`=>` Arrhenius concept of acids and bases becomes useful in case of ionization of acids and bases as mostly ionizations in chemical and biological systems occur in aqueous medium.
`=>` Strong acids like perchloric acid `color{red}((HClO_4))`, hydrochloric acid `color{red}((HCl))`, hydrobromic acid `color{red}((HBr))`, hyrdoiodic acid `color{red}((HI))`, nitric acid `color{red}((HNO_3))` and sulphuric acid `color{red}((H_2SO_4))` are termed strong because :
•They are almost completely dissociated into their constituent ions in an aqueous medium, thereby acting as proton (H+) donors.
`=>` Similarly, bases like lithium hydroxide `color{red}((LiOH))`, sodium hydroxide `color{red}((NaOH))`, potassium hydroxide `color{red}((KOH))`, caesium hydroxide `color{red}((CsOH))` and barium hydroxide `color{red}(Ba(OH)_2)` are strong because:
•They almost get completely dissociated into ions in an aqueous medium giving hydroxyl ions, `color{red}(OH^–)`.
`=>` According to Arrhenius concept they are strong acids and bases as they are able to completely dissociate and produce `color{red}(H_3O^+)` and `color{red}(OH^–)` ions respectively in the medium. Alternatively, the strength of an acid or base may also be gauged in terms of Brönsted- Lowry concept of acids and bases, wherein a strong acid means a good proton donor and a strong base implies a good proton acceptor. Consider, the acid-base dissociation equilibrium of a weak acid `color{red}(HA),`
`color{red}(undersettext(acid)(HA (aq)) + undersettext(base)(H_2O(l)) ⇌ undersettext(conjugate acid)(H_3 O^+ (aq)) +undersettext(conjugate base)(A^- (aq)))`
•If `color{red}(HA)` is a stronger acid than `color{red}(H_3O^+),` then `color{red}(HA)` will donate protons and not `color{red}(H_3O^+)`, and the solution will mainly contain `color{red}(A^–)` and `color{red}(H_3O^+)` ions. The equilibrium moves in the direction of formation of weaker acid and weaker base because the stronger acid donates a proton to the stronger base.
`=>` It follows that as a strong acid dissociates completely in water, the resulting base formed would be very weak i.e., strong acids have very weak conjugate bases.
`=>` Strong acids like perchloric acid `color{red}((HClO_4))`, hydrochloric acid `color{red}((HCl))`, hydrobromic acid `color{red}((HBr))`, hydroiodic acid `color{red}((HI))`, nitric acid `color{red}((HNO_3))` and sulphuric acid `color{red}((H_2SO_4))` will give conjugate base ions `color{red}(ClO_4^–, Cl, Br^(–), I^(–), NO_3^(–))` and `color{red}(HSO_4^(–))` , which are much weaker bases than `color{red}(H_2O)`.
`=>` Similarly a very strong base would give a very weak conjugate acid.
`=>` On the other hand, a weak acid say `color{red}(HA)` is only partially dissociated in aqueous medium and thus, the solution mainly contains undissociated `color{red}(HA)` molecules.
`=>` Typical weak acids are nitrous acid `color{red}((HNO_2))`, hydrofluoric acid `color{red}((HF))` and acetic acid `color{red}((CH_3COOH))`. It should be noted that the weak acids have very strong conjugate bases. For example, `color{red}(NH_2^(–), O^(2–))` and `color{red}(H^–)` are very good proton acceptors and thus, much stronger bases than `color{red}(H_2O).`
Certain water soluble organic compounds like phenolphthalein and bromothymol blue behave as weak acids and exhibit different colours in their acid `color{red}((HI n))` and conjugate base `color{red}((In– ))` forms.
`color{red}(undersettext(acid indicator colour A)(HIn(aq)) +H_2O (l) ⇌ undersettext(conjugate acid)(H_3O^+(aq))+undersettext(conjugate base colourB)( In^(-)(aq)))`
Such compounds are useful as indicators in acid-base titrations, and finding out `color{red}(H^+)` ion concentration.
`=>` Arrhenius concept of acids and bases becomes useful in case of ionization of acids and bases as mostly ionizations in chemical and biological systems occur in aqueous medium.
`=>` Strong acids like perchloric acid `color{red}((HClO_4))`, hydrochloric acid `color{red}((HCl))`, hydrobromic acid `color{red}((HBr))`, hyrdoiodic acid `color{red}((HI))`, nitric acid `color{red}((HNO_3))` and sulphuric acid `color{red}((H_2SO_4))` are termed strong because :
•They are almost completely dissociated into their constituent ions in an aqueous medium, thereby acting as proton (H+) donors.
`=>` Similarly, bases like lithium hydroxide `color{red}((LiOH))`, sodium hydroxide `color{red}((NaOH))`, potassium hydroxide `color{red}((KOH))`, caesium hydroxide `color{red}((CsOH))` and barium hydroxide `color{red}(Ba(OH)_2)` are strong because:
•They almost get completely dissociated into ions in an aqueous medium giving hydroxyl ions, `color{red}(OH^–)`.
`=>` According to Arrhenius concept they are strong acids and bases as they are able to completely dissociate and produce `color{red}(H_3O^+)` and `color{red}(OH^–)` ions respectively in the medium. Alternatively, the strength of an acid or base may also be gauged in terms of Brönsted- Lowry concept of acids and bases, wherein a strong acid means a good proton donor and a strong base implies a good proton acceptor. Consider, the acid-base dissociation equilibrium of a weak acid `color{red}(HA),`
`color{red}(undersettext(acid)(HA (aq)) + undersettext(base)(H_2O(l)) ⇌ undersettext(conjugate acid)(H_3 O^+ (aq)) +undersettext(conjugate base)(A^- (aq)))`
•If `color{red}(HA)` is a stronger acid than `color{red}(H_3O^+),` then `color{red}(HA)` will donate protons and not `color{red}(H_3O^+)`, and the solution will mainly contain `color{red}(A^–)` and `color{red}(H_3O^+)` ions. The equilibrium moves in the direction of formation of weaker acid and weaker base because the stronger acid donates a proton to the stronger base.
`=>` It follows that as a strong acid dissociates completely in water, the resulting base formed would be very weak i.e., strong acids have very weak conjugate bases.
`=>` Strong acids like perchloric acid `color{red}((HClO_4))`, hydrochloric acid `color{red}((HCl))`, hydrobromic acid `color{red}((HBr))`, hydroiodic acid `color{red}((HI))`, nitric acid `color{red}((HNO_3))` and sulphuric acid `color{red}((H_2SO_4))` will give conjugate base ions `color{red}(ClO_4^–, Cl, Br^(–), I^(–), NO_3^(–))` and `color{red}(HSO_4^(–))` , which are much weaker bases than `color{red}(H_2O)`.
`=>` Similarly a very strong base would give a very weak conjugate acid.
`=>` On the other hand, a weak acid say `color{red}(HA)` is only partially dissociated in aqueous medium and thus, the solution mainly contains undissociated `color{red}(HA)` molecules.
`=>` Typical weak acids are nitrous acid `color{red}((HNO_2))`, hydrofluoric acid `color{red}((HF))` and acetic acid `color{red}((CH_3COOH))`. It should be noted that the weak acids have very strong conjugate bases. For example, `color{red}(NH_2^(–), O^(2–))` and `color{red}(H^–)` are very good proton acceptors and thus, much stronger bases than `color{red}(H_2O).`
Certain water soluble organic compounds like phenolphthalein and bromothymol blue behave as weak acids and exhibit different colours in their acid `color{red}((HI n))` and conjugate base `color{red}((In– ))` forms.
`color{red}(undersettext(acid indicator colour A)(HIn(aq)) +H_2O (l) ⇌ undersettext(conjugate acid)(H_3O^+(aq))+undersettext(conjugate base colourB)( In^(-)(aq)))`
Such compounds are useful as indicators in acid-base titrations, and finding out `color{red}(H^+)` ion concentration.