SALT

LESSON TOPIC ONE: SALTS

LEARNING OBJECTIVES

In this topic, you will learn about:

·         The meaning of the term salt

·         Types of salts

·         Methods of preparation of salts

·         Uses of salts

·         Characteristics of some salt

DEFINITION OF A SALT

A salt can be defined in the following ways:

1.      A salt is the compound formed, when the replaceable hydrogen atoms of an acid have been completely or partially replaced by a metallic or ammonium ions. For instance when the hydrogen ion, H⁺ from hydrochloric acid, HCl is replaced by sodium ion, Na⁺, sodium chloride, NaCl is formed. HCl becomes NaCl

2.      A salt is the compound formed when the positive metallic radical of a base combines with the negative acidic radical of an acid. For example, sodium ion in sodium hydroxide combine with chloride ion in hydrochloric acid to form sodium chloride salt:

NaOH_(aq)   +  HCl_(aq)  →  NaCl_(aq)  +  H2O_(l)      

 

TYPES OF SALTS

There are five main types of salts:

v  Normal salts

v  Acid salts

v  Basic salts

v  Double salts

v  Complex salts

 

NORMAL SALTS: normal salts are salts formed when all the replaceable hydrogen atoms in an acid have been completely replaced by metallic or ammonium ions. For examples:

HCl a monobasic acid forms NaCl (a normal salt)

H₂SO₄ a dibasic acid forms K₂SO₄ (a normal salt)

H₃PO₄ a tribasic acid forms Ca₃(PO₄)₂ (a normal salt)

Generally, monobasic acids form only normal salt because they have only one replaceable hydrogen atom. They are neutral to litmus paper. However, a few normal salts such as sodium trioxocarbonate(iv), aluminium chloride and sodium sulphide, when undergo hydrolysis (dissolves) in water gives an alkaline or acidic solution.

 

ACID SALTS: Acid salts are salt formed when the replaceable hydrogen atom in an acid are partially replaced by metallic or ammonium ions. Acid salts are generally produce by dibasic or tribasic acids. For example, if only one of the two hydrogen atoms in H₂SO₄ is replaced by a sodium atom, an acid salt NaHSO₄ is formed.

 

H₂SO₄    +  NaOH  →  NaHSO₄  +  H₂O

 

Hence, acid salts results when there is an insufficient supply of metallic ions to replace all the replaceable hydrogen atoms in the acid. Acids with two replaceable hydrogen atoms can form only one acid salt while those with three replaceable hydrogen atoms can form two different acid salts.

An acid salt still has acidic properties eg. It turns blue litmus red. However, in the presence of excess metallic ions, the remaining replaceable hydrogen ions become replace, converting the acid salt to a normal salt.

 

NaHSO_4(aq)   +  NaOH(aq)   →     Na₂SO_4(aq)   +  H₂O_(l)

 

BASIC SALTS: Basic salts are salts that contain hydroxide ion because of insufficient supply of acid needed for the complete neutralization of the base.

Zn(OH)_2(aq)   +  HCl_(aq)  →   Zn(OH)Cl   +   H₂O_(l)

 

Basic salts have the properties of base, eg. It turns red litmus paper blue and will react with excess acid to form a normal salt and water.

 Zn(OH)Cl_(aq)   +   HCl_(aq)   →  ZnCl_2(aq)   +   H₂O_(l)

  Basic salt                                  normal salt   

 

Double salts: Double salts are salt formed when hot concentrated equimolar aqueous solutions of two simple salts mixed together in the ratio of their relative molecular masses and allowed to cool. For example, a solution of ammonium tetraoxosulphate (vi),(NH₄)₂SO₄, mixed together in the right proportion with a solution of iron (III) tetraoxosulphate (VI),Fe₂(SO₄)₃, results in the formation of ammonium iron II tetraoxosulphate VI dodecahydrate, (NH₄)_2.Fe₂(SO4)₃.12H₂O.

Thus, the general chemical formula for double salts is: X₂SO₄.Y(SO₄)₃

Where X  = Na+, K+ or NH4+  (monovalent ions)

            Y  = Al3+. Fe3+ or Cr3+   (trivalent ions)

Double salts ionizes to produce three different types of ions in solution. Usually, two of these ions are positively charged while the other is negatively charged.

K₂SO₄.Al₂(SO₄)₃.24H₂O the ions are:  K⁺, Al³⁺ or SO₄^2-

Double salts are generally referred to as alum.

 

COMPLEX SALTS: Complex salts are salts that contain complex ion, an ion in which a metal ion forms dative bonds with anions or neutral molecules. For example: a solution of iron II salt mixed with a solution of potassium cyanide, KCN, result in the formation of potassium hexacyanofferate II, K₄Fe(CN)₆, a complex salt made up of potassium ion, K⁺ and the complex ion, [Fe(CN)₆]^4-.

Another example of complex salt is sodium tetrahydroxozincate II.

Na₂Zn(OH)_4(aq)    ↔  2Na⁺_(aq)  +  [Zn(OH)₄]^2-

 

PREPARATION OF SALTS

Salts are prepared in variety of ways. The method for preparing a particular salt depends largely on its solubility in water and stability to heat. Soluble salts are usually prepared by methods which involve crystallization while insoluble salts are prepared by methods which involve precipitation. Hence, it is essential to be familiar with the solubility rules.

 

 

 

SOLUBILITY RULES

1.      All salts of sodium, potassium and ammonium are soluble in water.

2.      All trioxonitrate (V) salts are soluble in water.

3.      All chlorides are soluble in water, except those of lead, mercury and silver that are insoluble in water.

4.      All tetraoxosulphate (VI) salts are soluble in water except those of lead and barium. Calcium tetraoxosulphate (VI) is only slightly soluble in water.

5.      All trioxosulphate (IV) salts are insoluble in water, except those of sodium, potassium and ammonium that are soluble.

6.      All trioxocarbonate (IV) salts are insoluble in water, except those of sodium, potassium and ammonium that are soluble in water.

7.      All sulphides are insoluble in water, except those of sodium, potassium and ammonium that are soluble in water.

8.      All acid salts are soluble in water.

 

PREPARATION OF SOLUBLE SALTS

1.      Action of dilute acid on a metal: a salt can be prepared by the direct displacement of the hydrogen atom in an acid by a metal more reactive than hydrogen.

Zn_(s)   +  HCl_(aq)   →  ZnCl₂   +  H_2(g)

Mg_(s)   +  H₂SO_4(aq)     →   MgSO_4(aq)   +  H_2(g)

2.      By neutralization reaction: salts are prepared by reacting an alkalis against an appropriate acid.

2NaOH(aq)  +   H₂SO_4(aq)     →    Na₂SO_4(aq)  +   2H₂O_(l)

3.      Action of dilute acid on an insoluble base: the dilute acid is heated gently in a beaker and the base is added a little at a time until the base is in excess. The excess base is then filtered off, leaving the salt formed in the solution.

H₂SO_4(aq)     +   CuO_(s)    →   CuSO_4(aq)  +  H₂O_(l)

4.      Action of dilute acid on trioxocarbonate (IV): an acid will react on a trioxocarbonate (IV) to produce a salt, water and carbon(IV) oxide

H₂SO_4(aq)     +  CuCO_3(s)   →    CuSO_4(aq)   +  H₂O_(l)    +  CO_2(g)

PbCO3(s)     +   2HNO3(aq)   →    Pb(NO3)2(aq)   +  H₂O_(l)    +  CO_2(g)

 

PREPARATION OF INSOLUBLE SALTS

·         Double decomposition: the insoluble salts can be prepared by precipitation from solution (double decomposition). This method requires two soluble compounds (salts) which are mixed together, one containing metallic radical and the other acidic radical of the required insoluble salts. During double decomposition, there is an exchange of ionic radicals resulting in the precipitation of the required salt, which is then obtained by filtration. For example, if a solution of barium chloride is added to a solution of any tetraoxosulphate VI, an insoluble white precipitate of barium tetraoxosulphate VI is formed.

BaCl_2(aq)  +  Na₂SO_4(aq)    →   BaSO_4(s)  +  NaCl_(aq)

·         Direct combination of constituent elements: this method is use for preparing insoluble salts containing two elements only (binary salts). Some metallic chloride and sulphide salts are prepared by this method.

Fe_(s)  +  S_(s)          →       FeS_(s)

Hg_(s)  +   Cl_2(g)     →       HgCl_2(s)

2Fe_(s)  +  3Cl_2(g)   →       2FeCl_3(s)

USES OF SALTS

s/n 1 2 3 4 5 6 7 8 9       10 11   12

Names of salts  Ammonium chloride Calcium trioxocarbonate IV Calcium chloride Fused calcium chloride Calcium tetraoxosulphate VI Copper tetraoxosulphateVI Magnesium tetraoxosulphate VI Potassium trioxonitrate V Sodium chloride       Zinc chloride Zinc tetraoxosulphate (VI) heptahydrate and zinc sulphide Magnesium trioxocarbonate (IV)

Used in washing powder and as an electrolyte in dry cell as a medicine to neutralize acidity in the stomach as an antifreeze as a drying agent for making plaster of paris in dyeing and calio printing as a laxative for making gunpower, matches and as soil fertilizers as preservatives for meat, fish and perishable food. In soap making and to separate soap from glycerine For lowering the freezing point of water in refridgerating systems and to melt ice in freezers In petroleum refining For manufacturing of white paint   Producing toothpaste, rubber and paints.

 

 

CHARACTERISTICS OF SOME SALTS

The following phenomena are the characteristic properties of salts:

1.      Efflorescence: it is a process whereby some hydrated salt loses its water of crystallization completely or partially when exposed to atmosphere to form a lower hydrated or anhydrous salt. Such hydrated salts are said to be efflorescent. Example, sodium tetraoxosulphate (VI) decahydrate, Na₂SO₄.10H₂O .

Na₂SO₄.10H₂O_(s)     →   Na₂SO₄    +    10H₂O_(g)

Efflorescent salts lose weight and their crystalline form when heated or exposed to atmosphere, to become anhydrous and powdery

2.      Deliquescence: it is a phenomenon whereby, a compound absorbs moisture (or water vapour) from the atmosphere to form a saturated solution. Such a salt is said to be deliquescent salts. Examples, fused calcium chloride, CaCl_2, magnesium chloride, MgCl_2, iron III chloride, FeCl₃ and calcium trioxonitrate (V), Ca(NO₃)₂. Sodium hydroxide and potassium hydroxide are deliquescent alkalis, while phosphorus (V) oxide, P2O5 is a deliquescent oxide. Deliquescent salts gain weight when exposed to the atmosphere. They have great affinity for water.

3.      Hygrscopy: it is a phenomenon in which a compound absorbs moisture from the atmosphere without forming a solution; rather they become moist or sticky. Hygroscopic liquids absorb water from atmosphere and become diluted and as well increases its volume. Such a compound is said to be hygroscopic.  Examples of hygroscopic compounds are calcium oxide, CaO, copper II oxide, CuO, anhydrous CuSO_4, Na₂NO₃, concentrated H₂SO₄ ( the only liquid hygroscopic substance). Hygroscopic compounds have great affinity for water and they gain weight when exposed to the atmosphere.

4.      Drying agents: drying agents are substances that have strong affinity for moisture or water. They can easily remove water or moisture from another compound. They may be either hygroscopic or deliquescent. In chemical processes they are called dehydration agent or desiccants because of their great affinity for water. They are popularly used in the laboratory as drying agent for gases, liquids and solids. Examples, concentrated H₂SO_4, CaO(s), and fused CaCl_2(s) are used for drying gases. Anhydrous MgSO₄ and fused CaCl_2(s) are used for drying organic solvents such as alcohol. Fused CaCl₂, CaO, and silica gel are used as desiccant in desiccators for drying solids. A desiccant is a drying agent usually placed in the desiccators (an apparatus used to keep solids dry).

A drying agent cannot be used if it reacts with the substance to be dried, eg conc. H₂SO₄ cannot be used to dry ammonia gas because they react to form ammonium tetraoxosulphate (VI).

2NH_3(g)   +     H₂SO₄     →   (NH₄)₂SO_4(aq)

 

 

COMMON DRYING AGENTS AND THE GASED THEY DRY

S/N	DRYING AGENT	GASES
1	CONC. Tetraoxosulphate VI acid	All gases expect ammonia and hydrogen sulphide gas
2	Fused calcium chloride	All gases expect ammonia
3	Phosphorous (V) oxide	All gases expect ammonia
4	Calcium oxide	Suitable for ammonia in particular
5	Silica gel	Suitable for all gases

 

EVALUATION QUESTIONS

1.      Define the term salt

2.      List three types of salt and give one example of each type

3.      Explain why tetraoxosulphate  (VI) able to produce two types of salt, unlike trioxonitrate (V) acid.

4.      Write an equation for the preparation  of  (a). calcium trioxonitrate (V) by neutralization  (b). barium tetra oxosulphate (IV) by double decomposition

5.      Write briefly on (a). deliquescence (b). hygroscopy

 

 

VISUAL VIDEO CLIP (PART ONE)

 

 (PART TWO)

 

PART THREE