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Ionization Of Acids And Bases

What is Ionization?

Ionization of a compound refers to a process in which a neutral molecule splits into charged ions when exposed to a solution. The compound which is dissolved in a solvent produces positive and negative ions called electrolytes and the degree of ionisation is defined as the ratio of a number of molecules undergoing dissociation to a total number of molecules.

𝞪 = number of molecules undergoing dissociation/ total number of molecules

Where 𝞪 is called as the degree of ionization

Table Of Contents

Arrhenius concept of Acid and base ionization

According to Arrhenius theory, acids are the compounds that dissociate in an aqueous medium to generate hydrogen ions,

\(\begin{array}{l}H^+\end{array} \)
(aq). On the other hand, bases are the compounds which furnish hydroxyl ions,
\(\begin{array}{l}OH^{-}\end{array} \)
(aq) in an aqueous medium.


Ionization of Acids and Bases

Arrhenius theory plays an important role in explaining the ionization of acids and bases as mostly ionization takes place in an aqueous medium. On the basis of the degree of ionization of acids and bases, we can determine the strength of acids and bases. The degree of ionization differs for different acidic and basic compounds. Some acids such as perchloric acid (

\(\begin{array}{l}HClO_4\end{array} \)
), hydrochloric acid (
\(\begin{array}{l}HCl\end{array} \)
) dissociate completely into their constituent ions in the aqueous medium. These acids are termed strong acids. The ionization of acids yields hydrogen ions, thus, these compounds act as proton donors. Similarly, some bases like lithium hydroxide (
\(\begin{array}{l}LiOH\end{array} \)
), sodium hydroxide (
\(\begin{array}{l}NaOH\end{array} \)
) too completely dissociate into their ions in an aqueous medium. These bases are termed strong bases. Ionization of these bases yields hydroxyl ions (
\(\begin{array}{l}OH^-\end{array} \)

Hence, the degree of ionization of acids and bases depends on the degree of dissociation of compounds into their constituent ions. Strong acids and bases have a high degree of ionization in comparison to weak acids and bases. Also, a strong acid implies a good proton donor, whereas a strong base implies a good proton acceptor. For example, dissociation of weak acid HA

\(\begin{array}{l}HA ~(aq)~ + ~H_2O~ (l)~ ⇌~H_3O^+~(aq)~ + ~A^-~ (aq)\end{array} \)

Explanation of Arrhenius acid and base ionization

This reaction implies that acid dissociation equilibrium is dynamic in nature where the transfer of proton can take place in both forward and backward directions. If

\(\begin{array}{l}HA\end{array} \)
has a higher tendency to donate proton in comparison to
\(\begin{array}{l}H_3O^+\end{array} \)
\(\begin{array}{l}HA\end{array} \)
acts as a strong acid in comparison to
\(\begin{array}{l}H_3O^+\end{array} \)
. Since the stronger acid donates a proton to the stronger base. The equilibrium moves in the direction of the formation of a weaker acid and weaker base. The strong acids generally have a weaker conjugate bases whereas strong bases have weaker conjugate acids. This is because of the high degree of ionization of strong acids and bases.

Determination of Ionisation constant of acid and base

From the above discussion we conclude that when an Arrhenius acid ionizes, it splits into the following manner.

HA (aq) + H2O (l) ⇌ H3O+(aq) + A(aq)

So according to equilibrium constant;

\(\begin{array}{l} K = \frac{[H_{3}O^{+}] [A^{-}]}{[HA] [H_{2}O]}\end{array} \)

K [H2O] = [H3O+] [A] / [HA]

Ka = [H3O+] [A] / [HA]

Where Ka is known as the ionization constant of acid, for stronger acid Ka value will be higher.

Similarly for Arrhenius base

BOH (aq) + H2O (l) ⇌ B+ (aq) + OH(aq)

So according to equilibrium constant;

\(\begin{array}{l} K = \frac{[B^{+}] [OH^{-}]}{[BOH] [H_{2}O]}\end{array} \)

K [H2O] = [B+] [OH] / [BOH]

Kb = [B+] [OH] / [BOH]

Where Kb is known as the ionization constant of the base, for a stronger base, the Kb value will be higher.

Examples of Acid and Base ionization

The equation for ionization of CH3COOH and NH4OH.

Ionization of acetic acid in aqueous solution: CH3COOH + H2O  ⇋  H3O+ + CH3COO

Ionization of ammonium hydroxide in aqueous solution; NH4OH + H2O  ⇋  NH4+ + OH

Frequently Asked Questions – FAQs

What happens when acids and bases are ionized?

According to Arrhenius’s theory, acids are the electrolytes that ionize in an aqueous medium to generate hydrogen ions, H+(aq). On the other hand, bases are the electrolytes which furnish hydroxyl ions, OH(aq) in an aqueous medium.

How do you know if something ionizes completely?

From the degree of ionization value, we can find out whether the compound is completely ionized or not. The degree of ionization means the ratio of the number of molecules undergoing dissociation to the total number of molecules. If the degree of dissociation value is 1, it suggests that the molecule is completely ionized.

Is HCl strong or weak acid?

HCl is a strong acid because it dissociates almost completely, and it has a high degree of dissociation or ionization value.

What is the percent ionization formula?

The percentage of ionization of weak acid/base is equal to the ratio of the concentration of the ionized weak acid/base to the initial concentration of acid/ base multiplied by 100.

What is base ionization?

When dissolved in water, a base is an ionized substance to produce hydroxide ions (OH) and its conjugate acid. The equilibrium constant for the ionization of the base is represented by Kb. Strong bases have higher Kb values than weak bases.

For detailed discussions on the ionization of acids and bases in an aqueous medium, download BYJU’S – The Learning App.

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