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Chemistry

 

Hydrolysis is reaction involving the breaking of bond in a molecule using in water thereby forming ions in solution. It can be referred to as the reverse of neutralization reaction. Hydrolysis often leads to change in the pH of salt. Most normal salts can undergo hydrolysis to form acidic or basic solutions.

  • Salt formed from a strong base and strong acid dissolve in water and the resulting salt solution will be neutral. Indicating that the solution will not break apart (No hydrolysis occurred). Examples:
  • NaCl(s) + H2O(l) NaOH(aq) + HCl(aq)

             Strong       Weak

Other examples of normal salts that will dissolve in water to produce neutral solutions of pH = 7 are; KCl, KNO3, NaNO3, Na2SO4, K2SO4 etc.

  • Salts formed from a strong acid and a weak base dissolves in water and the bonds in the salt solution will break apart and the solution becomes acidic. Examples:
  • NH4Cl(aq) + H2O(l) HCl(aq) + NH4OH(aq)

            Strong      Weak

Other examples of normal salts that dissolve in water to produce acidic solutions (pH less than 7) are; Ca(NO3)2, FeCl3, AlCl3, ZnCl2, FeSO4 etc.

  • Salts from a strong base and a weak acid hydrolyze to form a basic solution. Example;
  • Na2CO3(aq) + H2O(l) 2NaOH(aq) + H2CO3(aq)

               Strong          Weak

Other examples of normal salts that are basic in solution (pH greater than 7) are; K2CO3, Na2S, K2S, NaCN, CH3COONa etc.

  • Salts from a weak acid and a weak base will hydrolyze but pH of the solution is dependent on the equilibrium constants of acid (Ka) and base (Kb). If the Ka value is greater than the Kb value, then the resulting solution is acidic and vice versa. Example:

CH3COONH4(aq) + H2O(l)  CH3COOH(aq) + NH4OH(aq)

                                                   Weak               Weak

Other examples are (NH4)2CO3, (NH4)2S etc

GENERAL PROPERTIES OF SALTS

  • Hard, brittle solid due to strong ionic bonding.
  • High boiling and melting point
  • Soluble in water.
  • They are electrolytes in molten/solution states. (Solid salts do not conduct electricity).

PHYSICAL PROPERTIES OF SALTS

  • EFFLORESCENCE: This is the process whereby a hydrated salt loses its water of crystallization when partially or completely exposed to the atmosphere to form a lower hydrated salt or an anhydrous salt. Such salts are said to be efflorescent. Examples of such salts are
  • Sodium trioxocarbonate (IV) decahydrate, Na2CO3.10H2O
  • Copper (II) tetraoxosulphate (VI), CuSO4.5H2O
  • Iron (II) tetraoxosulphate (VI), FeSO4.7H2O etc.

Efflorescent salts lose weight and their crystalline appearance when heated or exposed to the atmosphere.

Na2CO3.10H2O(s)  Na2CO3.H2O(s) + 9H2O(g)

CuSO4.5H2O(s)  CuSO4(s) + 5H2O(g)

  • DELIQUESCENCE: This is the process whereby a compound absorbs moisture from the atmosphere to form a saturated solution. Such salts are said to be deliquescent. Examples are: CaCl2, MgCl2, FeCl3, NaCl, Ca(NO3)2, etc. NaOH and KOH are deliquescent alkalis. Deliquescent compounds gain weight on exposure to the atmosphere. They are soluble readily in water.
  • HYGROSCOPY: This is a phenomenon in which a compound absorbs moisture and becomes sticky. If it’s a liquid, it absorbs moisture and dilutes itself (it will increase in volume). Examples are; CaCl2, ZnCl2, NaCl, CuSO4 etc. CaO is also hygroscopic. H2SO4 is a hygroscopic liquid.

DRYING AGENTS

Deliquescence and hygroscopic compounds are used as drying agents because of their high affinity for water. They are also called dehydrating agents or desiccants. A drying agent is not used if it reacts with the substance to be dried e.g. conc. Tetraoxosulphate (VI) acid cannot be used to dry ammonia gas because both can react together.

Other examples of desiccants are; silica gel (for all gases), calcium chloride (for all gases except ammonia), calcium oxide (for ammonia), phosphorus (V) oxide (for all gases except ammonia) etc.

                                                                SALTS

A salt is a compound formed when the replaceable hydrogen atoms in an acid are wholly or partially replaced by a metallic ion or ammonium ion. Example:

a.       When the hydrogen atom in HCl is replaced by sodium ion (Na+), Sodium Chloride (NaCl) is formed.

b.       When two of the replaceable hydrogen atoms in H2SO4 are replaced by potassium ions (K+), potassium tetraoxosulphate(VI) [K2SO4] is formed.

c.       When one of the hydrogen atoms in H2SO4 is replaced by potassium ion (K+), potassium hydrogen tetraoxosulphate (VI) [KHSO4] is formed.

d.       When the hydrogen atom in trioxonitrate (V) acid is replaced by sodium ion (Na+), sodium trioxonitrate (V) [NaNO3] is formed.

 

TYPES OF SALT

1.       NORMAL SALT: This is a salt formed when all the hydrogen atoms in an acid are completely replaced by a metallic ion or ammonium ion. Examples:

a.       Sodium chloride, NaCl formed from HCl (Monobasic acid)

HCl(aq) + NaOH(aq)  NaCl(aq) + H2O(l)

b.      Potassium tetraoxsulphate (VI), K2SO4 formed from H2SO4 (dibasic acid)

H2SO4(aq) + 2KOH(aq)  K2SO4(aq) +2 H2O(l)

c.       Calcium tetraoxphosphate (V), Ca3(PO4)2 formed from H3PO4 (tribasic acid)

2H3PO4(aq) + 3Ca(OH)2(aq)  Ca3(PO4)2(aq) +6H2O(l)

d.      Ammonium chloride, NH4Cl formed from HCl (monobasic acid)

2HCl(aq) + Na2O(s)  2NaCl(aq) + H2O(l)

N:B – A monobasic acid forms only one type of salt which is a normal salt.

2.       ACIDIC SALT: This is the salt formed when the hydrogen atoms in a polybasic acid is partially replaced by a metallic ion. Example:

a.       Potassium hydrogen tetraoxosulphate (VI), KHSO4.

H2SO4(aq) + KOH(aq)  KHSO4(aq) + H2O(l)

b.      Sodium hydrogen tetraoxosulphate (VI), NaHSO4.

2H2SO4(aq) + Na2O(aq)  2NaHSO4(aq) + H2O(l)

c.       Sodium dihydrogen tetraoxophosphate (V), NaH2PO4.

H3PO4(aq) + NaOH(aq)  NaH2PO4(aq) + H2O(l)

N:B – Acid salts are acidic because they dissolve to yield hydrogen ion (H+) in water.

NaHSO4 Na+ + H+ + SO42−

 

3.       BASIC SALT: This is the salt formed by partial replacement the hydroxide ions in a metallic hydroxide. Examples:

a.       Lead hydroxide trioxonitrate (VI), Pb(OH)NO3.

HNO3(aq) + Pb(OH)2(s)  Pb(OH)NO3(aq) + H2O(l)

b.      Zinc hydroxide chloride, Zn(OH)Cl.

NaOH(aq) + ZnCl  Zn(OH)Cl(s) + NaCl(aq)

c.       Magnesium hydrogen trioxonitrate (V), Mg(OH)NO3.

NaOH(aq) + Mg(NO3)2(aq)­  Mg(OH)NO3(s) + NaNO3(aq)

4.       DOUBLE SALT/MIXED SALT: Double salts are formed when an acid is partially neutralized by a base and then completely neutralized by another base. Also, double salts occur when hot concentrated aqueous solutions of two simple salts are mixed and cooled to form one compound (usually the resulting crystalline compound [double salt] is hydrated i.e. contains water of crystallization). Examples:

a.       Ammonium iron (II) tetraoxosulphate (VI) dodehydrate, (NH4)2Fe(SO4)2.12H2O.

This is formed from cooling mixed solutions of ammonium tetraoxosulphate (VI) and iron(II)tetraoxosulphate (VI) salts. This is iron(II) alum, a purple crystalline solid.

b.      Potassium alum:  Potassium aluminium tetraoxosulphate (VI), KAl(SO42.12H2O.

This is a white crystalline solid often used as a coagulant.  

N:B – Double salts ionize to produce three different kinds of ions in solution. Two cations and an anion.

KAl(SO4)2 K+ + Al3+ + 2SO42−

(NH4)2Fe(SO4)2 ⇌2 NH4+ + Fe2+ + 2SO42−

 

5.       COMPLEX SALT: Complex salt is a salt that contain complex ion. It usually contains two different types of ions in a solution. One ionizes and the other which do not ionize when in solution but form the complex ions i.e. ions consisting of charged group of atoms. Complex salts are also called co-ordination compounds.

Examples:

1.       Sodium tetrahydroxozincate (II), Na2Zn(OH)4.

This is formed from sodium salt and zinc oxide or hydroxide.

2NaCl(aq) + Zn(OH)2(s) + 2H2O(l)  Na2Zn(OH)4(aq) + 2HCl(aq)

2.       Potassium hexacyanoferrate (II), K4Fe(CN)6.

This could be gotten from iron (II) salt and potassium cyanide.

6KCN + FeSO4  K4Fe(CN)6 + K2SO4

N:B – Complex salts exhibit the properties of the complex ion formed when in solution.

Na2Zn(OH)4 2Na+ + [Zn(OH)4]2−

K4Fe(CN)6 4K+ + [Fe(CN)6]4−

 

                                                                 SALTS

A salt is a compound formed when the replaceable hydrogen atoms in an acid are wholly or partially replaced by a metallic ion or ammonium ion. Example:

a.       When the hydrogen atom in HCl is replaced by sodium ion (Na+), Sodium Chloride (NaCl) is formed.

b.       When two of the replaceable hydrogen atoms in H2SO4 are replaced by potassium ions (K+), potassium tetraoxosulphate(VI) [K2SO4] is formed.

c.       When one of the hydrogen atoms in H2SO4 is replaced by potassium ion (K+), potassium hydrogen tetraoxosulphate (VI) [KHSO4] is formed.

d.       When the hydrogen atom in trioxonitrate (V) acid is replaced by sodium ion (Na+), sodium trioxonitrate (V) [NaNO3] is formed.

 

TYPES OF SALT

1.       NORMAL SALT: This is a salt formed when all the hydrogen atoms in an acid are completely replaced by a metallic ion or ammonium ion. Examples:

a.       Sodium chloride, NaCl formed from HCl (Monobasic acid)

HCl(aq) + NaOH(aq)  NaCl(aq) + H2O(l)

b.      Potassium tetraoxsulphate (VI), K2SO4 formed from H2SO4 (dibasic acid)

H2SO4(aq) + 2KOH(aq)  K2SO4(aq) +2 H2O(l)

c.       Calcium tetraoxphosphate (V), Ca3(PO4)2 formed from H3PO4 (tribasic acid)

2H3PO4(aq) + 3Ca(OH)2(aq)  Ca3(PO4)2(aq) +6H2O(l)

d.      Ammonium chloride, NH4Cl formed from HCl (monobasic acid)

2HCl(aq) + Na2O(s)  2NaCl(aq) + H2O(l)

N:B – A monobasic acid forms only one type of salt which is a normal salt.

2.       ACIDIC SALT: This is the salt formed when the hydrogen atoms in a polybasic acid is partially replaced by a metallic ion. Example:

a.       Potassium hydrogen tetraoxosulphate (VI), KHSO4.

H2SO4(aq) + KOH(aq)  KHSO4(aq) + H2O(l)

b.      Sodium hydrogen tetraoxosulphate (VI), NaHSO4.

2H2SO4(aq) + Na2O(aq)  2NaHSO4(aq) + H2O(l)

c.       Sodium dihydrogen tetraoxophosphate (V), NaH2PO4.

H3PO4(aq) + NaOH(aq)  NaH2PO4(aq) + H2O(l)

N:B – Acid salts are acidic because they dissolve to yield hydrogen ion (H+) in water.

NaHSO4 Na+ + H+ + SO42−

 

3.       BASIC SALT: This is the salt formed by partial replacement the hydroxide ions in a metallic hydroxide. Examples:

a.       Lead hydroxide trioxonitrate (VI), Pb(OH)NO3.

HNO3(aq) + Pb(OH)2(s)  Pb(OH)NO3(aq) + H2O(l)

b.      Zinc hydroxide chloride, Zn(OH)Cl.

NaOH(aq) + ZnCl  Zn(OH)Cl(s) + NaCl(aq)

c.       Magnesium hydrogen trioxonitrate (V), Mg(OH)NO3.

NaOH(aq) + Mg(NO3)2(aq)­  Mg(OH)NO3(s) + NaNO3(aq)

4.       DOUBLE SALT/MIXED SALT: Double salts are formed when an acid is partially neutralized by a base and then completely neutralized by another base. Also, double salts occur when hot concentrated aqueous solutions of two simple salts are mixed and cooled to form one compound (usually the resulting crystalline compound [double salt] is hydrated i.e. contains water of crystallization). Examples:

a.       Ammonium iron (II) tetraoxosulphate (VI) dodehydrate, (NH4)2Fe(SO4)2.12H2O.

This is formed from cooling mixed solutions of ammonium tetraoxosulphate (VI) and iron(II)tetraoxosulphate (VI) salts. This is iron(II) alum, a purple crystalline solid.

b.      Potassium alum:  Potassium aluminium tetraoxosulphate (VI), KAl(SO42.12H2O.

This is a white crystalline solid often used as a coagulant.  

N:B – Double salts ionize to produce three different kinds of ions in solution. Two cations and an anion.

KAl(SO4)2 K+ + Al3+ + 2SO42−

(NH4)2Fe(SO4)2 ⇌2 NH4+ + Fe2+ + 2SO42−

 

5.       COMPLEX SALT: Complex salt is a salt that contain complex ion. It usually contains two different types of ions in a solution. One ionizes and the other which do not ionize when in solution but form the complex ions i.e. ions consisting of charged group of atoms. Complex salts are also called co-ordination compounds.

Examples:

1.       Sodium tetrahydroxozincate (II), Na2Zn(OH)4.

This is formed from sodium salt and zinc oxide or hydroxide.

2NaCl(aq) + Zn(OH)2(s) + 2H2O(l)  Na2Zn(OH)4(aq) + 2HCl(aq)

2.       Potassium hexacyanoferrate (II), K4Fe(CN)6.

This could be gotten from iron (II) salt and potassium cyanide.

6KCN + FeSO4  K4Fe(CN)6 + K2SO4

N:B – Complex salts exhibit the properties of the complex ion formed when in solution.

Na2Zn(OH)4 2Na+ + [Zn(OH)4]2−

K4Fe(CN)6 4K+ + [Fe(CN)6]4−

 

 

TEACHER: MISS EMELDA OKPALANWEZE

 

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