from a liquid solution. The equivalent weight is the mass of silver chloride which contains 1 gram of silver metal.
Polymer chemists react long molecules with active side groups to form strong crosslinked polymers. The reaction activity or affinity can be measured in equivalent weights. Resins of equal weight will produce the same degree of crosslinking within the same families of polymers.
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