What is Arrhenius Acid?

The compounds which can increase the concentration of ${\text{H}}^{+}$when dissolved in water are termed as Arrhenius acid. Simply speaking the compounds which are capable of donating H+ ions can be considered as Arrhenius acid. They are the compounds which have importance in both organic chemistry and inorganic chemistry.

What is Arrhenius' Acid-Base Theory? 

Arrhenius' theory was postulated by a Swedish scientist named Svante Arrhenius in 1887. This theory states that any substance dissociates in presence of water to produce electrically charged species namely ions, atoms, or molecules. If the substance on dissociation yields hydrogen ion ${\text{H}}^{+}$or hydronium ion $\left({\text{H}}_3{\text{O}}^{+}\right)$then the substance is classified as ‘Arrhenius acid’ and if a substance on dissociation yields hydroxide ions${\text{OH}}^{-}$, then the substance is classified as ‘Arrhenius base’.

In other words, if a substance has the tendency to easily liberate hydrogen ion or a proton in solutions of water, or if a substance has the tendency to increase the concentration of ${\text{H}}^{+}$ ions in a solution of water, it is defined as ‘Arrhenius acid’ and if a substance has a tendency to easily liberate hydroxide ion in an aqueous solution or if a substance has the tendency to increase the concentration of ${\text{OH}}^{-}$ ions in solutions of water, then it is termed as ‘Arrhenius base’.

For example, hydrogen chloride or hydrochloric acid $\left(\text{HCl}\right)$dissolves in water to liberate ${\text{H}}^{+}$ and ${\text{Cl}}^{-}$ions. This increases the concentration of ${\text{H}}^{+}$ ions in the aqueous solution of $\text{HCl}$. Thus, $\text{HCl}$is an Arrhenius acid.

Sodium hydroxide $\left(\text{NaOH}\right)$dissolves in water to liberate ${\text{Na}}^{+}$and ${\text{OH}}^{-}$ ions. This makes the ${\text{OH}}^{-}$ion concentration high in the aqueous solution of $\text{NaOH}$. Thus, $\text{NaOH}$ is classified as an Arrhenius base.

Examples of Arrhenius Acid  

According to Arrhenius' theory of acids and bases, the acids are the substances which increase the concentration of ${\text{H}}^{+}$ ion in the aqueous solution or liberate ${\text{H}}^{+}$ ions easily. This indicates that a substance to act as an Arrhenius acid should have at least one hydrogen atom present in it.

Some of the examples of the Arrhenius acid are $\text{HBr}$(hydrogen bromide), $\text{HF}$(hydrogen fluoride), ${\text{HNO}}_{\text{3}}$(nitric acid), ${\text{H}}_{\text{2}}{\text{SO}}_{\text{4}}$(sulphuric acid), $\text{HI}$(hydrogen iodide), ${\text{HClO}}_{\text{4}}$(perchloric acid), ${\text{H}}_{\text{3}}{\text{PO}}_{\text{4}}$(phosphoric acid), ${\text{H}}_{\text{2}}{\text{CO}}_{\text{3}}$(carbonic acid), ${\text{CH}}_{\text{3}}\text{COOH}$(acetic acid), ${\left(\text{COOH}\right)}_{\text{2}}$(oxalic acid),$\text{HCN}$(hydrogen cyanide), $\text{HCOOH}$(formic acid), ${\text{H}}_{\text{3}}{\text{PO}}_{\text{3}}$(Phosphoric acid) etcetera.

Arrhenius acid act as a good electrolyte as it dissociates to its respective ions in the aqueous solutions. Keeping it similar to the general acid properties, Arrhenius acid also neutralizes bases and turns litmus paper into red. 

What is the Basicity of an Arrhenius Acid?

The number of replaceable hydrogen ions present in an Arrhenius acid is termed as the basicity of that particular Arrhenius acid.  

For example, ${\text{H}}_{\text{3}}{\text{PO}}_{\text{4}}$ ionizes in the presence of aqueous solution to liberate three hydrogen ions $\left({\text{H}}^{+}\right)$and one phosphate ion$({\text{PO}}_4{}^{3-})$. Thus, the basicity of the phosphoric acid is three as there are three replaceable hydrogen ions in it.

It is not necessary that the number of replaceable hydrogens should be equal to the number of hydrogens present in the molecule. 

For example, ${\text{H}}_{\text{3}}{\text{PO}}_{\text{4}}$ (phosphoric acid) is a tribasic acid whereas ${\text{H}}_{\text{3}}{\text{PO}}_{\text{3}}$ (phosphorous acid) is a dibasic acid and ${\text{H}}_{\text{3}}{\text{PO}}_{\text{2}}$ (hypo phosphorous acid) is a monobasic acid.

The structure of ${\text{H}}_{\text{3}}{\text{PO}}_{\text{4}}$contains three $\text{-OH}$groups because of which three hydrogen atoms act as replaceable hydrogen atoms, but in the structure of phosphorous acid only two $\text{-OH}$groups are present because of which only two hydrogen atoms act as the replaceable hydrogen atoms and in hypo phosphorous acid only one $\text{-OH}$group is present indicating that there is only one replaceable hydrogen atom in it. Thus, phosphoric acid (${\text{H}}_{\text{3}}{\text{PO}}_{\text{4}}$) acts as tribasic Arrhenius acid whereas, phosphorous acid (${\text{H}}_{\text{3}}{\text{PO}}_{\text{3}}$) acts as dibasic Arrhenius acid and hypo phosphorous acid (${\text{H}}_{\text{3}}{\text{PO}}_{\text{2}}$) acts as monobasic Arrhenius acid.

Thus, basicity is not related to the number of hydrogen atoms present in the molecule. It is based on the number of replaceable hydrogens present in the molecule depending upon its structure.  

Based on Arrhenius acids, some of the monobasic acids are${\text{HCl, HBr, HI, HF, HNO}}_{\text{3}}{\text{, H}}_{\text{3}}{\text{PO}}_{\text{2}}$, ${\text{HClO}}_{\text{4}}{\text{, H}}_{\text{3}}{\text{BO}}_{\text{3}}{\text{, CH}}_{\text{3}}{\text{COOH, HCOOH, HCN, H}}_{\text{2}}{\text{CO}}_{\text{3}}$,${\text{H}}_{\text{3}}{\text{PO}}_{\text{3}}$ etcetera., dibasic acids are${\text{H}}_{\text{2}}{\text{SO}}_{\text{4}}{\text{, H}}_{\text{3}}{\text{PO}}_{\text{3}}\text{, }{\left(\text{COOH}\right)}_{\text{2}}{\text{, H}}_{\text{2}}{\text{SO}}_{\text{3}}$etcetera, tribasic acids are ${\text{H}}_{\text{3}}{\text{PO}}_{\text{4}}$, citric acid${\text{(C}}_{\text{6}}{\text{H}}_{\text{8}}{\text{O}}_{\text{7}}\text{)}$, Arsenic acid (${\text{H}}_{\text{3}}{\text{AsO}}_{\text{4}}$) and ${\text{H}}_{\text{4}}{\text{P}}_{\text{2}}{\text{O}}_{\text{7}}$(pyrophosphoric acid) is a tetrabasic acid.

How does Arrhenius Acid Act as an Electrolyte?

Arrhenius acid dissolves in aqueous solutions to liberate its respective ions. If an acid dissociates completely in its aqueous solution, then it is said to be a strong acid. This indicates that if fifty molecules of an acid (${\text{HNO}}_{\text{3}}$) are dissociated in an aqueous solution then there should be fifty molecules of ${\text{H}}^{+}$ and ${\text{NO}}_{\text{3}}{}^{\text{-}}$ions each and there should not be any $\text{HCl}$molecule in the solution. This type of dissociation is seen in the case of strong acids.

Thus, the acids which dissociate completely in an aqueous solution are said to be strong acids. For example, ${\text{HCl, HBr, HI, HClO}}_{\text{4}}{\text{, HNO}}_{\text{3}}{\text{, H}}_{\text{2}}{\text{SO}}_{\text{4}}$etcetera.

The acids which do not dissociate completely in an aqueous solution i.e., only few molecules dissociate into their respective ions and most of the molecules remain as such i.e., as a molecule and not ions, are termed as weak acids.

For example, ${\text{CH}}_{\text{3}}{\text{COOH, H}}_{\text{3}}{\text{PO}}_{\text{4}}{\text{, H}}_{\text{2}}{\text{CO}}_{\text{3}}\text{, HF, HCN, HCOOH}$etcetera.

The type of acids which dissociate completely in their aqueous solutions conduct electricity with the help of ions liberated in the aqueous solutions. Thus, the strong acids have the tendency to acts as strong electrolytes as they dissociate completely in an aqueous solution and are present in the form of ions which helps in the conduction of electricity. 

What are the Limitations of Arrhenius' Theory for Acids?

The major point is that the substance which liberates H+ ions in an aqueous solution is an Arrhenius acid. But ${\text{H}}^{+}$ ion does not exist as such in aqueous solutions. It gets taken up by the water $({\text{H}}_{\text{2}}\text{O})$molecule and forms stable hydronium (${\text{H}}_{\text{3}}{\text{O}}^{\text{+}}$) ion.

Practice Problems

1. Which one is the strongest acid among the given acids? 

Solution: Among the given acids, ${\text{HClO}}_{\text{4}}$is the strongest acid followed by${\text{HClO}}_{\text{3}}{\text{, HClO}}_{\text{2}}$, and$\text{HClO}$. So the answer is option B

2. When Arrhenius acid is dissolved in water, which ion is released? 

Solution: When an Arrhenius acid dissolves or dissociates in water it releases hydrogen ions i.e., ${\text{H}}^{+}$ ions. So the answer is option C.

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for;     

B.Sc Biochemistry

B.Sc biology       

M.Sc Biology

M.S. Biomolecular chemistry

M.S. Biotechnology