Difference Between Monoprotic and Polyprotic Acids

Monoprotic vs Polyprotic Acids
 

Acids are defined in several ways by various scientists. Arrhenius defines an acid as a substance that donates H₃O⁺ ions in the solution. Bronsted- Lowry defines a base as a substance that can accept a proton. Lewis acid definition is far common than the above two. According to it, any electron pair donator is a base. According to the Arrhenius or Bronsted-Lowry definition, a compound should have a hydrogen and the ability to donate it as a proton to be an acid. However, according to Lewis, there can be molecules, which don’t possess hydrogen, but can act as an acid. For example, BCl₃ is a Lewis acid, because it can accept an electron pair. An alcohol can be a Bronsted-Lowry acid because it can donate a proton but, according to Lewis, it will be a base.

Regardless of the above definitions, we normally identify an acid as a proton donor. Acids have a sour taste. Lime juice, vinegar are two acids we come across at our homes.  They react with bases producing water, and they also react with metals to form H₂, thus increase metal corrosion rate. Acids can be categorized into two, based on their ability to dissociate and produce protons. Strong acids like HCl, HNO₃ are completely ionized in a solution to give protons. Weak acids like CH₃COOH partially dissociate and give fewer amounts of protons. K_a is the acid dissociation constant. It gives an indication of the ability to lose a proton of a weak acid. To check whether a substance is an acid or not we can use several indicators like litmus paper or pH paper. In the pH scale, from 1-6 acids are represented. An acid with pH 1 is said to be very strong, and as the pH value increases, acidity is decreased.  Moreover, acids turn blue litmus to red.

Monoprotic Acid

When one molecule of acid dissociates in an aqueous solution, if it gives a single proton, then that acid is said to be a monoprotic acid. HCl and nitric acid (HNO₃) are some examples for monoprotic mineral acids. Following is the dissociation for HCl in the aqueous medium to give out one proton.

HCl   →  H⁺ + Cl^–

Other than the mineral acid, there can be monoprotic organic acids too. Usually when there is one carboxylic group, that acid is monoprotic. For example, acetic acid, benzoic acid, and a simple amino acid like glycine are monoprotic.

Polyprotic Acid

Polyprotic acids contain more than one hydrogen atoms, which can be donated as protons when they are dissolved in an aqueous medium. Specifically, if they are donating two protons, we call them as diprotic and, if giving three protons, triprotic, etc. Hydrogen sulfide (H₂S) and H₂SO₄ are diprotic acids, which give out two protons. Phosphoric acid (H₃PO₄) is a triprotic acid. In most cases, polyprotic acids do not fully dissociate and give out all the protons simultaneously. Dissociation constants for each dissociation varies. For example, in phosphoric first dissociation constant is 7.25×10⁻³, which is a larger value. So the full dissociation takes place. Second dissociation constant is 6.31×10⁻⁸,and the third one is 3.98×10⁻¹³, which are less favorable dissociations than the first one.