Bases And Salts SS1 Chemistry Lesson Note

BASES AND ALKALIS

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

An alkali 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 (O2 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.

PbO2(s) +   4HCl (aq)        PbCl2 (aq) +    2H2O (l)   +   Cl2 (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 acid, we have the strength of a base. The strength of a base can either be weak or strong.

1. Weak Base: A weak base is a base that ionizes slightly in an aqueous solution to produce positively charged metallic ions and negatively charged hydroxide ions e.g. CaO, NH3.

NH3 (g) + H2O(l)  >  NH4+(aq) OH-(aq)

2. Strong Bases: Strong bases are base that ionizes completely in an aqueous solution to produce positively charged metallic ions and negatively charged hydroxide ions 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 forms of the caustic alkalis of NaOH and KOH are corrosive.

CHEMICAL PROPERTIES OF ALKALIS

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

NaOH(aq)   +    HCl(aq)         NaCl(aq)       +    H2O(l)

MgO(s)      +   2HNO3(aq)     Mg(NO3)2(aq)   +   H2O(l)

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

NaOH (aq)   +      NH4Cl (aq)       NaCl (aq)    +     H2O(l)   +     NH3(g)

Ca (OH)2(aq)  +  (NH4)2SO4(aq)       CaSO4(aq)   +    2H2O(l)  +   2NH3(g)

USES OF ALKALIS/BASE

1. NaOH

i. Used in the manufacture of soap, Na salts and plastic

ii. Used in petrol refining.

2. KOH

i. Used in the manufacture of liquid soap

ii. Used in dyeing and electroplating.

3. Ca(OH)2

i. Used in the manufacture of  mortar, cement and plaster

ii. Used in dissolving acidic soil.

4. Mg(OH)2

i. Used in the manufacture of toothpaste

ii. Used as a laxative.

5. Aqueous NH3

i. Used for bleaching cloth

ii. Used as detergent.

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 ions (OH-).

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

HCl(aq)   +   NaOH(aq)          NaCl(aq)    +    H2O(l)

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

H3OCl(aq)        +      KOH(aq)            KCl(aq)    +    H2O(l)

(H3O)2SO4(aq)   +      2NaOH(aq)          Na2SO4(aq)    +     4H2O(l)

H3O+(aq)    +   OH-(aq)  >  2H2O(l)

OXONIUM ION

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

H+    +   H2O > H3O+

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

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)

H2SO4(aq) + KOH(aq) → KHSO4(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 ions in the acid have 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)

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

2. Acid salts: Acid salts are formed when the replaceable hydrogen ions in the acids are only partially replaced by a metal e.g. NaHSO4, Na2HPO4, NaH2PO4, NaHCO3. They can be produced 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

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 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 an 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 ions 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 salts 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 ions i.e. ions 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 (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 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 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 dry cells (Leclanche cell).
2. CaCO3 is used as medicine to neutralise acidity in the stomach.
3. CaCl2 is used as an antifreeze while fused CaCl2 is used as a drying agent and also a as 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.

EFFLORESCENCE, DELIQUESCENCE AND HYGROSCOPIC

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

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 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 e.t.c

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: these 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 effect physical change. Drying agents are different from dehydrating agents which remove 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)