DISSOCIATION OF ACIDS AND BASES (Acid Strength and Base Strength)

Different acids differ in their ability to donate protons. A strong acid is one, that is almost completely dissociated (strong electrolytes) i.e. an acid that produces large number of (H+) ions in aqueous solution is said to be a strong acid. Typical examples of strong acids are hydrochloric acid (HCL) nitric acid (HHO3) and sulphuric acid (H2SO4).

A weak acid is one, that is only partially dissociated (weak-electrolytes). Only a small fraction of the weak acids transfer a proton to water. Typical examples of weak acids are nitrous acid (HNO2), phosphoric acid (H3PO4), hydro fluoric acid (HF) carbonic acid (H2CO3) acetic acid (CH3-COOH) and formic acid (HCOOH).

For example: In strong acid, greater is the extent of ionization in water.

HCl + H2O < ----- > H3O+ + Cl+ (complete ionization in dilute solution)

HF + H2O < ----- > H3O+ + F- (8.5% in 0.1 M solution)

CH3COOH + H2O < ----- > H3O+ + CH3COOO- (1.3% in 0.1 M solution)

Similarly, the strong base is one, that is almost dissociated completely (strong electrolytes), that is, a base which yields large number of (OH-) ions in aqueous solution, is said to be a strong base. Most metal hydroxides, such as NaOH, KOH and Ba (OH)2 are strong electrolytes and strong bases.

A weak base is one, that is only partially dissociated (weak electrolytes); weak bases dissociate to a small extent in water to yield (OH-) ions.

Typical examples of weak bases are, NH4OH, Mg (OH)2 and Be(OH)2, etc. Hence, the relative strength of weak bases may be measured by the extent to which they dissociate in water to yield hydroxide ions (OH-).

 

9.4 SALTS

A salt is ionic compound produced when an acid is neutralized by a base. For example, sodium hydroxide neutralizes hydrochloric acid to form sodium chloride (salt) and water.

NaOH (aq) + HCI (aq) ------à NaCl (aq) + H2O (l)

Similarly potassium hydroxide neutralizes nitric acid to form potassium nitrate and water

KOH (aq) + HCl (aq) ------à KNO3 (aq) + H2O (l)

On the basis of their chemical nature, salts can be divided into three groups.

1. Normal Salts    2. Acidic salts.     3. Basic salts.

1. Normal Salts:

Salts which are formed by the complete neutralization of an acid by a base e.g. NaCl, NaNO3, K2SO4 etc are normal salts. These salts do not have replaceable hydrogen, atoms or hydroxyl groups.

2. Acidic Salts:

Salts which are formed by the partial neutralization of an acid by a base e.g. NaHSO4, KHCO3, etc are acidic salts.

NaOH (aq) + H2SO4 (aq) ------à NaHSO4 (aq) + H2O (l)

          KOH (aq) + H2CO3 (aq) ------à KHCO3 (aq) + H2O (l)

 These salts contain replaceable hydrogen ion. They react further with bases to form normal salts.

3. Basic Salts:

Salts, which are formed by the partial neutralization of a base by an acid E.g. Mg (OH) Cl, Zn (OH) Cl, etc, are basic salts.

Mg (OH)2 + HCl (aq) ------à Mg(OH)Cl + H2O (l)

These salts have replaceable hydroxyl groups. They can further react with acids to form normal salts.

 

DOUBLE SALTS

The crystalline compounds which are obtained, when two specific salts are crystallized together are known as double salts. These salts have definite chemical composition. These compounds usually have definite number of water molecules with them. Typical examples of double salts are:

 

Potash Alum K2SO4 Al2 (SO4)3. 6H2O

Chrome Alum K2SO4.Cr2 (SO4)3. 6H2O

Carnalite KC1. MgCl2. 6H2O

Mohr's Salt FeSO4 . (NH4)2 SO4. 6H2O

9.4.1 Some important Commercial Preparation and uses of Salts.

1.    Sodium carbonate: (Na2CO3.10 H3O)…………………………………………
Today sodium carbonate (Na, CO3) is commercially prepared by the Solvay process or ammonia soda process.
The raw materials are lime stone (CaCO3), sodium chloride (NaCl), ammonia (NH3) and water.
The process involves the following steps:

                      i.        Lime stone [CaCO3] is heated to yield calcium oxide (quicklime CaO) and the CO2 gas.

                  CaCO3(s)
---- ΔH--à CaO(s) + CO2 (g)

 

                     ii.        This (CO2) is passed into aqueous solution of ammonia, and the ammonium bicarbonate is produced.

                  
NH3 (g) + CO2 (g) + H2O (l) ------à NH4HCO3 (aq).

 

                    iii.        This (NH4HCO3) reacts with aqueous. cold solution of (NaCl) at 15°C, called Brine to yield, sodium bicarbonate (NaHCO3), which is not soluble at low temperature (15°C) and this precipitates out.

Na+Cl-(aq) +NH+4HCO-3(aq)   at 15°Cbrine
à  NH+4Cl-(aq) + NaHCO3(s)

 

                   iv.        This (NaHCO3) on heating yields sodium carbonate.

                2NaHCO3(s)   AH ΔH
à Na2CO3(s) + H2O (l) + CO2 (g)
The ammonia (NH3) which is used as a raw material in 2nd step discovered by reacting (CaO) with NH4Cl.

      2NH4Cl (aq) +CaO(s)
------à 2NH3 (g) + CaCl2 (aq) + H2O(l)

Anhydrous sodium carbonate (Na2CO3) is known as soda-ash and sodium carbonate decahydrate (Na2CO3.10H2O) is commonly known as washing soda.

Uses of Sodium Carbonate (Na2CO3):

Sodium carbonate is soluble in water and has many important applications.

a.    It is used in the softening of water. Sodium carbonate furnishes carbonate ion (CO3) to precipitate calcium and magnesium ions,

                         CaCl2 + Na2CO3
------à CaCO3 + 2NaCl
                        MgCl2 + Na2CO3
------àMgCO3 + 2NaCl

 

b.    It is used as cleaning agent, and in making of soap, detergents and paper.

c.    It is used in making ordinary glass, which is used in bottles,

     E.g.         Na2SiO3(s) + SiO2(s)
------à Na2SiO3(s) + CO2T

                                                                        (Water glass)

2.    Sodium Hydrogen carbonate (NaHCO3) (Baking Soda): Preparation:
Sodium hydrogen carbonate (NaHCO3) or baking soda is formed by "Solvay process", but mostly it is prepared by passing the stream of CO2 through concentrated aqueous (Na2CO3) solution.

Na2CO3(s) + CO2 (g) + H2O (l) ------à NaHCO3(s)

Uses:

1.    Baking soda is used in the preparation of baking powder.

2.    In the preparation of effervescent drinks and fruit salts.

3.    In medicines to remove acidity of stomach (i.e. as Antacid)

4.    In fire extinguishers.

3.    Copper Sulphate (CuSO4.5H2O):
Copper sulphate or cupric sulphate which is also known as blue vitrol or blue stone may be prepared by reacting copper scraps with dilute sulphuric acid in the presence of air.

2Cu(s) + 2H2SO4 (aq) + O2 (g)
------à 2CuSO4 (aq) + 2H2O

It can also be prepared by the treatment of CuO or CuCO3 with dilute sulphuric acid (H2SO4).

CuO + dilute H2SO4 ------à CuCO4 + H2O

CuSO3+dilute H2SO4 ------à CuCO4 + CO2 + H2O

Uses of Copper Sulphate:

1.    In textiles (mordant), tanning, electric batteries, hair dyes and in electroplating.

2.    As germicide, insecticide, preservative for wood, paper pulp.

3.    In calico printing, making synthetic rubber and copper salts e.g. scheels, green paint,

4.    In paint and varnish industry.

5.    A mixture of copper sulphate and milk lime is used to kill fungus and molds.

4.    Magnesium Sulphate (MgSO4.7H2O) (Epsom-Salt):
Preparation:
It is prepared by the action of H2SO4 and magnetite or dolomite, but nowadays it is prepared by heating kiesserite under pressure with water.
 

                      i.        MgCO3+H2SO4 H2SO4 ------à MgSO4 + H2O + CO2 (g)
Magnetite

 

                     ii.        MgCO3 .CaCO3+2H2SO4 ------à MgSO4 + CaSO4+ 2H2O + 2CO2 (g)
Dolomite

 

                    iii.        MgSO4.H2O + 6H2O   pressure > MgSO4 .7H2O
Kiesserite                            (Epsom salt)

Uses:

1.    It is used as a mild purgative in medicines.

2.    In dyeing and tanning processes.

3.    In making fire proof fabrics.

4.    As a filler in paper industry.

5.    In manufacture of ceramics, glazed tiles and match boxes.

5.    Potash Alum   (K2SO4.Al2 (SO4)3.24H2O):
If equi-molecular quantities of potassium sulphate and aluminum sulphate are dissolved in water and the solution is allowed to evaporate. Crystals of (K2SO4.Al2 (SO4)3.24H2O) which is called ordinary alum or potash alum are seperated out.

Uses: 

1.    Alum is used in dyeing as mordant to fix insoluble dye to fibre.

2.    It is also used in tanning leather.

3.    In sizing paper.

4.    In purifying water.

5.    As an antiseptic and as a mouth wash.

6.    It also used in medicines.

 
    back to previous page  
     
 
 
Home | About Us | Notes | Animation | Career Guidance | Tuition | Wisdom | Contact Us
© All Copyright Reserved to www.mynoteslibrary.com