Hydrolysis of Salt

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In today’s class, we will be talking about the hydrolysis of salt. Enjoy the class!

HYDROLYSIS OF SALT

Some salts undergo hydrolysis in water to give an acidic or alkaline medium (solution) e.g. Na₂CO₃, NaHCO₃, AlCl₃, Na₂S, NH₄Cl, CH₃COONa etc.

Na₂CO₃ + H₂O    →  NaOH   +   H₂CO₃.

AlCl₃   +   H₂O    →  Al (OH)₃  +   HCl.

Na₂S    +   H₂O  →  NaOH    +    H₂S

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

USES OF SALTS

1. NH₄Cl is used as an electrolyte in a dry cell (Leclanche cell)
2. CaCO₃ is used as medicine to neutralise acidity in the stomach
3. CaCl₂ is used as antifreeze while fused CaCl₂ is used as a drying agent and also in the desiccator.
4. CaSO₄ is used for making plaster of Paris.
5. CuSO₄ is used in dyeing and calico printing.
6. MgSO₄ is used as a laxative.
7. KNO₃ is used for making gunpowder, matches and soil fertilizer.
8. NaCl is used for preserving food and in glazing pottery.
9. ZnCl₂ is used in petroleum refining

EVALUATION

1. Define salt?
2. List the five main types of salts giving two examples each
3. Name four salts and state the use of each of them

SOLUBILITY RULE

S/NO	SOLUBLE SALTS	INSOLUBLE SALTS
1.	All Na+, K+ and NH4+ salt
2.	All trioxonitrate (v)
3.	All chloride except	PbCl2, HgCl2 and AgCl are soluble in hot water.
4.	Trioxocarbonate (iv) of Na+, K+ and NH4+	All other trioxocarbonate (iv).
5.	Trioxosulphate (vi) of Na+, K+, NH4+ & Cu2+	All other trioxosulphate (IV)
6.	Sulphide of Na+, K+ and NH4+	All other sulphides.
7.	All tetraoxosulphate (vi) except	PbSO4, BaSO4 and CaSO4 are slightly soluble in H2O
8.	All hydrogen trioxocarbonate (iv)
S/NO	SOLUBLE BASE/ALKALIS	INSOLUBLE BASE/ALKALIS
1.	K2O, Na2O are very soluble MgO, CaO are slightly soluble	Other oxides are insoluble
2.	NaOH, KOH, Ca(OH)2 are very soluble Mg (OH)2 is slightly soluble.	Other hydroxides are insoluble.

METHOD OF PREPARATION OF SALTS

The method of preparing a particular salt depends on its:

1. Solubility in water
2. Stability to heat.

It is necessary, therefore, for us to become familiar with the simple rules of solubility indicated above. Knowing the solubility of the salt enables us to determine which method to be used.

SOLUBLE SALT

Soluble salts can be prepared by the following method:

1. The action of dilute acid on a metal.
2. Neutralization of an acid by an alkali
3. The action of dilute acid on an insoluble base.
4. The action of dilute acid on trioxocarbonate (IV).

RECOVERING SOLUBLE SALTS FROM SOLUTION

This can be done by:

1. Heating to dryness: This is used to prepare soluble salts which are not destroyed or decomposed by heat e.g. most chlorides such as NaCl, ZnCl₂, FeCl₂ and FeCl₃ are recovered by heating.
2. Crystallization: This is used to prepare salt which is easily decomposed or destroyed by dry heating. All trioxonitrate (V) and tetraoxosulphate (VI) are recovered by crystallization.

INSOLUBLE SALTS

Insoluble salts can be prepared by the following method:

1. Double decomposition or precipitation.

Pb (NO₃)_2(aq)    +    2NaCl _(aq)      →  2NaNO_{3 (aq)}   + PbCl_2(s)

AgNO_{3 (aq)}       +      NH₄Cl _(aq)     →  NH₄NO_{3 (aq)}   + AgCl_(s)

2. Direct combination of 2 elements.

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

2Fe(_s) +   3Cl_2(g)    →  2FeCl_3(s)

ANHYDROUS AND HYDRATED SALT

• Anhydrous salts: are salts which do not contain water and cannot be crystallized out from the aqueous solution.
• Hydrated salts/salts with water of crystallization: are salts which combine chemically with water. The water molecule is loosely held to the salt molecule and when heated, such salt loses their water of crystallization. The water attached is known as the water of crystallization e.g

Cu(NO₃)₂.3H₂O:  Copper (ii) trioxonitrate (v) trihydrate.

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

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

Salts without water of crystallization	Salts with water of crystallization
NaCl	CuSO4.5H2O
KNO3	FeSO4.7H2O
KMnO4	ZnSO4.7H2O
(NH4)2SO4	Na2CO3.10H2O
K2SO4	Cu(NO3)2.3H2O
Pb(NO3)2	MgSO4.7H2O
AgNO3	Zn(NO3)2.6H2O.

Calculation of water of crystallization

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 the water of crystallization.

Solution

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

.         Mass of water molecule              Molar mass of water molecule

    14                =    (90+18x)

(14-9.99)                    18x

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 = H₂C₂0₄.2H₂0.

(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

.            126

= 3600

.    126

= 28.57%

EFFLORESCENCE, DELIQUESCENCE AND HYGROSCOPIC

When a certain compound is exposed to the air, they either lose their water of crystallization or they absorb moisture from their surroundings. The term efflorescent, deliquescent and hygroscopic are used to describe such compound.

EFFLORESCENCE: are substances which on exposure to air, lose some or all of their water of crystallization. The phenomenon or process is efflorescence. There is a 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 etc.

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

HYGROSCOPIC: are substances which absorb moisture on exposure to the atmosphere without forming a solution. If they are solids, no solution will be formed but if a liquid absorbs water, it gets diluted. There is little or no difference in mass e.g. Conc. H₂SO₄, NaNO₃, CuO, CaO and anhydrous Na₂CO₃.

DRYING AGENTS

These are substances which have a high affinity for water or moisture. They are either deliquescent or hygroscopic. They remove water molecules to affect 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	Gases
Conc. H2SO4	All gases except NH3 & H2S
Fused CaCl2	All gases except NH3
CaO or quicklime	For ammonia
P2O5	All gases except ammonia
Silica gel	All gases

Salts are usually dried in a desiccator.

EVALUATION

1. Using balanced equations, state two methods of preparing: (a) Soluble salt (b) insoluble salt
2. How can soluble salts be recovered from their solution?

GENERAL EVALUATION/REVISION

1. Calculate the percentage of water in sodium trioxocarbonate (iv) heptahydrate
2. What is the number of molecules in 6.4g of sulphur (iv) oxide (N_A=6.0X10²³/mol)
3. Write an equation to show the acid formed when phosphorus (v) oxide is dissolved in cold water and name the acid formed
4. Differentiate between a base and an alkali
5. Define: Efflorescence, Deliquescence and Hygroscopy

READING ASSIGNMENT

New School Chemistry for Senior Secondary Schools by O.Y Ababio pages 100-101 and 108-115.

WEEKEND ASSIGNMENT

1. The two types of bonds that exist in H₃O⁺ are a. covalent and ionic b. co-ordinate covalent and covalent c. metallic and ionic d. polar covalent and metallic
2. How many moles of hydrogen ions are there in 50cm³ of 0.20moldm^-3 H₂SO₄? A.  0.01    B.    0.02   C.    0.10      D.   0.20
3. Which of these is not recovered through dry heating (evaporation)?  A.   NaClO₃    B.   NH₄NO₃    C.   CuHSO₄         D.   NaHCO₃.
4. Which pH value indicates a basic solution? A. -1    B.    3    C.    9     D.    7
5. All common gases are dried using P₂O₅ except A.   NO₂     B.   NH₃     C.    SO₂    D.    H₂S

THEORY

1. Give the reason for each of the following:
2. Sodium salts cannot be prepared by double decomposition
3. Na₂CO_3(aq) which is a salt solution, turns red litmus blue.
4. 1.34g of hydrated Na₂SO₄ was heated to give an anhydrous salt weighing 0.71g.
5. Calculate the number of molecules of water of crystallization
6. Give the formula of the hydrated salt [Na=23, S=32, O=16, H=1].

 

In our next class, we will be talking about Carbon and its Properties.  We hope you enjoyed the class.

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