Acid-Base Theories: Arrhenius Concept of Acids and Bases

 Introduction and Explanation:

In a chemical change, When the rates of the forward and reverse reactions become equal to each other, the concentrations of the reactants and the products remain constant. This is called chemical equilibrium. Equilibrium is dynamic in nature; this means that the reactant changes to the product and the product changes to the reactant.

• Let us consider a reversible reaction, A + B ⇌ C + D.
• As time passes, the concentration of the product goes on increasing, and the concentration of the reactant decreases.
• Due to this, the rate of the forward reaction decreases, and the rate of the reverse reaction increases.
• Then at a particular stage, the two reactions occur at the same rate, and the system reaches a stage of equilibrium.

Graphical Representation

Ionic Equilibrium

• On the ability to conduct electricity, the substances are categorized into two.
• One that conducts electricity in its aqueous solutions is called an electrolyte.
• While the other that does not conduct electricity is called a non-electrolyte.

• Electrolytes are of two types, that is, strong and weak electrolytes.
• In water, strong electrolytes are ionized completely, while weak electrolytes are partially ionized.
• When sodium chloride is dissolved in water, it dissociates into a sodium ion and chloride ion completely.
• But acetic acid contains both unionized acetic acid molecules and some acetate ions and hydronium ions.
• 100% ionization takes place in the case of sodium chloride (strong electrolyte) and in the case of acetic acid (weak electrolyte), only 5% ionization takes place.

• In weak electrolytes, equilibrium is established between ions and the unionized molecules.
• This type of equilibrium in an aqueous solution involving ions is called ionic equilibrium.

Acids

• According to Arrhenius’s theory, acids are substances (or species) that dissociate (or break) in water to give hydrogen ions H⁺ (aq).
• A proton, H⁺, is highly reactive. So, in an aqueous solution, it cannot exist freely.
• Therefore, it combines with the oxygen atom of a water molecule (solvent) to give trigonal pyramidal hydronium ion, H3O+.

Example – i

HX (aq) → H⁺ (aq) + X^– (aq)

                OR

HX (aq) + H₂O (l) → H₃O⁺ (aq) + X^– (aq)

HCl (aq) → H⁺ (aq) + Cl^– (aq)

                OR

HCl (aq) + H₂O (l) → H₃O⁺ (aq) + Cl^– (aq)

Example – ii

HX (aq) → H⁺ (aq) + X^– (aq)

               OR

HX (aq) + H₂O (l) → H₃O⁺ (aq) + X^– (aq)

HNO₃ (aq) → H⁺ (aq) + NO₃^– (aq)

                OR

HNO₃ (aq) + H₂O (l) → H₃O⁺ (aq) + NO₃^– (aq)

Example – iii

HX (aq) → H⁺ (aq) + X^– (aq)

              OR

HX (aq) + H₂O (l) → H₃O⁺ (aq) + X^– (aq)

H₂SO₄ (aq) → 2 H⁺ (aq) + SO₄^2- (aq)

              OR

H₂SO₄ (aq) + H₂O (l) → H₃O⁺ (aq) + SO₄^2- (aq)

Arrhenius Concept Of Acids And Bases

Bases

• According to Arrhenius theory, bases are substances (or species) that generate hydroxyl ions OH^– (aq).
• Like H⁺, the hydroxyl ion in an aqueous solution also exists in the hydrated form.

Example – i

MOH (aq) → M⁺ (aq) + OH^– (aq)

                           OR

MOH (aq) + H₂O (l) → OH^– (aq) + M⁺ (aq)

NaOH (aq) → Na⁺ (aq) + OH^– (aq)

                          OR

NaOH (aq) + H₂O (l) → Na⁺ (aq) + OH^– (aq)

Example – ii

MOH (aq) → M⁺ (aq) + OH^– (aq)

                           OR

MOH (aq) + H₂O (l) → OH^– (aq) + M⁺ (aq)

KOH (aq) → K⁺ (aq) + OH^– (aq)

                           OR

KOH (aq) + H₂O (l) → K⁺ (aq) + OH^– (aq)

Example – iii

MOH (aq) → M⁺ (aq) + OH^– (aq)

                           OR

MOH (aq) + H₂O (l) → OH^– (aq) + M⁺ (aq)

Mg (OH)₂ (aq) → Mg²⁺ (aq) + 2 OH^– (aq)

                           OR

Mg (OH)₂ (aq) + H₂O (l) → Mg²⁺ (aq) + 2 OH^– (aq)

Hydronium Ion

• The hydrogen ion is a proton with high reactivity due to its instability and very small size (~10^-15 m radius); it combines with the water molecule with one of the two available lone pairs on it, giving H₃O⁺.

Advantages:

This theory is used to explain

1. Strength of acid and bases

It can be determined by the extent to which it dissociates to give H⁺ or hydroxide ions.

HCl (aq) → H⁺ (aq) + Cl^– (aq)

NaOH (aq) → Na⁺ (aq) + OH^– (aq) acids and bases in aqueous medium.

2. Arrhenius could not explain acid-base reactions that do not occur in aqueous solutions.

For example, the reaction between gaseous ammonia and hydrogen chloride is an example of an acid-base reaction.

NH₃ (g) + HCl (g) ⇌ NH₄Cl (s)

Limitations

• This concept is applicable only to aqueous solutions.
• Ammonia is a base, but it does not contain hydroxyl ions. Arrhenius’s concept does not give any explanation for it.

Summary

Equilibrium

• In a chemical change, the rates of the forward and reverse reactions become equal to each other, at this stage the concentrations of the reactants and the products remain constant. This is called chemical equilibrium.
• Equilibrium is dynamic in nature
• In weak electrolytes, equilibrium is established between ions and the unionized molecules. This type of equilibrium in an aqueous solution involving ions is called ionic equilibrium.
• Example of ionic equilibrium – Acetic acid contains both unionized acetic acid molecules and some acetate ions and hydronium ions.

Arrhenius Concept of Acids

• According to Arrhenius’s theory, acids are substance (or species) that dissociates (or break) in water to give hydrogen ions H⁺ (aq).
• A proton, H⁺ is highly reactive. So, in an aqueous solution, it cannot exist freely.
• Therefore, it combines with the oxygen atom of a water molecule (solvent) to give trigonal pyramidal hydronium ion, H₃O⁺

HX (aq) → H⁺ (aq) + X^– (aq)

OR

HX (aq) + H₂O (l) → H₃O⁺ (aq) + X^– (aq)

Arrhenius Concept of Bases

• According to Arrhenius theory, bases are substances (or species) that generate hydroxyl ions OH- (aq).
• Like H+, the hydroxyl ion in aqueous solution also exists in the hydrated form.

MOH (aq) → M⁺ (aq) + OH^– (aq)

OR

MOH (aq) + H₂O (l) → OH^– (aq) + M⁺ (aq)