Acids and Bases

Acids and Bases

Matters can be classified in many ways. Acids and bases are another way of classification of matters. Most of the reactions taking place in water solutions are in acid or base mediums. Definitions of acids and bases are given by Arrhenius and Bronsted -Lowery. They define acids and bases in different ways. Now we examine these definitions by one one.

1) Definition of Arrhenius :

Arrhenius defines acids as " in water solutions matters that  give H⁺ ion are called acid “. Examples of acids are;

HCl(aq) → H⁺(aq) + Cl^-

HNO₃(aq) → H⁺(aq) + NO₃^-(aq)

H₃PO₄(aq) → 3H⁺(aq) + PO₄-3(aq)

H₂SO₄(aq) → 2H⁺(aq) + SO₄-2(aq)

As you can see from the examples, acids give H+ ion to solution. On the contrary, some of the acids like SO₂, P₂O₅, SO₃ do not include H atom, but their solutions with water shows acidic property. Solutions of these matters with water becomes;

SO₂(g) + H₂O(l) → H₂SO₃(aq)

CO₂(g) + H₂O(l) → H₂CO₃(aq)

P₂O₅(s) + 3H₂O(l) → 2H₃PO₄(aq)

These non metal oxides are called “anhydrous acids “.

Arrhenius defines bases as " in water solutions matters that  give OH^- ion are called bases “. Examples of bases are;

KOH(s) → K⁺(aq) + OH^-(aq)

Ba(OH)₂(s) → Ba⁺²(aq) + 2OH^-(aq)

NaOH(s) → Na⁺(aq) + OH^-(aq)

Ca(OH)₂(s) → Ca⁺²(aq) + 2OH^-(aq)

As you can see from the examples, bases give OH^- ion to solution. On the contrary, some of the bases like NH₃, MgO, CaO do not include OH^- ion, but their solutions with water includes OH^- ion and they show basic property. Look at these examples given below;

NH₃(aq) + H₂O(l) → NH₄⁺(aq) + OH^-(aq)

K₂O(s) + H₂O(l) → 2K⁺(aq) + 2OH^-(aq)

MgO(s) + H₂O(l) → Mg⁺²(aq) + 2OH^-(aq)

These matters are called “anhydrous bases “.

2) Definition of Bronsted-Lowry :

There are some limitations in Arrhenius definition. For example, it can not explain anhydrous acids and bases. Thus, Bronsted-Lowry state another definition for acids and bases. They define acids and bases like;

“Acids are matters that donates H⁺ and bases are matters that accept H⁺ ion.”

HCl(g) + NH₃(g) → NH₄⁺(s) + Cl^-(s)

In this reaction, HCl donates H⁺ ion so it is acid and NH₃ accepts H⁺ ion, it is base.

CO₃^-2 + H₂O → HCO₃^-+ OH^-

In this reaction one acid and one base react to give another acid and base. H₂O acid donates H⁺ ion and becomes OH^- base, CO₃^- base accepts H⁺ ion and becomes HCO₃^- acid. OH^- is the conjugate base of H₂O and HCO₃^- is conjugate acid of CO₃^-2. List given below shows some common acid and base conjugates;

Example: According to Bronsted-Lowry definition, which ones of the following statements are true for following reaction.

HCO₃^- + HSO₄^- → H₂CO₃ + SO₄^-2

I. Acidic property of HSO₄^- ion is higher than acidic property of HCO₃^-

II. SO₄^-2 ion shows both acidic and basic property.

III. Acidic property of H₂CO₃ is higher than HSO₄^- ion.

Solution:

In given reaction, HSO₄^- and H₂CO₃ are acid and HCO₃^- and SO₄^-2 are base. Since HSO₄^- ion donate H⁺ ion to HCO₃^-, acidic property of it is high. I is true.

Acids should have H⁺ ion in its structure to become Bronsted-Lowry acid. Since there is no H⁺ ion in SO₄^-2, it does not behave like acid. II is false.

Third statement  can be true or false, we can not say it is true by looking at this reaction.

Example: Which ones of the following couples are acid in given reaction?

H₂PO₄^- + HSO₃^- ↔ HPO₄^- + H₂SO₃

I. H₂PO₄^- and H₂SO₃

II. HSO₃^- and H₂SO₃

III. HSO₃^- and HPO₄^-

IV. H₂PO₄^- and HPO₄^-

Solution:

H₂PO₄^- donates H⁺ and becomes HPO₄^- so it shows acidic property.

H₂SO₃ donates H⁺ and becomes HSO₃^- so it shows acidic property.

Couple given in I is true.

Acids and Bases Exams and Problem Solutions