Bases

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 (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.

Metal Solubility Decomposition by heat.

K

Na

Ca The hydroxides of these metals are soluble in water and are alkalis. Hydroxide of sodium and potassium can

not be decomposed by heat.

Mg, Al, Zn

Fe, Pb, Cu These metals form hydroxides which are insoluble in water. They are amphoteric except the three hydroxides of Magnesium, iron, and copper. Decomposed on heating to form oxide and water.

Hg, Ag, Au. Hydroxides of these metals do not exist.

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.

NH3 (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.

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/BASES

Names Uses

NaOH Used in the manufacture of soap, Na salts and plastic

Used in petrol refining.

KOH Used in the manufacture of liquid soap

Used in dyeing and electroplating.

Ca(OH)2 Used in manufacture of mortar, cement and plaster of paris

Used in dissolving acidic soil.

Mg(OH)2 Used in the manufacture of toothpaste

Used as a laxative.

Aqueous NH3 Used for bleaching cloth

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 ion (OH-).

Neutralization can be majorly defined as the process whereby an acid react completely with an alkalis/bases to form salt and water

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

Also, 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)

Likewise, 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)2SO4(aq) + 2NaOH(aq) Na2SO4(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 alkali giving examples

2. State three physical properties of alkalis

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

4. State 3 uses of bases

5. Define the term neutralization with one equation

6. What is the difference between a base and an alkali?