Introduction
Ionic equilibrium is the equilibrium established between the unionized molecules and the ions in a solution of weak electrolytes.
Arrhenius concept
According to Arrhenius concept, An acid is a substance which dissociate in aqueous solution to give H⁺ ions and base is a substance with dissociates in aqueous solution to give (OH)⁻ ions.
Thus according to the acidic properties of substance in the solution is due to the presence of H⁺ Ions and basic properties due to the presence of (OH)⁻ ions.
For acid:HX ⇆ H⁺ + X⁻For base:MOH ⇆ M⁺ + OH⁻He also clarify the strength of acid and base in term of dissociation power and classify the acid and base in terms of strength.
Strong acid
Acid such as HCl, H₂SO₄, HNO₃ are completely dissociate in solution of so they are called strong acid.
Weak acid
Acid such H₃PO₄, CH₃COOH, HCOOH are feebly or partially dissociates into the aqueous solution so they are called weak acid.
Strong base
Base like NaOH, KOH are completely dissociates in aqueous. So they are strong base.
Weak Base
Base like NaOH, KOH are completely dissociates in aqueous. So they are strong base.
Limitation of Arrhenius concept
- This concept is applicable to the acid base behavior only in a aqueous medium but can’t explain in non-aqueous a medium that means Gas is not acid according to this concept.
- This concept falls to fall explain the acidic character of CO₂,SO₂ as well as basic character of MgO.
Bronsted-Lowry concept
This theory was proposed by JN Bronsted and Tom lowry (England) in 1929. According to this concept, An acid is a substance that can donate proton (H⁺) and base is a substance that can accept proton (H⁺). Simply proton donor are acid and proton acceptor base.
For eg.HCl+ NH₃ → Cl⁻+NH₄⁺Here, HCl donate H⁺ and acts as acid and NH₃ accept H⁺ from HCl and acts as base.
This theory is explained in terms of conjugate pair in above reaction, HCL donate proton and form Cl ion again, this Cl ion have tendency to accept proton and acts as a base. Thus, a base obtained from an acid having capacity to accept proton is called conjugate base. Here HCl and Cl are conjugate acid and base pair.
Also NH₃ accept proton and form NH₄⁺. Again this NH₄⁺. Again this NH₄⁺ ion have tendency to donate proton and acts as acid. Thus an acid obtained from a base having capacity to donate proton is called conjugate acid. Here NH₃ and NH₄⁺ are conjugate base-acid pair.
According to this concept, water is an amphoteric compound which can be illustrated by the two reaction given below.H₂O+NH₃ → OH⁻+NH₄HCl+H₂O → Cl⁻+H₃O⁺According to this concept, strength of acid and base depend upon its tendency to donate the proton or accept the proton.HCl + H₂O ⇆ Cl⁻ + H₂O⁺Here, HCl has strong tendency donate proton. So, it is strong acid but Cl⁻ obtained from HCl has less tendency to accept the proton.So it is weak base that means conjugate of strong acid and base is always weak and vise versa.
Advantages
- This concept explains the basic character of the substance like Na₂CO₃, NH₃ which does not contain OH⁻ ions.
- It can explain the acid or base in aqueous as well as non-aqueous medium.
- This concept isn’t limited to neutral molecules but also include the ionic species that acts as acid and base.
Limitation of Bronsted concept
- It cannot explain the relation between acid oxides like SO₃, SO₂, CO₂ and basic oxides like MgO,CaO, BaO, etc. which takes place in absence of solvent.
CaO + SO₃ → CaSO₄ - Substance like BF₃, AlCl₃, etc. do not have any hydrogen hence cannot give a proton but are known to behave as acids.
Lewis concept
Gilbert Newton Lewis in 1928 proposed a new theory regarding acid and base on the basis of electronic theory of valency.
According to this concept,”An acid is a substance that accept a lone pair of electrons through co-ordinate bond and base is a substance that donates a pair of electron through co-ordination bond “need a photoSimply acid are electron pair accepter and base are electron pair donar.
Thus,from this concept it id evident that any unshared pair of electron containing substances are lewis base where as the substance having vacant orbital and that can accommodate pair of electron are lewis acid.NH₃(base) need a photo BF₃(acid)
Lewis bases maybe:
- Negatively charged species.
Eg. Cl⁻, SO₄⁻⁻, CO₃⁻⁻, etc. - Neutral molecules having unshared pair of electrons.
Eg. NH₃, H-O-H, R-OH, R-O-R, etc.
Since they have more electrons or electron rich species. So Lewis base are also called nucleophile.
lewis acid maybe:
- positively charged species:
Eg.NH4⁺, Ca++, Al+++, etc. - Neutral molecules having incomplete octet.
Eg.AlCl₃,BF₃,etc. - Molecules having multiple bonds containing dissimilar electronegative atom.
Eg.CO₂,SO₂,SO₃,etc.
Since,there is electron deficient in such species.So, lewis acid are also electrophile.
Limitation of Lewis concept
- This concept doesn’t explain the strength of aced and base.
- It does not explain the acid-base behavior of substance like HCl which do not form co-ordinate covalent compound.Hence, Hcl cannot be considered as lewis acid.
- Since, acid-base reaction are very fast but the formation of Co-ordinate covalent compound is very slow. Hence, this concept does not fit for the acid base reaction
Hydrolysis of salt
As we know that aced react with base to give salt and water. Some salts when dissolved water interact with it and break down to its parent acid and base. This reaction is called Hydrolysis of salt.CuSO₄ + H₂O → Cu(OH)₂ + H2SO₄
CuSO₄ ⇆ Cu⁺⁺ + SO₄⁻⁻
H₂O⇆ H⁺ + OH⁻
Simply it is defined as the interaction of ions of salt with water resulting in the formation of acid and base.
It helps to predict the nature of salt whether it is acidic or basic or neutral.
Questions ???
- Predict the aqueous solution pf Na₂CO₃?
- Why aqueous solution of FeCl₃ acidic?
Ans 1: Sodium carbonate is a salt of strong base(NaOH) and weak acid(H₂CO₃)Na₂CO₃ + H₂O → H₂CO₃ + NaOHThe aqueous solution is Na₂CO₃ is basic.
Ans 2: aqueous solution of FeCl₃ is acidic because it undergoes hydrolysis and forms HCl.This strong acid release H⁺ ions in the solution,Hence the resulting solution is acidic and turns blue litmus into red.FeCl₃ + H2O → HCl + Fe(OH)₃