Acids, Bases, and Ionisation

1.0What is Ionization?

Ionization is the process by which a neutral molecule splits into charged ions when dissolved in a solvent, usually water. In the context of acids and bases, ionisation refers to the dissociation of these compounds into their respective ions. This process is fundamental in understanding chemical equilibrium, conductivity, and the behavior of solutions in acid-base chemistry.

• Acids ionize to produce hydrogen ions (H⁺).
• Bases ionize to produce hydroxide ions (OH⁻).

The extent of ionization determines whether an acid or base is strong (almost complete ionization) or weak (partial ionization), affecting their chemical properties and reactivity.

2.0Acids Bases Ionisation Formula

Degree of Ionization (α)

$\alpha =\frac{\text{Number of molecules ionized}}{\text{Total number of molecules dissolved}}$

For a weak acid (HA) with initial concentration (C) and degree of ionisation ($\alpha$):

$$\begin{gathered} [{\text{H}}^{+}]=C\alpha \\ Ka=\frac{C{\alpha }^2}{1-\alpha } \end{gathered}$$

For very weak acids, ($1-\alpha \approx 1$), so: $Ka\approx C{\alpha }^2$

Similarly, for weak bases: $K_b=\frac{C{\alpha }^2}{1-\alpha }\approx C{\alpha }^2$

pH and pOH Formulas

$$\begin{gathered} \text{pH}=-\log [{\text{H}}^{+}] \\ \text{pOH}=-\log [{\text{OH}}^{-}] \\ \text{pH}+\text{pOH}=14 \end{gathered}$$

3.0Arrhenius Concept of Acid and Base Ionization

The Arrhenius concept of acid and base ionization provides a foundational understanding of how acids and bases behave in aqueous solutions. According to the Arrhenius theory, acids are substances that dissociate in an aqueous medium to release hydrogen ions, H⁺ (aq). In contrast, bases are compounds that dissociate in water to produce hydroxide ions, OH⁻ (aq).

The normal reaction: HCl + KOH → H₂O + KCl

(Dissociation)

Breakdown of the strong acid and base into ions

• HCl → H⁺ + Cl⁻
• KOH → K⁺ + OH⁻

Ionic Equation : H⁺ + Cl⁻ + K⁺ + OH⁻ → H₂O + K⁺ + Cl⁻

• Net Ionic Equation: H⁺ + OH⁻ → H₂O

This ionization process is crucial in determining the chemical properties and strength of acids and bases. The degree of ionization—that is, the fraction of molecules that actually split into ions—varies among different compounds and directly influences whether an acid or base is classified as strong or weak.

For instance, certain acids such as hydrochloric acid (HCl) and perchloric acid (HClO₄) ionize completely in water, releasing a high concentration of H⁺ ions. These are known as strong acids. Their complete ionization means they act as excellent proton donors in solution. Similarly, bases like sodium hydroxide (NaOH) and lithium hydroxide (LiOH)also dissociate fully, providing a substantial amount of OH⁻ ions, and are thus termed strong bases.

In contrast, weak acids (such as acetic acid, CH₃COOH) and weak bases (like ammonia, NH₃) only partially ionize in water. This limited ionization results in lower concentrations of H⁺ or OH⁻ ions in solution. Therefore, the degree of ionization is a direct measure of acid or base strength: strong acids and bases have high degrees of ionization, while weak acids and bases have much lower degrees.

To summarize, the Arrhenius concept explains that:

• Acids ionize in water to donate protons (H⁺ ions).
• Bases ionize in water to accept protons, typically by furnishing OH⁻ ions.

This theory helps clarify why strong acids and bases are highly effective in chemical reactions, while weaker ones only partially contribute to ion formation in aqueous environments.

Acids bases ionisation examples:

• Strong acid: HCl → H⁺ + Cl⁻
• Strong base: NaOH → Na⁺ + OH⁻
• Weak acid: HA + H₂O ⇌ H₃O⁺ + A⁻

4.0Ionization Processes of Acids and Bases

Ionization of Acids

When acids dissolve in water, they ionize to produce hydrogen ions (H⁺), which associate with water molecules to form hydronium ions (H₃O⁺):

General Equation: $\text{HA}+\text{H}2\text{O}\rightleftharpoons \text{H}3{\text{O}}^{+}+{\text{A}}^{-}$

Example:

• Acetic acid (CH₃COOH) ionizing in water: $[\text{CH}3\text{COOH}+\text{H}2\text{O}\rightleftharpoons \text{H}3{\text{O}}^{+}+\text{CH}3{\text{COO}}^{-}]$

Ionization of Bases

Bases ionize in water to produce hydroxide ions (OH⁻):

General Equation:$\text{BOH}\rightleftharpoons {\text{B}}^{+}+{\text{OH}}^{-}$

Example:

• Ammonia (NH₃) acting as a base: $\text{NH}3+\text{H}2\text{O}\rightleftharpoons {\text{NH}}_4^{+}+{\text{OH}}^{-}$

5.0Explanation of Arrhenius Acid and Base Ionization

Arrhenius Acid Ionization

When an Arrhenius acid dissolves in water, it ionizes to release H⁺ ions. These ions typically associate with water molecules to form hydronium ions (H₃O⁺).

General equation: $\text{HA}\to {\text{H}}^{+}+{\text{A}}^{-}\text{or}\text{HA}+\text{H}2\text{O}\to \text{H}3{\text{O}}^{+}+{\text{A}}^{-}$

Example:

$\text{HCl}\to {\text{H}}^{+}+{\text{Cl}}^{-}$

$\text{HNO}3\to {\text{H}}^{+}+\text{NO}{3}^{-}$

Arrhenius Base Ionization

When an Arrhenius base dissolves in water, it ionizes to produce OH⁻ ions.

General equation:

$\text{BOH}\to {\text{B}}^{+}+{\text{OH}}^{-}$ or, for bases like ammonia: $\text{NH}3+\text{H}2\text{O}\to {\text{NH}}_4^{+}+{\text{OH}}^{-}$

Example:

$\text{NaOH}\to {\text{Na}}^{+}+{\text{OH}}^{-}$

$\text{KOH}\to {\text{K}}^{+}+{\text{OH}}^{-}$

6.0Determination Of Ionisation Constant Of Acid And Base

The ionisation constant quantifies the strength of an acid or base by measuring the extent of its ionisation in water.

Ionisation Constant of Acid (Ka)

For a weak acid, HA, the ionization in water can be represented as:

$\text{HA}+\text{H}2\text{O}\rightleftharpoons \text{H}3{\text{O}}^{+}+{\text{A}}^{-}$

The equilibrium expression is:

$Ka=\frac{[\text{H}3{\text{O}}^{+}][{\text{A}}^{-}]}{[\text{HA}]}$

A higher Ka value indicates a stronger acid.

Ionisation Constant of Base (Kb)

For a weak base, BOH, the ionization is:

$\text{BOH}\rightleftharpoons {\text{B}}^{+}+{\text{OH}}^{-}$

The equilibrium expression is: $K_b=\frac{{\text{B}}^{+}][{\text{OH}}^{-}]}{[\text{BOH}]}$

A higher Kb value indicates a stronger base.

Relationship with Water Ionization (Kw)

The ionic product of water: $Kw=[{\text{H}}^{+}][{\text{OH}}^{-}]=1.0\times 10^{-14},\text{at 25°C}$

For a conjugate acid-base pair: $Ka\times Kb=Kw$

7.0Examples of Acid and Base Ionization

Example 1: Ionisation of Hydrochloric Acid (HCl)

HCl is a strong acid and ionizes completely in water: $\text{HCl}\to {\text{H}}^{+}+{\text{Cl}}^{-}$

If 0.1 M HCl is dissolved in water, $([{\text{H}}^{+}]=0.1)M,and(\text{pH}=1).$

Example 2: Ionisation of Acetic Acid (CH₃COOH)

Acetic acid is a weak acid and ionizes partially: $\text{CH}3\text{COOH}+\text{H}2\text{O}\rightleftharpoons \text{CH}3{\text{COO}}^{-}+\text{H}3{\text{O}}^{+}$

Given $(Ka=1.8\times 10^{-5})$ and initial concentration (C = 0.1) M: $Ka=C{\alpha }^2\Rightarrow \alpha =\sqrt{\frac{K_a}{C}}$

$\alpha =\sqrt{\frac{1.8\times 10^{-5}}{0.1}}\approx 0.013$

So, only about 1.3% of CH₃COOH molecules are ionized.

Example 3: Ionisation of Sodium Hydroxide (NaOH)

NaOH is a strong base and ionizes fully: $\text{NaOH}\to {\text{Na}}^{+}+{\text{OH}}^{-}$

For 0.01 M NaOH, $([{\text{OH}}^{-}]=0.01)M,(\text{pOH}=2),so(\text{pH}=12).$

Example 4: Ionisation of Ammonia (NH₃)

NH₃ is a weak base and ionizes partially: $\text{NH}3+\text{H}2\text{O}\rightleftharpoons \text{NH}{4}^{+}+{\text{OH}}^{-}$

Given $(Kb=1.8\times 10^{-5})and(C=0.1)$ M:

$Kb=C{\alpha }^2\Rightarrow \alpha =\sqrt{\frac{Kb}{C}}$

$\alpha =\sqrt{\frac{1.8\times 10^{-5}}{0.1}}\approx 0.013$

8.0Degree of Ionization and Factors Affecting It

Degree of Ionization (α): The fraction of the total number of molecules that ionize in solution.

$\alpha =\frac{\text{Number of ionized molecules}}{\text{Total number of molecules}}$

Factors Affecting Ionization:

1. Nature of the Acid or Base: Strong acids/bases ionize completely (α ≈ 1), while weak acids/bases ionize partially (α < 1).
2. Concentration: Dilution increases ionization for weak acids and bases due to decreased ion pairing.
3. Temperature: Higher temperatures generally increase ionization.
4. Presence of Common Ion: The common ion effect suppresses ionization.

9.0Sample Questions for Acids, Bases and Ionisation

Q1: What is the acids bases ionisation formula?

For acids: $(Ka=\frac{[{\text{H}}^{+}][{\text{A}}^{-}]}{[\text{HA}]})$

For bases: $(Kb=\frac{[{\text{B}}^{+}][{\text{OH}}^{-}]}{[\text{BOH}]})$

Q2. What is the relationship between Ka, Kb, and Kw?

Ans: For a conjugate acid-base pair: ( Ka \times Kb = K_w = 1.0 \times 10^{-14} ) at 25°C.