CHM 221: Inorganic Chemistry I
2nd Semester 2020/2021
Lecture Note 2
Concepts of Acids and Bases
Introduction
The concept of an acid, and a base are ancient ones that modern
chemical science has adopted and refined. The term acid originates from the
Latin word acidus, meaning sour, while the term alkali is an Arabic word for
the ashes that comes from burning certain plants. Thus, Acids and bases
were first recognised as specific classes of compounds because of the
distinctive properties exhibited by their aqueous solutions. Some properties of
acids and bases are listed in table 2.1 below:
Table 2.1 Properties of acids and bases
Over the years, Chemists have proposed various definitions of acids and
bases. These definitions, otherwise known as acid-base concepts have tried
to cover many acid-base reactions, redox reactions, and complex formation. It
should be noted that each definition is correct when applied to the specific
situation for which it was meant. These concepts include Arrhenius concepts,
Bronsted-Lowry concept and Lewis concept.
Arrhenius Concept
This concept was presented in 1887 by the Swedish Chemist Svante
Arrhenius. According to this concept all substances which give H+ ions when
dissolved in water are called acids while those which ionise in water to furnish
OH- ions are called bases. i.e.
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Thus, according to this concept, substances like HCl, H2SO4, HNO3,
CH3COOH etc. are acids since they give H+ ions, when dissolved in water.
While substances like NaOH, NH4OH, KOH, Ca(OH)2 etc. are bases, since
they give OH- ions in water.
Acid-base neutralisation reactions taking place in water
The reaction between an acid and a base is termed neutralisation, i.e.,
neutralisation involves the reaction between H+ and OH- ions to form water.
However, according to the Arrhenius concept, free H+ ions do not exist in
water. They combine with solvent molecules, i.e., hydrated to form hydronium
or hydroxinium (H3O+) ions. For example, the ionisation of HCl acids in
aqueous solutions should be represented as:
Thus, the formation of salt(NaCl) and H20 from neutralisation reaction between
HCl and NaOH can be represented below:
Basicity or Protoclty of an Acid
The basicity or protocity of an acid is the number of hydrogen ions which can
be furnished by one molecule of the acid upon dissociation. An acid liberating
1, 2 or 3 H+ ions are respectively known as monobasic, dibasic and tribasic
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(monoprotic, diprotic and triprotic). Polyprotic or polybasic acids are acids that
possess more than one dissociable hydrogen ion eg., H2SO4 and H3PO4.
Their dissociation usually takes place in steps.
It should be noted that basicity of an acid is not necessarily the number of
hydrogen atoms present in one molecule of the acid but the number of
hydrogen ions furnished by one molecule.
Acidity or Hydroxicity of a Base
Similarly, the acidity or hydroxicity of a ,base is the number of hydroxyl ions
produced from one molecule of the base on dissociation. A base liberating 1,
2 or 3 OH- ions are respectively known as monoacidic or monohydroxic,
diacidic or dihydroxic, triacidic or trihydroxic. Sodium hydroxide (NaOH),
ammonium hydroxide (NH4OH), potassium hydroxide (KOH) are monoacidic
or monohydroxic bases while calcium hydroxide [Ca(OHh] barium hydroxide
[Ba(OH] are diacidic or dihydroxic bases.
Strength of Acids and Bases
The strength of an acid or a base in Arrhenius concept is based on the extent
of ionisation or the equilibrium constant.
The equilibrium constant, Ka for the acid, HX, is :
The equilibrium constant, Kb for the acid, BOH, is :
The acids which are highly ionised in aqueous solutions, i.e., giving large
number of hydrogen ions are known as strong acids. The value of Ka for
strong acids is higher than 0.1. HCIO4, HNO3, H2SO4, HCl, etc., are very
strong acids. The value of Ka is very small for weak acids such as CH3COOH,
H3PO3, H2CO3, H3BO3, etc. For example, Ka value of CH3COOH is 1.8 x l0-5 at
25ºC. In other words, in the case of weak acid, the ionisation is only slight and
the major part remains as undissociated molecules.
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The bases which are highly dissociated in aqueous solution to produce large
number of OH- ions are called strong alkalies. The value of Kb for strong
alkalies is higher than 0.1. NaOH, KOH, Ba(OH)2, etc., are strong bases. The
value of Kb is low for weak bases such as NH4OH, Al(OH)3, Fe(OH)3, etc.
Applications (or advantages) of Arrhenius concept
The Arrhenius concepts has aided the understanding of the following:
i.
Acidic character of aqueous solutions of non-metallic oxides e.g,
CO2, SO2, SO3. etc. This is because these oxides can give off H+
ions in water. E.g.
ii.
Basic character of aqueous solutions of metallic oxides e.g., CaO,
Na2O etc. and compounds like NH3, N2H4. This is because these
oxides can give of OH- ions in water.
iii.
iv.
Acid-base neutralization reaction taking place in water.
The catalytic properties of acids in many reactions can be explained
by saying the H+ ions are available from the acids.
Limitations of Arrhenius concept
i.
It is applicable only to aqueous solutions. For the acidic or basic
properties, the presence of water is necessary. Dry HCl for example,
cannot act as an acid.
ii.
According to the concept, acid-base neutralisation reactions take
place only in water and hence cannot explain such reactions
occurring in other solvents or in the gas phase. E.g., the formation of
NH4Cl (s) by the combination of NH3 (g) and HCl(g) cannot be
explained by the Arrhenius concept.
iii.
The concept does not explain acidic or basic properties of acids and
bases in non-aqueous solvents like liquid NH3 or SO2
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