Bases and Salts

CONTENT

• Bases
• Salts
• Efflorescence, deliquescence and hygroscopic

BASES AND ALKALIS

A base is a substance which will neutralize an acid to yield a salt and water only. Most oxide and hydroxide of metals are bases e.g. Na₂O, K₂O, MgO, NaOH, KOH e.t.c.

An alkalis is a basic hydroxide which is soluble in water NaOH, KOH, Ca(OH)₂. A basic oxide (or hydroxide) is a metallic oxide (or hydroxide) which contains ions (O^2‑ or OH^‑) and will react with an acid to form a salt and water only.

Note: An exception to this definition is the reaction of lead (IV) oxide with hydrochloric acid to produce lead (ii) chloride (a salt), water and chlorine gas.

PbO_2(s) +   4HCl _(aq)     –>    PbCl_{2 (aq) +}    2H₂O _(l)   +   Cl_{2 (g)}

From the equation above, PbO₂ is not a base.    The nature of the hydroxides of the metals varies according to the position of the metal in the electrochemical series, as illustrated below.

STRENGTH OF BASE

Like an acid, we have strength of a base. The strength of a base can either be weak or strong.

Weak base: Weak base are base that ionizes slightly in aqueous solution to produce positively charged metallic ion and negatively charged hydroxide ion e.g. CaO, NH₃.

NH_{3 (g)}    +   H₂O _(l)     –>    NH₄⁺(_aq) OH^–_(aq)

Strong bases: Strong base are base that ionizes completely in aqueous solution to produce positively charged metallic ion and negatively charged hydroxide ion e.g. Na₂O, K₂O.

Na₂O_(s)   +      H₂O _(l)       –>    2NaOH_(aq)

K₂O_(s)      +     H₂O _(l)       –>   2KOH_(aq)

NaOH _(aq)    –>    Na⁺_(aq)    +       OH^–_(aq)

KOH _(aq)       –>   K⁺_(aq)         +       OH^–_(aq)

PHYSICAL PROPERTIES OF ALKALIS

1. Alkalis have a bitter taste.

2. Alkalis are soapy to the touch.

3. Alkalis turn red litmus blue.

4. Concentrated form of the caustic alkalis of NaOH and KOH are corrosive.

CHEMICAL PROPERTIES OF ALKALIS

1. Reaction with acid: All base react with acid to form salt and water only.

 –>2. Displacement of volatile ammonia from ammonium salt by a non volatile alkali: if an ammonium salt is warmed with an alkali (in the presence of water) ammonia gas is liberated

 –>Ca (OH)_2(aq)  +  (NH₄)₂SO_4(aq)    –>    CaSO_4(aq)   +    2H₂O_(l)  +   2NH_3(g)

USES OF ALKALIS/BASE

EVALUATION

1. Define alkali giving examples

2. State three physical properties of alkalis

3. Using balanced equations, state two chemical properties of bases

4. State the uses of bases

NEUTRALIZATION REACTION

Neutralization reaction can be defined in three major ways.

1. In terms of acid and the base present.

2. In terms of H⁺ ion and OH^– present in the acid and base.

3. In terms of oxonium ions (H₃O⁺) and hydroxide ion (OH^–).

Neutralization is the process whereby an acid react completely with an alkalis/bases to form salt and water.

HCl_(aq)   +   NaOH_(aq)       –>    NaCl_(aq)    +    H₂O_(l)

Neutralization is the combination of hydrogen ion (H⁺) and hydroxide ions (OH^–) to form water molecules. A salt is also form at the same time.

H⁺_(aq)    +     OH^–    –> H₂O_(l)

Neutralization can also be defined as the combination of oxonium ions (H₃O⁺) and hydroxide ions (OH^–) to form water molecule. A salt is also formed at the same time.

H₃OCl_(aq)        +      KOH_(aq)    –>  KCl_(aq)    +    H₂O_(l)

(H₃O)₂SO_4(aq)   +      2NaOH_(aq)     –> Na₂SO_4(aq)    +     4H₂O_(l)

H₃O⁺_(aq)              +      OH^–_(aq)      –> 2H₂O_(l)

OXONIUM ION

In aqueous solution, the hydrogen ion become associated with a water molecule to form oxonium ion (H₃O⁺).This is an example of a coordinate covalent combination.

H⁺    +   H₂O    –>  H₃O⁺

During neutralization, oxonium ion H₃O⁺ behaves as hydrogen ion and thus reacts with hydroxide ion (OH^–) to form water molecules.

EVALUATION

1. Define the term neutralization.

2. Write TWO balanced equations to show neutralization reactions.

SALTS

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

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

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 ion e.g. NaCl, K₂SO₄, Na₃PO₄, NaNO₃ etc. Normal salts are neutral to litmus

HCl_(aq)   +    NaOH_(aq)          –>     NaCl_(aq)  +    H₂O_(l)

H₂SO_4(aq)  +  KOH_(aq)             –>    K₂SO_4(aq)  +   H₂O_(aq)

2. Acid salts: Acid salts are formed when the replaceable hydrogen ion in the acids are only partially replaced by a metal e.g. NaHSO4, Na2HPO4, NaH2PO4, NaHCO3. They can be produce from acids which contain more than one replaceable hydrogen ion. Acids with two replaceable hydrogen ions can form only one acid salt while acid with three replaceable hydrogen ions can form two different acid salts

H2SO4(aq)    +    NaOH(aq)  –>NaHSO4(aq)  +  H2O(l)

2H3PO4(aq)   +   3NaOH(aq)  –>NaH2PO4(aq)  + Na2HPO4(aq)  +3H2O(l)

Acid salts turn blue litmus red. Acid salts can be converted to normal salt if the remaining replaceable hydrogen ions in the acid salt are replaced in with metallic ions.

KHSO4(aq)   +    KOH(aq) –> K2SO4(aq)  +  H2O(l)

3. Basic salts: Basic salts are formed when only part of the hydroxide ions of a base are replaced by the negative ions from an acid. It can occur when there is insufficient supply of acid for complete neutralization of the base e.g Zn(OH)Cl , Mg(OH)Cl, Mg(OH)NO3, Bi(OH)2NO3 e. t .c.

Zn(OH)2(aq)   +   HCl(aq) –>Zn(OH)Cl(aq)  +   H2O(l)

Because of the presence of hydroxide ion in the salt, it has basic properties. Basic salts turn red litmus blue. Basic salts react with excess acid to form a normal salt and water only.

Mg(OH)NO3(aq)   +  HNO3(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 contains complex ion i.e ion consisting of a charged group of atom e.g. Na2Zn(OH)4, K4Fe(CN)6, NaAl(OH)4.

Na2Zn(OH)4: Sodium tetrahydroxozincate (ii)

K4Fe(CN)6 : Potassium hexacyanoferrate (iii)

NaAl(OH)4: Sodium tetrahydroxoaluminate (iii)

Na2Zn(OH)4  –>2Na+   +   [Zn(OH)4]2-

K4Fe(CN)6  –>   4K+    +     [Fe(CN)6]4-.

HYDROLYSIS OF SALT

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

Na2CO3 + H2O  –>     NaOH   +   H2CO3.

AlCl3   +   H2O  –>    Al (OH)3  +   HCl.

Na2S    +   H2O  –>    NaOH    +    H2S

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

USES OF SALTS

1. NH4Cl is used as an electrolyte in 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 dessicator.

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 sulphide.

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 oxide 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. 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).

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 are 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)  –>    2NaNO3 (aq)   + PbCl2(s)

AgNO3 (aq) +      NH4Cl (aq)  –>   NH4NO3 (aq)   + AgCl(s)

2. Direct combination of 2 elements.

Fe(s)    +    S(s)  –>      FeS(s)

2Fe(s) +   3Cl2(g)  –>   2FeCl3(s)

ANHYDROUS AND HYDRATED SALT

Anhydrous salts: are salts which do not contain water and cannot be crystallized out from 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 lose their water of crystallization. The water attached is known as 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 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)

4.0118x

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 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.

EFFLORESCENTS: 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

DELIQUESCENTS: 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: 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 absorb 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 high affinity for water or moisture. They are either deliquescent or hygroscopic. They remove water molecules 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.

NH3(g) –> +      H2SO4(aq) –>  (NH4)2SO4(aq)

H2S(g)     +      H2SO4(aq) –>   2H2O(l)  +    SO2(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 dessicator.

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 (NA=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:

a. Sodium salts cannot be prepared by double decomposition

b. Na2CO3(aq) which is a salt solution, turns red litmus blue.

2.  1.34g of hydrated Na2SO4 was heated to give an anhydrous salt weighing

0.71g.

a. Calculate the number of molecules of water of crystallization

b. Give the formula of the hydrated salt [Na=23, S=32, O=16, H=1].