SALT I

 

 

SALT I

 What is a Salt?

A salt is a compound consisting of positive metallic ion and a negative ion derived from an acid.

e.g. NaCl (Sodium chloride), Na2SO4 ( Sodium tetraoxosulphate(vi)), ZnCl ( Zinc chloride)

NaCl         Na+ + Cl-     ; Na2SO4    2Na+  +  SO2-4    ; ZnCl2       Zn2+  +  2Cl-

ii. A salt is the compound formed when all or part of ionizable hydrogen of an acid is replaced by metallic or ammonium ions.

Eg. NH4Cl: Ammonium Chloride ;  NaHSO4; Sodium hydrogen tetraoxosulpahte(vi) Ca(NO3)2; Calcium trioxonitrate(v)

2HCl(aq) + 2K(s)       2KCl(aq) + H2(g)

iii. Most salts, are products of neutralization reaction. E.g Acid + Base Salt + Water

H3O+ + OH-               (Salt) + 2H2O

H3O+ Cl- + K+OH-            K+Cl- + 2H2O

(H3O+)2SO2-4 + K+OH-         K+2SO2-4 + 4H2O

 Types Of Salts

Five main types of salts are:

1. Normal salt  2. Acid salt   3.  Basic salt   4.  Double salt,  5.  Complex salt.

1. Normal salt: Normal salt is formed when all the replaceable hydrogen ions in an acid have been completely replaced by metallic ions of a base.

Metallic ions of a base.

E.g NaCl, CaCl2, MgSO4, Na2SO4. A normal salt contains no replaceable hydrogen, H+ or hydroxyl, OH-, ion. Aqueous solution of normal salt is neutral to litmus and it has a pH of 7.

2. Acid salt: An acid salt is formed when the replaceable hydrogen ions in the acid are partially replaced by a metal. An acid salt contains replaceable hydrogen ion E.g NaHCO3; sodium hydrogen trioxocarbonate(iv). NaHSO4; Sodium hydrogen tetraoxosulphate(vi) Ca(HCO3)2; Calcium hydrogen trioxocarbonate(iv).

A solution of an acidic salt turns blue litmus paper red, and it can react with excess alkali to form

a normal salt and water. Eg.

NaHSO4(aq) + NaOH(aq)             NaSO4(aq) + H2O(l)

Ca(HCO3)2(aq) + Ca(OH)2(aq)                 2CaCO3(aq) + 2H2O

3. Basic salt: A basic salt is formed when a base reacts with a limited quantity of an acid. That is, the oxonium ion, H3O+, is insufficient for the complete neutralization of the base. A basic salt therefore contains replaceable hydroxyl ions, OH-.

Ca(OH)2 + HCl(aq)              Ca2+(OH)- Cl- + H2O

Examples of basic salt are:

Ca(OH)Cl (Calcium hydroxide chloride ); Zn(OH)Cl (Zinc ii hydroxide chloride) ; Mg(OH)Cl ( Magnesium (ii) hydroxide chloride)

A basic salt turns red litmus blue and reacts with excess acid to form a normal salt and form a

normal salt and water.

Mg(OH)Cl(aq) +  HCl(aq)                   MgCl2(aq)    +     H2O

                           Acid                       Normal salt     Water

4. Double salt: Double salt is a salt which ionizes to yield three different types of ions in solution.

Two of than are positively charged (metallic ions) while the other is negatively charged. Eg

(NH4)SO4 FeSO4 .6H2O Ammonium iron(ii) tetraoxosulphate(vi) hexahydrate

KAl(SO4)2 .12H2O Ammonium potassium tetroxosulphate (vi) dodecahydrate

5. Complex salt: This is a salt that contains complex ions. A complex ion is the ion consisting of a charged group of atoms. Eg  Na2Zn(OH)4: Sodium tetrahydroxozinate(ii) K4Fe(CN)6: Potassium hexacyanoferrate(ii)

The complex salts ionize as follows to produce the complex ions:

(Zn(OH)4)2- Tetrahydroxozincate (ii) ion   and   (Fe(CN)6)-4 Hexacyanoferrate (ii) ion

Na2Zn(CN)6(aq)              2Na+(aq) + (Zn(OH)4)2-(aq)

K4Fe(CN)6(aq)                 4K+(aq) + (Fe(CN)6)-4(aq)

 Preparation of Soluble Salts

Note: A method adopted for the preparation of a particular salt is dependent on two factors.

These are:

1. The solubility of the salt in water

2. The stability of the salt to heat.

The methods used in the preparation of soluble salts are:

i. Reaction of an acid with a metal which is more reactive than hydrogen e.g Na, Ca, Zn,Mg and Fe.

Ca(s) + 2HCl           CaCl2(aq)  +  H2(g)

Ii. Neutralization reaction: Reaction between an alkali and acid

NaOH(aq)  + HNO3(aq)           NaNO3(aq)  +  H2O(l)

iii. Neutralization reaction: reaction between a hot dilute acid and an insoluble base.

H2SO4(aq)  +  CuO(s)            CuSO4(aq)  +  H2O(l)

iv. Reaction between a dilute acid and trioxcarbonate (iv)

H2SO4(aq) + CaCO3(s)             CaSO4(aq) + H2O(l)  +  CO2(g)

2HCl(aq)  +  ZnCO3(s)               ZnCl2(aq)  +  H2O(l)  +   CO2(g)

The soluble salts can then be recovered from the solution by either evaporation to dryness or crystallization.

Preparation of Insoluble Salts

Insoluble salt can be prepared by:

i. Direct combination of the constituent elements.

2Fe(s)    +   3Cl2(s)            2FeCl3(s)

Fe(s)     +     S(s)                FeS(s)

ii. Double decomposition reaction.

Two appropriate insoluble salts react to precipitate the required insoluble salt. The latter is then

filtered, washed and dried. These insoluble salts that can be prepared by this method are; PbCl2,

PbI2, CaCO3, AgCl and BaSO4

BaCl2(aq) + Na2SO4(aq)             BaSO4(s) +     2NaCl(aq)

Soluble      Soluble                Insoluble      Soluble

White ppt

AgNO3(aq)    +    KCl(aq)         AgCl(s) +     KNO3(aq)

Soluble           Soluble     Insoluble     Soluble

White ppt

Na2CO3(aq)   +     CaCl2(aq)     CaCO3(s)  +      2NaCl(aq)

Soluble             Soluble     Insoluble        Soluble

White ppt

Pb(NO3)2(aq)    +   2KI(aq)       PbI2(s)  +      2KNO3(aq)

Soluble              Soluble     Insoluble   Soluble

White ppt

Above reactions can be represented ironically as follows:

Ba2+(aq) + SO2-4(aq)                    BaSO4(s)       ;    Ag+(aq) + Cl-(aq)                         AgCl(s)

Ca2+(aq) + CO32-(aq)                 CaCO3(s)         ;     Pb2+(aq) + 2I-(aq)        PbI2(s)

 

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