Dibasic acids, also known as diprotic acids, can release two hydrogen ions when reacting with a base. Monoprotic acids only release one, while polyprotic acids can release more than two. The number of hydrogen atoms in an acid molecule does not determine its strength. Acidic salts can be either acidic or basic, depending on the hydrogen atom originating from an acid. The number of hydrogen atoms in an acid molecule does not necessarily indicate its proticity.
In acid-base chemistry, a dibasic acid is one that can deliver two positively charged hydrogen ions, or protons, when it reacts with a base. A more modern term for this type of acid is diprotic acid. An acid and a base normally react to form a salt and water. Water results from the positively charged hydrogen ion from the acid reacting with the negatively charged hydroxide ion from the base: H+ + OH- → H2O. In a dibasic acid molecule there are two hydrogen atoms available to react, and thus two types of salt can be formed, one of which will be an acid salt containing one hydrogen atom.
An acid that can only deliver one proton is known as a monoprotic or monobasic acid. Examples are hydrochloric acid (HCl) and nitric acid (HNO3). There are also polyprotic acids which can supply more than two protons, for example phosphoric acid H3PO4, which is triprotic. There is no relationship between the number of hydrogen atoms in the acid molecule and the strength of the acid – this depends on the extent to which the hydrogen in the molecule breaks down into hydrogen ions in solution. An acid with one hydrogen atom that splits easily will be stronger than one with two hydrogen atoms that don’t; for example, hydrochloric acid (HCl) – a monobasic acid – is a much stronger acid than dibasic carbonic acid (H2CO3).
The terms monobasic and dibasic are rarely applied to acids these days, but old chemistry textbooks may use them. Acids are now generally described as monoprotic, diprotic, triprotic, and so on. The term dibasic can still be seen in the context of bases, for example calcium hydroxide (Ca(OH)2) can be described as dibasic, as it has two hydroxide groups which can combine with an H+ ion from an acid to form water .
Sulfuric acid, one of the best known and most widely used acids, is a good example of dibasic acid. It can form two types of salts, known as sulfates and hydrogen sulfates, sometimes called bisulfates. Carbonic acid is another common dibasic acid which can, likewise, form carbonates and hydrogen carbonates. The latter are often called bicarbonates; sodium bicarbonate, or sodium bicarbonate, is the best known of these. These acidic salts can be acidic, as in sodium hydrogen sulfate, or basic, as in sodium hydrogen carbonate; the term simply indicates that the salt contains a hydrogen atom originating from an acid.
Not all hydrogen atoms in an acid molecule are necessarily available to form H+ ions and react with bases. It is therefore not possible to tell whether an acid is monoprotic, diprotic or polyprotic simply by counting the hydrogen atoms in the molecule. This is especially true for organic acids, which can have relatively complex structures involving hydrogen in other roles. An example is tartaric acid (C4H6O6). There are six hydrogen atoms in the molecule, but it is so structured that only two of them can split into hydrogen ions in solution; it is therefore a dibasic acid.
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