Acids and Bases Thursday, May 30, 2019
Arrhenius and Solvent system concept of acids and bases with examples and Utility of these concepts with related questions answers
What are the acids? Any of the classes of substances whose aqueous solutions are characterized by the sour taste, the ability to turns blue litmus, and the ability to react with bases and certain metals to form salts.
What are the bases? In chemistry, a base is a substance that in aqueous solution is slippery to touch, that's bitter, changed the color of the indicator, (litmus to blue) reacts with an acid to form salts. The concepts of acid and base are developed one after another tend to make the definitions more and more broad-based. We need to familiar with the following five concepts : 1. 2. 3. 4.
Arrhenius concept. Solvent system concept. Protonic concept Lewis concept
Arrhenius concept of acids and bases Arrhenius was one of the early exponents of electrolytic dissociation theory to define acids and bases. His classification of acids and bases was based on the theory that acids when dissolved in water, dissociate hydrogen ions and anions whereas bases when dissolved in water dissociate into hydroxyl ions and cations. Thus HCl is acid and NaOH is a base and the neutralization process can be represented by a reaction involving the combination of H⁺ and OH⁻ ions to form H₂O. HCl → H⁺ + Cl⁻ NaOH → Na⁺ + OH⁻ H⁺ + OH⁻ ⇆ H₂O
Acid-base neutralization reaction Arrhenius concept of acids and bases was highly useful in explaining the acid-base reaction in an aqueous medium. The heat liberated during neutralization of strong acid (HCl, HClO₄, HNO₃, HBr, HI, and H₂SO₄) and strong base (NaOH, KOH, RbOH, and Ca(OH)₂) is the same, namely 13.4 kcal/mole (56 KJ/mole) indicating that the ion participating in the reaction must be the same (that is H⁺ and OH⁻) and that other ions (Na⁺, K⁺, Rb⁺, Ca⁺², Cl⁻, Br⁻, I⁻, ClO₄⁻ or SO₄⁻²) take no part. This can be explained as follows. HA(aq) + BOH(aq) ⇆ BA(aq) + H₂O Where HA and BOH are strong electrolytes and completely dissociate in the Solution. (H⁺+A⁻)+( B⁺+OH⁻)⇆(B⁺+A⁻)+ H₂O Canceling likes ions, we have, H⁺ + OH⁻ ⇆ H₂O Since the neutralization of all strong acids and bases used to the formation of 1-mole water from H⁺ ion and OH⁻ ion. The enthalpy change (ΔH) will also be the same.
Limitation of Arrhenius concept 1. According to this concept, HCl is regarded as an acid only when dissolved in water and
not in some other solvent such as benzene or when it exists in the gaseous state. 2. It cannot account for the acidic and basic character of the materials in non-aqueous solvents, as for example, NH₄NO₃, in liquid NH₃ acts as an acid, though it does not give H⁺ ions. Similarly, many organic materials in NH₃, which does not give OH⁻ ions at all, are actually known to show basic character. 3. The neutralization process limited to those reactions which can occur in aqueous solutions only, although the reactions involving salt formation do occur in many other solvents and even in the absence of solvents. 4. It cannot explain the acidic character of certain salts such as AlCl₃ in aqueous solution.
Solvent system concept The non- aqueous solvent molecules may also dissociate into two oppositely charged ions. We consider the solvent H₂O, its characteristic cation and anion are H⁺ and OH⁻ respectively are, H₂O ⇆ H⁺ + OH⁻
Since we know that a bare proton will readily polarize other anions or molecules we write an H⁺ as H₃O⁺ indicating that it is a solvated proton that exists in the solution. So that the overall dissociation of solvent will be: H₂O + H₂O ⇆ H₃O⁺ + OH⁻ Thus all those compounds which can give H₃O⁺ ions in H₂O will act as acids and all those compounds which can give OH⁻ ions in H₂O will behave as bases. NH₃ + NH₃ ⇆ NH₄⁺ + NH₂⁻ Thus those compounds which give NH₄⁺ ions in liquid NH₃ will act as acids and all those compounds which can give NH₂⁻ ions in liquid NH₃ acts as bases. Thus the dissociation (or auto ionization) of non-aqueous solvents is directly responsible for the nature of the chemical reactions that can be initiated in such solvents. According to the solvent system concept, An acid is a substance which by dissociation in the solvent forms the same cation as does the solvent itself due to auto-ionization. A base is one that, gives on dissociation in the solvent the same anion as does the solvent itself on its ionization.
Auto-ionization of solvents Auto-ionization of some protonic and non-protonic solvents are given below
Protonic solvent H₂O
+ H₂O
Acid
Base
⇆ H₃O⁺ Acid
+ OH⁻ Base
NH₃ + NH₃ ⇆ NH₄⁺ + NH₂⁻ CH₃COOH + CH₃COOH ⇆ CH₃COOH₂⁺ + CH₃COO⁻
Non-protonic solvents
SO₂ Acid
+ SO₂ Base
⇆ SO⁺²
+ SO₃⁻²
Acid
Base
BrF₃ + BrF₃ ⇆ BrF₂⁺ + BrF₄⁻ N₂O₄ + N₂O₄ ⇆ 2NO⁺ + 2NO₃⁻
Neutralization reactions Just as with the Arrhenius definition, neutralization is a reaction between an acid and a base to produce salt and solvent. Neutralization of some non-aqueous solvents are,
Neutralization reaction in liquid NH₃ Dissociation of solvent NH₃ + NH₃ ⇆ NH₄⁺ + NH₂⁻
Dissociation of acid NH₄Cl ⇆ NH₄⁺ + Cl⁻
Dissociation of base KNH₂ ⇆ K⁺ + NH₂⁻
Thus the acid-base neutralization reaction is NH₄Cl Acid
+ KNH₂
⇆ KCl
Base
Salt
+ 2NH₃
Neutralization reaction in liquid SO₂ Dissociation of solvent SO₂ + SO₂ ⇆ SO⁺² + SO₃⁻²
Dissociation of acid SOBr₂ ⇆ SO⁺² + 2Br⁻
Solvent
Dissociation of base K₂SO₃ ⇆ 2K⁺ + SO₃⁻²
Thus the acid-base neutralization reaction SOBr₂ Acid
+ K₂SO₃ Base
⇆ 2KBr Salt
+ 2SO₂ Solvent
Acids and bases
Utility of the solvent system concept Evidently, this concept of the solvent system can be used to explain the acid-base reactions occurring in aqueous and non-aqueous solvents(protonic or non-protonic both). Thus this theory can simply be said to be an extension of the Arrhenius theory.
Limitations of the solvent system concept 1. This theory does not consider a number of acid-base reactions included in the protonic definition. 2. It limits acid-base phenomena to the solvent system only. Thus it does not explain the acid-base reactions which may occur in the absence of solvent. 3. It can not explain the neutralization reactions occurring without the presence of ions. Ask a question related to this topic Labels: Acids and bases, Inorganic chemistry, Recent post
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