Acid and Bases

Acids, Bases, and Salts

The diverse tastes of substances like lemon (tangy) and mango (sweet) are due to varying amounts of acids, bases, and salts in their chemical composition. Understanding these fundamental substances involves exploring concepts from different scientific perspectives, including those proposed by Arrhenius, Bronsted-Lowry, and Lewis.

Experimental Definitions

Acids, Bases, and Salts:Traditionally, acids, bases, and salts were characterized through experimental tests involving their aqueous solutions.

• Acids: Substances whose water solutions taste sour, turn blue litmus paper red, and neutralize bases.
• Bases: Substances whose aqueous solutions taste bitter, turn red litmus paper blue, and neutralize acids.
• Salts: Neutral substances whose aqueous solutions do not affect litmus paper.

Faraday's and Liebig's Contributions:

• Faraday: Classified acids, bases, and salts as electrolytes, meaning they conduct electricity when dissolved in water.
• Liebig: Proposed that acids are compounds containing hydrogen, which can be replaced by metals.

Characteristics of Acids

Acidity: Acidity is a defining property of acids, characterized by a sour taste. Examples of common acids include:

• Citric Acid: Found in citrus fruits like lemons and oranges.
• Ascorbic Acid: Also present in citrus fruits.
• Tartaric Acid: Found in tamarind paste.

The word "acid" originates from the Latin word "acidus," meaning sour. Acids turn blue litmus paper red but do not change the color of red litmus paper. They also liberate hydrogen gas (H₂) when reacting with certain metals.

Characteristics of Bases

Basicity: Bases turn red litmus paper blue, and the blue litmus paper remains unchanged. They have a bitter taste and a soapy feel. Common examples of bases include:

• Sodium Bicarbonate: Used in cooking (baking soda).
• Sodium Hydroxide: Commonly found in household bleach.

Characteristics of Salts

Salt Formation: Salts are formed by the neutralization reaction between an acid and a base. For example, sodium chloride (NaCl), or common table salt, is produced when hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH).

Ionic Structure: Sodium chloride consists of positively charged sodium ions (Na⁺) and negatively charged chloride ions (Cl⁻), held together by electrostatic forces. When dissolved in water, these ions separate and become hydrated by water molecules, allowing them to move freely in solution.

Ionization and Dissociation

Dissociation: Dissociation refers to the separation of ions from an ionic crystal when a solid ionic compound dissolves in water.

Ionization: Ionization is the process by which a neutral molecule breaks into charged ions when dissolved in a solution. The extent of ionization depends on the strength of the bonds between ions and the degree of solvation (interaction with solvent molecules).

Modern Concepts of Acids and Bases

1. Arrhenius Concept:

• Acids: Substances that produce hydrogen ions (H⁺) when dissolved in water.
• Bases: Substances that produce hydroxide ions (OH⁻) when dissolved in water.

For example:

HA→ H⁺+ A^–(Acid)

BOH→B⁺+ OH^–(Base)

Arrhenius proposed that acid-base reactions are characterized by the formation of H⁺ and OH⁻ ions in aqueous solutions.

Limitations of Arrhenius Concept:

• The presence of water is essential for the behavior of acids and bases (e.g., dry HCl cannot act as an acid).
• The concept does not explain acid-base behavior in non-aqueous solvents.
• Neutralization is only defined for reactions in aqueous solutions, although salt formation can occur without a solvent.
• The concept cannot explain the acidic behavior of certain salts like aluminum chloride (AlCl₃) in water.
• A separate explanation is needed to define the basic nature of substances like ammonia (NH₃).

2. Bronsted-Lowry Concept:

• Acids: Any hydrogen-containing substance (molecule, anion, or cation) that can donate a proton (H⁺).
• Bases: Any substance (molecule, cation, or anion) that can accept a proton.

This concept introduced the idea of conjugate acid-base pairs, where the acid donates a proton to the base, and the roles can reverse in the reaction.

Example: H₂O + HCl⇔ H₃O⁺+ Cl^–

• HCl donates a proton to water (H₂O), acting as an acid.
• Water (H₂O) accepts the proton, acting as a base.
• In the reverse reaction, H₃O⁺ acts as an acid and Cl⁻ as a base.

Limitations of Bronsted-Lowry Concept:

• It does not explain reactions in non-protonic solvents (e.g., COCl₃, SO₂, N₂O₄).
• Cannot explain reactions between acidic and basic oxides that occur without solvent (no proton transfer).
• Substances like BF₃ and AlCl₃ do not contain hydrogen but still behave as acids.

3. Lewis Concept:

• Acids: Electron-pair acceptors.
• Bases: Electron-pair donors.

The Lewis theory broadens the definition of acids and bases, complementing the model of oxidation-reduction reactions, where electron transfer plays a key role.

Example:

• Al(OH)₃+ 3H⁺→ Al³⁺+ 3H₂O (Aluminium hydroxide is acting as a base)
• Al(OH)₃+ OH^–→ Al(OH)^4-(Aluminium hydroxide is acting as an acid)

The Lewis concept explains why non-metal oxides like carbon dioxide (CO₂) dissolve in water to form acids such as carbonic acid (H₂CO₃).

Limitations of Lewis Concept:

• The Lewis concept includes all coordination reactions and compounds, which is not always accurate.
• It does not provide insight into the relative strength of acids and bases.
• The concept does not align with the traditional acid-base reaction model.
• It does not address the behavior of protonic acids like HCl.