Tamilnadu Samacheer Kalvi 12th Chemistry Notes Chapter 8 Ionic Equilibrium Notes
Acid: The term ‘acid’ is derived from the latin word ‘acidus’ meaning sour, which turns the blue litmus into red.
Base: Base tastes bitter and turns the red litmus to blue.
Arrhenius Concept:
- An acid is a substance that dissociates to give hydrogen ions in water.
- A base is a substance that dissociates to give hydroxyl ions in water.
Limitations of Arrhenius Concept:
- Arrhenius theory does not explain the behaviour of acids and bases in non aqueous ‘ solvents such as acetone, Tetrahydrofuran etc.
- This theory does not account for the basicity of the substances like ammonia (NH3) which do not possess hydroxyl group.
Lowry – Bronsted Theory :
- An acid is defined as a substance that has a tendency to donate a proton to another substance.
- A base is a substance that has a tendency to accept a proton form other substance.
Limitations of Lowry – Bronsted Theory :
Substances like BF3, AlCl3 etc., that do not donate protons are known to behave as acids.
Lewis concept:
- An acid is a species that accepts an electron pair (Lewis acid)
- Base is a species that donates an electron pair. (Lewis base)
Strong acid : A strong acid is the one that is almost completely dissociated in water.
Weak acid : A weak acid is only partially dissociated in water.
Auto ionisation of water : The pure water itself has a little tendency to dissociate.
i.e., one water molecule donates a proton to an another water molecule. This is known as auto ionisation of water.
pH scale : The term pH is derived from the French word ‘Purissance de hydrogene’ meaning, the power of hydrogen. It is defined as the negative logarithm of base 10 of the molar concentration of the hydronium ions present in the solution.
Ostwald’s dilution law : Ostwald’s dilution law relates the dissociation constant of the weak acid (Ka) with its degree of dissociation (α) and the concentration (c).
Degree of dissociation (α): It is the fraction of the total number of moles of a substance that dissociates at equilibrium.
Common ion effect: The dissociation of the weak acid is suppressed in the presence of a salt containing an ion common to the weak electrolyte. It is called the Common ion effect.
Buffer solution : Buffer is a solution which consists of a mixture of a weak acid and its conjugate base (or) a weak base and its conjugate acid. This buffer solution resists drastic changes in its pH upon addition of a small quantities of acids (or) bases, and this ability is called buffer action.
Types of Buffer Solution : There are two types of buffer solutions.
- Acidic buffer solution : a solution containing a weak acid and its salt.
Example : solution containing acetic acid and sodium, acetate - Basic buffer solution : a solution containing a weak base and its salt.
Example : Solution containing NH4OH and NH4Cl
Buffer index : Buffer index ‘β’ is defined as the number of gram equivalents of acid or base added to one litre of the buffer solution to change its pH by unity.\(\beta=\frac{\mathrm{dB}}{\mathrm{d}(\mathrm{pH})}\)
Here,
dB = number of gram equivalents of acid / base added to one litre of buffer solution.
d(pH) = The change in the pH after the addition of acid / base.
Salt Hydrolysis : Salts completely dissociate in aqueous solutions to give their constituent ions. The ions so produced are hydrated in water. In certain cases, the cation, anion or both react with water and the reaction is called salt hydrolysis.
Solubility Product: It is defined as the product of the molar concentration of the constituent ions, each raised to the power of its stoichiometric co-efficient in a balanced equilibrium equation.
Molar solubility : The maximum number of moles of solute that can be dissolved in one litre of the solution.
Difference between Lewis acids and Lewis base:
| Lewis acids | ‘ Lewis bases |
| Electron deficient molecules such as BF3, AlCl3, BeF2 etc… |
Molecules with one (or) more lone pairs of electrons. NH3, H2O, R-O-H,R-O-R, R – NH2 |
| All metal ions (or) atoms Examples: Fe2+ ,Fe3+ ,Cr3+ ,Cu2+ etc… |
All anions F–,Cl– ,CN– , SCN–, SO42- etc… |
| Molecules that contain a polar double bond Examples : SO2, CO2, SO3 etc… |
Molecules that contain carbon – carbon multiple bond Examples: CH2 = CH2, CH = CH etc… |
| Molecules in which the central atom can expand its octet due to the availability of empty d – orbitals Example: SiF4, SF4, FeCl3 etc.. |
All metal oxides CaO, MgO, Na2O etc… |
| Carbonium ion (CH3)3 c+ |
Carbanion CH3– |
Kw values at different temperatures are given in the following table.
| Temperature (°C) | Kw |
| 0 | 1.14 xl0-15 |
| 10 | 2.95 x 1o-15 |
| 25 | 1.00 x 10-14 |
| 40 | 2.71x 10-14 |
| 50 | 5.30 x 10-14 |