IONIC EQUILIBRIA (AS)

ACIDS AND BASES

1. Types of acids and bases

Classified according to degree of dissociation

or ionisation in water

a. strong acids and bases :

undergoes complete ionisation

examples :

strong acids: HCl(aq) H+(aq) + Cl-(aq)

H2SO4 , HNO3

strong bases : NaOH , KOH

b. Weak acids and bases :

undergoes incomplete or partial

ionisation in water

only a fraction of the molecules ionise

most of it remains as molecules

examples

Weak acids :

CH3COOH CH3COO- + H+

H2CO3 , HNO2 , H2S

Weak base : aqueous NH3

NH3 + H2O OH- + NH4+

2. Neutralisation reactions of acids and

bases ,even weak acids / bases are

completely reacted

Example :

CH3COOH + NaOH CH3COO-Na+ + H2O

weak acid

a. CH3COOH CH3COO- + H+

b. NaOH added will neutralise H+ ,

[H+] decreases , equilibrium shifts right

more CH3COOH will ionise and so

reaction will continue until all the acid

molecules have reacted

Note : similar for reactions involving weak

bases

3. Strength of acids / bases vs concentration

a. Strength : depends on degree of

dissociation or ionisation

weak and strong

b. Concentration : depends on amount of

solute present in a certain volume of the

solution ( usually in 1 dm3 )

i) concentrated : contains many moles

of solute in 1 dm3 solution

ii) dilute : contains a few moles of solute

in 1 dm3 solution

c. There may be concentrated solution of a

weak acid or dilute solution of a strong

base etc

4.Definitions of acids and bases

a. Arrhenius definition :

Acids are substances that produces H+ ions

in water

Bases are substances that produces OH-

ions in water

b. Bronsted Lowry definition :

i) Acids are proton (H+) donors

ii) Bases are proton acceptors

iii) When an acid loses its H+ , it forms a base

(conjugate base)

When a base accepts a H+ , it forms an acid (

conjugate acid )

Eg : Acid base + H+

HCl H+ + Cl-

acid base

- H+

H2O + H+ H3O+

base acid

+ H+

5. An acid base reaction is a dynamic

equilibrium between 2 conjugate acid-

base pairs

Acid 1 + Base 2 Base 1 + Acid 2

- H+

+ H+

Acids 1 and acid 2 are usually of different strength

Eg :

HCl + H2O Cl- + H3O+

acid base base acid

+ H+

- H+

Exercise

Identify the acids and bases present in the

following reactions :

1. HCl + NH3 Cl- + NH4+

acid base base acid

2. HSO4- + H2O H3O

+ + SO42-

acid base acid base

3. CH3COO- + H2O CH3COOH + OH-

base acid acid base

4. H2O + CO32- OH- + HCO3

-

acid base base acid

5. HSO4- + H3O

+ H2SO4 + H2O

base acid acid base

Note :

H2O and HSO4- are amphoteric species

6. Kc values can be used to compare the strength of the 2 acids or bases present in the equilibrium • Eg : A + BH+ AH+ + B

base acid acid base

a. If Kc is large :

Position of equilibrium lies to the right

More products , more A and BH+ reacted

BH+ stronger acid / A is stronger base

b. If Kc is small ,

Position of equilibrium lies to the left

More AH+ and B reacted

AH+ is stronger acid / B is stronger base

Note:

In an equilibrium mixture containing 2

acids and bases,

a. the stronger acid and base will react

more

b. the strength can be determined based

on the position of equilibrium ( Kc value )

Past year Q (June 97/1/3)

a. What do you understand by the Bronsted

Lowry definitions of acids and bases?

b.For each of the reactions C and D,

i) identify the two acids and two bases present

ii) suggest , with reasons , which one ion or

molecule is the stronger acid , and which the

stronger base

C: NH3 + H2O NH4+ + OH-

Kc = 1.8 x 10-5 mol dm-3

D: C6H5O- + CH3COOH C6H5OH +

CH3COO-

Kc = 1.3 x 106 mol dm-3