SALTS at a glace

SALTS

A salt is a compound formed when all or part of the ionizable or replaceable hydrogen ion in an acid is replaced by a metallic or ammonium ion e.g.

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

ii.  H₂SO_4(aq) + KOH_(aq) → KHSO_4(aq) + H₂O_(l)

TYPES OF SALTS

There are five main types of salts namely:

1. Normal salt.

2. Acid salts

3. Basic salts

4. Double salts.

5. Complex salts.

1. Normal salts: are the salts formed when all the replaceable hydrogen ion in the acid has been completely replaced by a metal or an ammonium ion e.g. NaCl, K₂SO₄, Na₃PO₄, NaNO₃ etc. 

Normal salts are neutral to litmus and does not contain any replaceable hydrogen ion (H+) 

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

ii. H₂SO_4(aq) + KOH_(aq) → K₂SO_4(aq) +   H₂O_(aq)

2. Acid salts: Acid salts are salts formed when the replaceable hydrogen ions of an acid are only partially replaced by a metal or an ammonium ion. e.g. NaHSO₄, Na₂HPO₄, NaH₂PO₄, NaHCO₃. 

They are usually formed from acids which contain more than one replaceable hydrogen ion. Acids with two replaceable hydrogen ions can form only one acid salt while acids with three replaceable hydrogen ions can form two different acid salts.

H₂SO_4(aq) + NaOH_(aq) → NaHSO_4(aq)+ H₂O_(l)
                                         sodiumhydrogentetraoxosulphate (VI)

2H₃PO_4(aq) + NaOH_(aq) → NaH₂PO_4(aq) 
                                            monosodiumhydrogentetraoxophosphate (V)

NaH₂PO_4(aq)  + 2NaOH_(aq) → Na₂HPO_4(aq) +H₂O_(l)
                                         disodiumhydrogentetraoxophosphate (V)

Na₂HPO_4(aq + NaOH_(aq) → Na₃PO_4(aq) +H₂O_(l) 
                                       sodiumhydrogentetraoxophosphate (VI)                                        

Properties of Acid salts 

i. Acid salts turn blue litmus red. 

ii. Acid salts react with bases to form salts 

 KHSO_4(aq) + KOH_(aq) → K₂SO_4(aq) + H₂O_(l)

3. Basic salts: Basic salts that still contain replaceable hydroxide ions.

Thay are are formed when only part of the hydroxide ions of a base are replaced by the negative ions from an acid.

e.g Zn(OH)Cl,   Mg(OH)Cl, Mg(OH)NO₃, Bi(OH)₂NO₃ e.t.c.

i. Zn(OH)_2(aq) + HCl_(aq)  → Zn(OH)Cl_(aq) +  H₂O_(l)

ii. Ca(OH)(aq) + HNO3 →    Ca(OH)NO3(aq) + H2O(l)

Properties of basic salts 

i. basic salts turn red litmus blue. 

ii. basic salts react with more acid to form a normal salt and water only.

     Mg(OH)NO_3(aq) +HNO_3(aq) → Mg(NO₃)_2(aq) + H₂O_(l)

4. Double salts: Double salts are salt which ionize to produce three different types of ions in solution. Usually, two of these are positively charged (metallic or NH4+ ion) while the other is negatively charged e.g. (NH₄)2Fe(SO₄)₂.6H₂O, KAl(SO₄)₂.12H₂O, KCr(SO₄)₂.12H₂O.

(NH₄)2Fe(SO₄)₂.6H₂O:  Ammonium iron (II) tetraoxosulphate (VI) hexahydrate.

KAl (SO₄)₂.12H₂O:  Aluminium Potassium tetraoxosulphate (V) dodecahydrate (Potash alum).

KCr(SO₄)₂.12H₂O:   Chromium (III) Potassium tetraoxosulphate (VI) dodecahydrate (Chrome alum).

5. Complex salts: Complex salts contain complex ion i.e ion consisting of a charged group of atoms e.g. Na₂Zn(OH)₄, K₄Fe(CN)₆, NaAl(OH)₄.

Na₂Zn(OH)₄:     Sodium tetrahydroxozincate (II)

K₄Fe (CN)₆:      Potassium hexacyanoferrate (II)

NaAl(OH)₄:       Sodium tetrahydroxoaluminate (III)

Na₂Zn(OH)₄  → 2Na⁺  + [Zn(OH)₄]^2-

K₄Fe(CN)₆  → 4K⁺ + [Fe(CN)₆]^4-.

Properties of complex salts

i. they are soluble in water

HYDROLYSIS OF SALT

Some salts when dissolved in water, undergoes hydrolysis to give an acidic or alkaline solution.

 e.g. Na₂CO₃, NaHCO₃, AlCl₃, Na₂S, NH₄Cl, CH₃COONa e.t.c. It is like the reverse of neutralization. A salt dissolves in water to give the initial acid and alkali or hydroxide from which it was formed. for example 

1. Na₂CO_3(s) +H₂O_(l) → Na⁺ + CO3^2-
 From water   OH^-     

    H⁺
                       strong.    weak

                        base.      acid                                              pH < 7

2. AlCl_3(s) + H₂O_(l) → Al³⁺ + Cl^-
 From water             3(OH)^-    H⁺
                             weak.    strong

                               base      acid                                           pH < 7

3. (NH₄)₂CO_3(s) +H₂O_(l) ⇌ NH₄⁺ + CO₃^2-

     from water       OH^-        H⁺
                            Weak      weak

                             base.       acid                                         pH = 7

Hydrolysis of salt occurs when a salt reacts with water e.g, salt of strong acid and weak base to give an acidic solution. The change in pH of solution is due to hydrolysis.

USES OF SALTS

	SALT	USES
1.	NH4Cl	is used as an electrolyte in dry cell (Leclanché cell)
2.	CaCO3	is used as medicine to neutralize acidity in the stomach.
3.	CaCl2	i. is used as antifreeze while fused CaCl2 is used as a drying agent and also in desiccators. ii.  is used in dyeing and calico printing.
4.	CaSO4	is used for making plaster of Paris.
5.	MgSO4	is used as a laxative.
6.	KNO3	is used for making gunpowder, matches and soil fertilizer.
7.	NaCl	is used for preserving food and in glazing pottery.
8.	ZnCl2	is used in petrol

METHODS OF PREPARATION OF SALTS

The method of preparing  a salts in general depends on its:

i. Solubility in water

ii. Stability to heat.

It is important for us to know the simple rules of solubility indicated above. If we know  the solubility of a salt, it will enables us to determine which method will be used for its preparation

SOLUBLE SALT

Soluble salts can be prepared by any one of the following method:

1. Neutralization of an acid by an alkali

2. Action of dilute acid on a metal.

3. Action of dilute acid on an insoluble base.

4. Action of dilute acid on trioxocarbonate (IV).

OBTAINING SOLUBLE SALTS FROM SOLUTION

This can be done by:

1. Heating to dryness (Evaporation): This is used to recover soluble salts which do decomposed or destroyed by heat e.g. most chlorides such as NaCl, ZnCl2, FeCl2 and FeCl3 are recovered by heating.

2. Crystallization: This method is used to prepare salts which are easily decomposed or destroyed by heating to dryness. All trioxonitrate (V) salts and tetraoxosulphate (VI) are recovered by crystallization.

INSOLUBLE SALTS

Insoluble salts can be prepared by the following method:

1. Double decomposition or precipitation.

i. Pb (NO₃)_2(aq) + 2NaCl_(aq) → 2NaNO_3(aq) + PbCl_2(s)

ii. AgNO_3(aq) + NH₄Cl_(aq) →NH₄NO_3(aq) + AgCl_(s)

2. Direct combination of 2 elements.

i. Fe_(s) + S_(s) → FeS_(s)

ii. 2Fe_(s) + 3Cl_2(g) →2FeCl_3(s)

ANHYDROUS AND HYDRATED SALT

Anhydrous salts: These are salts which do not contain water of crystallization. They cannot be crystallized out from aqueous solution. 

Hydrated salts are salts which contain water of crystallization, when heated, such salt loses their water of crystallization.

Water Of Crystallization: This is a specific amount of water molecules that is embedded in crystals of salts as they form during crystallization.

Cu(NO₃)₂.3H₂O:  Copper (II) trioxonitrate (V) trihydrate.

MgSO₄.7H₂O:      Magnesium tetraoxosulphate (VI) heptahydrate.

FeSO₄.7H₂O:       Iron (ii) tetraoxosulphate (VI) heptahydrate.

Calculations of water of crystallization

1.  14g of hydrated H₂C₂O₄.xH₂O was heated to give an anhydrous salt weighing 9.99g.

(a). Calculate the value of x.

(b). Give the formula of the hydrated salt.

(c). Calculate the % of water of crystallization.

Solution

(a).  Mass of hydrated salt      = Molar mass of hydrated salt
       Mass of water molecule       Molar mass of water molecule
               

                mass of water lost = (14-9.99) = 4.01
                     14 = (90+18x)
                     4.01        18x

        14(18x) = 4.01 (90 + 18x)

        252x = 360.9 + 72.18x

        252x – 72.18x = 360.9

        179.82x = 360.9

        x =    360.9
                 179.82
                   x = 2.007

        x = 2 to the nearest whole number.

(b). Formula of hydrated salt = H₂C₂O₄.2H₂O.

(c) To calculate the % of water of crystallization:

% of water of crystallization = Mass of water x 100%
                                                         Total mass
                       = 36 x 100
                          (90 + 36)
            
                =   36    x   100   = 28.57%
                          126

   

EFFLORESCENCE, DELIQUESCENCE AND HYGROSCOPIC

When certain compounds are exposed to the air, they either lose some or all of their water of crystallization or they absorb moisture from their surroundings to become either moist or form solutions. The term efflorescent, deliquescent and hygroscopic are used to describe such compound/ or phenomenon.

EFFLORESCENCE: This is a phenomenon whereby some salts/ compounds when exposed to the atmosphere loss all or part of their water of crystallization.

EFFLORESCENT SAALTS: are substances which on exposure to air, lose some or all of their water of crystallization. The phenomenon or process is efflorescence. There is loss of weight or mass of the substances.

e.g Na₂CO₃.10H₂O →   Na₂CO₃.H₂O + 9H₂O

Other examples are Na₂SO₄.10H₂O, MgSO₄.7H₂O and CuSO₄.5H₂O e.t.c

DELIQUESCENCE: This a phenomenon whereby some salts when exposed to air absorbs so much water from the air that they form a solution.

DELIQUESCENTS SALTS: are substances that absorb so much water from air and form a solution e.g. NaOH, CaCl₂, FeCl₃, MgCl₂, KOH and P₄O₁₀. There is a gain in weight.

HYGROSCOPIC SUBSTANCES: are substances which absorb moisture on exposure to the atmosphere without forming a solution but only become sticky or wet. If they are solids, no solution will be formed but if a liquid, they absorb water and become diluted  e.g Conc. H₂SO₄, NaNO₃, CuO, CaO and anhydrous Na₂CO₃.

DRYING AGENTS

These are substances which have high affinity for water or moisture. They are either deliquescent or hygroscopic substances. They remove water molecules attached to wet substances to effect physical change. Drying agents are different from dehydrating agents which removes elements of water i.e hydrogen and oxygen atoms or intra-molecular water.

Drying agents which react with gases are not used to dry the gas e.g conc. H₂SO₄ is not used to dry NH₃ and H₂S gas.

NH_3(g) + H₂SO_4(aq) → (NH₄)₂SO_4(aq)

H₂S_(g) + H₂SO_4(aq) → 2H₂O_(l) + SO_2(g) + S_(s)

Drying agent For Gases

Concentrated H₂SO₄ is used to dry All gases except NH₃ & H₂S

Fused CaCl₂ is used to dry All gases except NH₃

CaO (quicklime) is used to dry Ammonia 

P₂O₅ All gases except Ammonia

Silica gel All gases

Salts are usually placed inside desiccators to dry

             a desicator

OBJECTIVE QUESTIONS

1. A substance is said to be hygroscopic if it absorbs 

a. water from the atmosphere to form a solution

b. heat from the surrounding 

c. carbon (iv) oxide from the atmosphere

d. moisture from the atmosphere 

2. The gas given off when NH4Cl is heated with an alkali is 

a. H₂

b. Cl₂

c. N₂ 

d. NH₃

3. A major factor considered in selecting a suitable method for preparing a simple salt is its

a. crystalline form 

b. melting point 

c. reactivity with dilute acids 

d. solubility in water

4. Which of the following salts solutions will have a pH greater than 7

a. NaCl_(aq)

b. Na₂CO_3(aq)

c. Na₂SO_4(aq)

d.NaHSO_4(aq)

5.  Which of the following compound will leave a metal residue when heated 

a. Cu(NO₃)₂

b. AgNO₃

c. K₂CO₃

d.CaCO₃

6. 

THEORY QUESTIONS

1. Give one example of the following salts 

i. Hydrated salt  

ii. Acidic salt

iii. Basic salt 

2.(a)(i) State two methods of preparing salts, giving one example in each case of a salt so prepared.

    (ii). What type of salt is each of the following?    NaH₂PO₄; (CH₃COO)₂Pb; KAl(SO₄).12H₂O

3.(a) Rock salt is an impure form of sodium chloride.

  (i). Outline a suitable procedure for preparing a pure sample of sodium chloride from rock salt.

 (b). Classify each of the following as normal salt/ acid salt/basic salt/double salt

(i). Sodium hydrogen trioxocarbonate (IV) 

(ii). Iron (III) chloride 

(iii). Sodium ethanoate