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Polybasic acids are those that can produce more than one hydrogen ion per molecule, with dibasic, tribasic, and so on denoting the number of hydrogen atoms that can be replaced. We can tell that some acids, such as sulphuric acid and phosphoric acid, have more than one ionizable ion per molecule by looking at them. Polybasic acids are acids that have a high level of ionization. Base refers to that substance that gives (OH-) ions when dissolved in water. Their strength depends on OH– ions concentration. The acidity of the base refers to the number of ionizable hydroxide ions (OH–) present in one molecule of the base. Bases can be classified as monoacidic base, diacidic base, triacidic base etc, on the basis of acidity.
Read Also: Haloform Mechanism Reaction
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Key Terms: Polybasic Acids, Poly Acidic Bases, Ionization, Ions, Bases, Acids, Hydrogen, Molecules, Atoms, Sulphuric Acid, Dissociation
Polybasic Acids
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Polybasic acids are those acids that can produce more than one hydronium per molecule. According to the number of hydrogens they can replace, these are referred to as dibasic, tribasic, and so on. Sulphuric acid and phosphorous acid are examples of polybasic acids.
Every day, we use a variety of acids and bases, such as vinegar or acetic acid in the kitchen, boric acid for laundry, baking soda for cooking, and washing soda for cleaning. Many acids that we do not consume in the home are used in laboratories, including acids such as HCl, H2SO4, and bases such as NaOH, KOH, and others.
Read More: Difference between Baking Soda and Baking Powder
Ionization of Polybasic Acids
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Let us consider the following ionization reaction of a typical polybasic acid.
H2X (aq) \(\rightleftharpoons\) H+ (aq) + HX- (aq)
HX– (aq) \(\rightleftharpoons\) H+ (aq) + X2- (aq)
Here is an example of dibasic acid dissociating into its constituent ions.
The equilibrium constant for the above reaction will be written as:
H2X (aq) \(\rightleftharpoons\) H+ (aq) + HX– (aq)
Ka1 = {[H +] [HX–]} / [H2X]
HX– (aq) \(\rightleftharpoons\) H+ (aq) + X2- (aq)
Ka2 = {[H+] [X2-]} / [HX–]
Hence, the dissociation constant of polybasic acid is the multiplication of dissociation constants of constituent ions.
Ka = Ka1 × Ka2 = {[H+] [HX–] / [H2X]} × {[H+] [X2-] / [HX–]}
= [H+]2 [X2-] / [H2X]
In this case, Ka1 and Ka2 are the first and the second ionization constants of the acid H2X. Similarly, for a tribasic acid, we have three ionization constants: Ka1, Ka2, & Ka3 &so on. Examples of some polybasic acids are H2SO4, H2S, H3PO4 so on.
Dissociation of Phosphoric Acid (Polybasic Acid)
Example: Ionization of H3PO4 is
H3PO4 (aq) \(\rightleftharpoons\) 2 H+ (aq) + SO42- (aq)
Ka = [H+]2 [SO42-] / [H3PO4]
The following considerations may help to explain how acid salts form. Most inorganic acids combine with bases in such a way that one atom of the acid is joined with one atom of a metallic oxide, resulting in monobasic acids.
Certain acids, such as pyrophosphoric, exist, with one atom capable of combining with two atoms of the base; these acids are thus referred to as dibasic acids. Tartaric acid and malic acid are two examples of this class of acids found in the vegetable and animal kingdoms.
Read More: Ionization Energy Formula
Poly Acidic Bases
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A base is defined as a substance that gives (OH-) ions when dissolved in water and their strength depends on OH– ions concentration. The acidity of the base refers to the number of ionizable hydroxide ions (OH–) present in one molecule of the base. Bases can be classified as the monoacidic base, diacidic base, triacidic base etc, on the basis of acidity.
Ionization of Poly Acidic Bases
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As an example, a few bases such as calcium hydroxide, aluminium hydroxide, we can see that they contain more than one ionizable ion per molecule of the base. Such acids are termed poly-acidic bases.
Similarly, ionization reaction of a typical poly-acidic base is as follows:
M(OH)2 (aq) \(\rightleftharpoons\) M2+ (aq) + 2 OH– (aq)
Kb = [M2+] [OH–]2} / [M(OH)2]
Examples: Ca (OH)2, Zn (OH)2 etc.
Ca (OH)2 (aq) \(\rightleftharpoons\) Ca2+ (aq) + 2 OH– (aq)
Kb = {[Ca2+] [OH–]2} / [Ca (OH)2]
Read More: Classification of Oxides
Things to Remember
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- Polybasic acids are those acids that can produce more than one hydronium per molecule. According to the number of hydrogens they can replace, these are referred to as dibasic, tribasic, and so on. Sulphuric acid and phosphorous acid are examples of polybasic acids.
- The ionization of a typical polybasic acid is given below:
- H2X (aq) \(\rightleftharpoons\) H+ (aq) + HX– (aq)
- HX – (aq) \(\rightleftharpoons\) H+ (aq) + X2- (aq)
- Many acids that we do not consume in the home are used in laboratories, including acids such as HCl, H2SO4, & bases such as NaOH, KOH, & others.
- The dissociation constant of polybasic acid is the multiplication of dissociation constants of constituent ions.
- As an example a few bases such as calcium hydroxide, aluminium hydroxide, we can see that they contain more than one ionizable ion per molecule of the base. Such acids are termed poly-acidic bases.
Sample Questions
Ques. What are Polyprotic acids and bases? (3 Marks)
Ans. Polyprotic acids are specific acids that can lose more than one proton per molecule in acid-base reactions. In other words, acids have more than one H + atom ionizable per molecule. Monoprotic acids contrast in the Monoprotic vs Polyprotic Acids and Bases portion.
Ques. What is the ionization of an acid? Do acids and bases ionize in water? (3 Marks)
Ans. The degree of ionization refers to acid or basic strength. Although strong acids are said to be completely ionized in water, weak acids are partially ionized.
Both acids and bases dissolve in water and thus increase the concentration of one of the self-ionizing components of water, either protons or hydroxide ions, thus suppressing water dissociation. Acidic solutions have a low pH and basic solutions have a high pH.
Ques. Is HCl a strong acid? (3 Marks)
Ans. Hydrochloric acid has a very low pH, high acidity, a higher H + ion concentration than OH ions, and when dissolved in water, it has a higher degree of ionization/dissociation of H + ions. HCl completely dissociates. Therefore, HCl is a strong acid.
Ques. What is a monobasic acid? (3 Marks)
Ans. In an acid-base reaction, a monobasic acid is an acid that can donate hydrogen ions only to the base. Therefore, a monobasic molecule has only one exchangeable hydrogen atom. This source is HCl and HNO3.
Ques. What is a weak dibasic acid? (3 Marks)
Ans. Making a mild dibasic acid solution will result in a solution containing a mixture of acids. Carbonic acid, H2CO3, is a classic example of a weak dibasic acid. Bicarbonate ions can also behave like acids. This ionizes and produces lower concentrations of hydronium and carbonate ions.
Ques. How does bond strength affect acidity? (3 Marks)
Ans. In general, the bond strength of an acid depends on the size of the "A" atom. The smaller the "A" atom is, the stronger the HA bond will be. As the atom moves along the path of the periodic table, it grows larger and the bond becomes weaker. In other words, the acid becomes stronger.
Ques. What is a Diprotic base? (3 Marks)
Ans. The acid-balance problem mentioned so far has been concentrated on classes of compounds known as monoprotic acid. Each of these acids has a single H + ion or proton and he can be given as he acts as a Bronsted acid. Diprotic acids, namely sulfuric acid (H2SO4), carbonic acid (H2CO3), hydrogen sulfide (H2S), chromic acid (H2CrO4), and oxalic acid (H2C2O4) have two acidic hydrogen atoms.
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