The Arrhenius acid and base concept is based on the ionic dissociation of substances in water or in an aqueous medium. According to this concept, an acid is any hydrogen-containing molecule that donates H+ ions in an aqueous medium is known as Arrhenius acid. Let an example of HCl, When HCl is dissolved in water it dissociates into its ions and gives H+ion so HCl is an Arrhenius acid.
HCl + H2O → H+ + Cl– + H2O → H3O+ + Cl–
Are all Hydrogen Containing Compounds are Acids?
All hydrogen-containing compounds are not acidic in nature because as we discussed earlier this concept is based on the ionic dissociation of substances in water so only those substances are Arrhenius acid which gives H+ ion in an aqueous solution. For example, C2H4 contains hydrogen but it is not Arrhenius acid because it does not give H+ ions in water.
Why HCl is 37%?
It is interesting that HCl is 37% by wt., not 100%. The reason behind this is that the HCl is a gas and basically we dissolved gas in water and HCl is not too soluble in water so only 37% HCl can be dissolved in water. why HCL is not 100% soluble in water? HCl is a covalent compound and the solubility of the covalent compound in water is not well, which means that the HCl is not soluble in water 100%.
A base is any hydrogen-containing molecule that donates OH– ions in an aqueous medium is known as Arrhenius base. Let an example of NaOH, When Sodium hydroxide NaOH is dissolved in water it dissociates into its ions and gives OH– ion so NaOH is an Arrhenius base.
NaOH + H2O → Na+ + OH– + H2O
Similarly, KOH is a base because it gives OH– ion in water, whereas C2H5OH is not a base.
Applications of Arrhenius Acid & Base
Why Non-metallic Oxides Acidic in Nature?
The non-metallic oxides are acid in nature. When we react to non-metallic oxides such as CO2, SO3, N2O5, P4O10, P2O5 with water, an acid is produced.
Non-metallic oxides + water → Acid
- When we react CO2 with water, carbonic acid is produce.
CO2 + H2O → H2CO3
Carbonic acid is a weak acid. When we dissolve it into aqueous, it gives H+ ions and acts as Arrhenius acid.
H2CO3 + H2O → H+ + HCO3–
HCO3– + H2O → H+ + CO32-
- When sulfur trioxide SO3 is react with water, sulfuric acid is produce.
SO3 + H2O → H2SO4
Sulfuric acid is a strong inorganic acid known as the king of acids. When we dissolve sulfuric acid in water or in an aqueous solution, it gives H+ ions and acts as Arrhenius acid.
H2SO4 + H2O → H+ + HSO4–
HSO4– + H2O → H+ + SO42-
- When nitrogen pentoxide N2O5 is react with water, nitric acid is produce.
N2O5 + H2O → 2HNO3
Nitric acid is a powerful oxidizing agent and is a strong acid. When we dissolve nitric acid in water or in an aqueous solution and HNO3 is acts as an Arrhenius acid.
2HNO3 → 2H+ + NO32-
So non-metallic oxides like CO2, SO3, N2O5 are Arrhenius acids.
Why Metallic Oxides are bases?
When metals react with oxygen, metallic oxides are formed. Metallic oxides are basic in nature because when it reacts with water it gives a base.
Metallic oxides + water → Base
- When CaO is react with water, it gives calcium hydroxide.
CaO + H2O → Ca(OH)2
Ca(OH)2 is a strong base. When we dissolve it into water it gives OH- ion in an aqueous solution and according to Arrhenius concept, it acts as a base.
Ca(OH)2 → Ca2+ + 2OH–
Strength of Acids
NH3 + H2O → NH4+ + OH–
Ammonia is less acidic in nature as compared to water. The order of strength of acids are given below:
CH4 < NH3 < H2O < HF
HF is a strong acid because of the high electronegativity of fluorine. Fluorine attracts the shared pair of electrons and HF gives H+ ions quickly.
Similarly, if we discussed the reaction above, Oxygen is more electronegative than nitrogen so water is more acidic in nature as compared to ammonia that’s why Ammonia does not give H+ ions quickly as water gives.
Limitations Of Arrhenius Acid & Base Concept
- According to this concept, when HCl is dissolved in water it is an acid but not in some other solvent like benzene.
- It cannot tell us about the acidic and basic character of substances in non-aqueous solvent such as liquid sulphur dioxide and liquid ammonia.
- This concept was not applicable to reactions in gaseous phase when no solvent is present.
- According to this concept, acids are those which gives H+ ion in aqueous solution but it cannot explain the acidic nature of non-metallic oxides like CO2.
- This concept is applicable only for aqueous solution and not explain the non-aqueous medium.