Acids, Bases and Salts

Acids, bases and salts find widespread occurrence in nature. Hydrochloric acid present in the gastric juice is secreted by the lining of our stomach in a significant amount of 1.2-1.5 L/day and is essential for digestive processes. Acetic acid is known to be the main constituent of vinegar. Lemon and orange juices contain citric and ascorbic acids, and tartaric acid is found in tamarind paste. As most of the acids taste sour, the word “acid” has been derived from a Latin word “acidus” meaning sour.

Acids are known to turn blue litmus paper into red and liberate dihydrogen on reacting with some metals. Similarly, bases are known to turn red litmus paper blue, taste bitter and feel soapy. When acids and bases are mixed in the right proportion they react with each other to give salts.

Ø Arrhenius concept of acid and bases:

According to Arrhenius theory, acids are substances that dissociates in water to give hydrogen ions and bases are substances that produce hydroxyl ions . The ionization of an acid can be represented by the following equations:

→ +

+ H₂O₍_l) → H₃ +

A bare proton, H⁺ is very reactive and cannot exist freely in aqueous solutions. Thus, it bonds to the oxygen atom of a solvent water molecule to give trigonal pyramidal hydronium ion, H₃O⁺ {[H (H₂O)]⁺} . In this chapter we shall use H⁺⁽^aq⁾ and H₃O^+(aq) interchangeably to mean the same i.e., a hydrated proton. Similarly, a base molecule like MOH ionizes in aqueous solution according to the equation:

MO → + O

The hydroxyl ion also exists in the hydrated form in the aqueous solution. Arrhenius concept of acid and base, however, suffers from the limitation of being applicable only to aqueous solutions and also, does not account for the basicity of substances like, ammonia which do not possess a hydroxyl group.

Ø Concept of Bronsted – Lowry for acid and base :

Danish chemist Bronsted and English chemist Lowry presented the concept of acid and base. They made H (Proton) as a base. According to their concept, the substance which gives a proton or donates a proton is called the acid and the substance which receives a proton or accepts a proton is called the base. Thus, acid is a proton donor and base is a proton acceptor. Let us take the dissociation reaction of hydrogen chloride in water.

We shall understand in detail in the first form the above reaction,

HCl	⇋	H⁺	+	Cl^-
Acid – 1		proton		conjugatebase-1

As it gives proton, HCI is an acid

H₂O⁺	+	H⁺	⇋	H₃O⁺
Base -2		proton		conjugate acids -2

As it accepts a proton, H₂0 is a base. Total reaction

HCl	+	H₂O	⇋	H₃O_(aq)	+	C
Acid -1		base-2		acid -2		base-1

Hence, it can be said that only transfer of proton takes place, it is not obtained free. Every acid will lose proton and so its conjugate base will be formed and every base will accept a proton and so its conjugate acid will be formed. Hence, this concept is known as proton transfer or conjugate acid-base concept.

NH₃	+	H₂O	⇋	N	+	O
Base-1		acid-2		base-2		acid-1

In the reaction, base NH₃ accepts a proton and forms conjugate acid NH₄⁺ ion and acid H₂0 loses proton and forms conjugate base OH^-.

Thus the concept of Bronsted - Lowry is found to be more applicable and acceptable than Arrhenius concept. Even then its limitations are also known. The difficulties are observed in the study of reactions in organic chemistry and complex salts. BF has no proton even then it acts as an acid. Hence, the third concept has come in to existence, which is known as Lewis acid-base concept. Proton is given importance in Bronsted-Lowry concept.

Ø Lewis concept of acids and bases:

G.N. Lewis in 1923 defined an acid as a species which accepts electron pair and base which donates an electron pair. As far as bases are concerned, there is not much difference between Brönsted-Lowry and Lewis concepts, as the base provides a lone pair in both the cases. However, in Lewis concept many acids do not have proton. A typical example is reaction of electron deficient species BF₃ with NH₃.

BF₃ does not have a proton but still acts as an acid and reacts with NH₃ by accepting its lone pair of electrons. The reaction can be represented by,

BF₃ + :NH₃ → BF₃:NH₃

Electron deficient species like AlCl₃, Co³⁺, Mg²⁺, etc. can act as Lewis acids while species like H₂O, NH₃ , OH^- etc. which can donate a pair of electrons, can act as Lewis bases.

Ø Ionisation of acid and base:

Arrhenius concept of acid and base is useful in understanding ionisation of acid and base in most of the chemical and biochemical reactions, ionisation is there in aqueous medium. Acids like HCIO₄, HCl, HNO₃, HBr, H₂SO₄, are called strong acids because they are completely ionised in aqueous solutions. Similarly bases like NaOH, KOH, Ba(OH)₂ , are strong bases because they are completely ionised in aqueous medium. The magnitude of ionisation determines the strength of acid or base. According to Bronsted - Lowry, the magnitude of accepting or donating a proton delides the strong or weak (HA) acid or base.

Let us take the following example:

HA₍_aq₎ + H₂O_(l) ⇋ H₃+

Thus, in above dissociation, equilibrium is attained and equilibrium is dynamic, that is the transfer of proton in forward and reverse reaction takes place continuously. If above reaction is more in forward reaction, then strength of acid will be more and if in reverse reaction, the strength of acid will be less and so their respective conjugate base will be weak and strong respectively.

Example: The conjugate bases of strong acids HCI, H₂SO₄, HNO₃ etc. namely CI^-,S and N will be weak bases. In the same way the conjugate acids of strong bases NaOH, KOH etc., namely Na⁺ and K⁺ etc. will be weak acids. Weak acid or base may not ionise completely and so equilibrium is obtained. Indicator like phenolphthalein is colourless in presence of acid and shows pink colour in presence of base.

Ø Ionic product of water:

Some substances like water are unique in their ability of acting both as an acid and a base. We have seen this in case of water. In presence of an acid, HA it accepts a proton and acts as the base while in the presence of a base, B^- it acts as an acid by donating a proton. In pure water, one H₂O molecule donates proton and acts as an acid and another water molecules accepts a proton and acts as a base at the same time.

	+		⇋		+	OH^-
acid		base		Conjugate acid		Conjugate base

The dissociation constant is represented by,

K = [H₃O⁺] [OH^-] / [H₂O]

The concentration of water is omitted from the denominator as water is a pure liquid and its concentration remains constant. [H₂] is incorporated within the equilibrium constant to give a new constant, K_w, which is called the ionic product of water.

The concentration of H⁺ has been found out experimentally as 1.0 × 10^-7 M at 298 K. And, as dissociation of water produces equal number of H⁺ and OH^- ions, the concentration of hydroxyl ions, the concentration of hydroxyl ions,

[OH^-] = [H⁺] = 1.0 × 10^-7 M.

Thus, the value of K_w at 298K

K_w = [H₃O⁺] [OH^-] = (1.0 × 10^-7)² = 1 × 10^-14 M².

The value of Kw is temperature dependent as it is an equilibrium constant.

The density of pure water is 1000 g / L and its molar mass is 18.0 g /mol. From this the molarity of pure water can be given as,

[H₂O] = (1000 g /L)(1 mol/18.0 g) = 55.55 M.

Therefore, the ratio of dissociated water to that of undissociated water can be given as:

10^-7 / (55.55) = 1.8 × 10^-9 or ˷ 2 in 10^-9

We can distinguish acidic, neutral and basic aqueous solutions by the relative values of the H₃O⁺ and OH^- concentrations:

Acidic: [H₃O⁺] > [OH^-]

Neutral: [H₃O] = [OH^-]

Basic: [H₃O⁺] < [OH^-]

Ø pH scale:

If we express the concentration of hydronium ion [H₃0⁺] in molarity then values like 10^-12 to 10^-2 are possible. Hence, scientist Sorensen found a scale which is called pH scale. According to him pH = -log₁₀ [H₃O⁺]. The values 10^-12 to 10^-2 shown above can be converted to +12 to +2 if calculated on the basis of this relation and plotting of graph can be easy.

The definition of pH can be given like this, "pH of a solution is the negative logarithm to the base 10, of the molar concentration of hydrogen or hydronium ion". According to thermodynamics, activity is more proper word instead of concentration but in dilute solutions activity and concentration can be considered to be the same. Now as seen earlier a solution containing 10^-7M [H₃0⁺] and [OH^-] is neutral. Hence,

pH = -log₁₀ [H₃O⁺] = -log₁₀10^-7 M = 7 and for acidic solution [H₃0⁺] > 10^-7 M, pH < 7 Similarly, for basic solution [H₃0⁺] < 10^-7 M, pH > 7, Hence, pH > 7. Therefore, it can be written as:

pH < acidic solution

pH > basic solution

pH = neutral solution

as seen below,

K_w = [H₃O⁺] [OH^-]

Putting the values, K_w =(10^-7)(10^-7)= (10^-14) and K_w = -)

∴ pK_w = 14

∴ pH + pOH = pK_w = 14

Temperature affects the value of pH, pOH, pK_w. The above discussion can be shown in the following table,

Concentration M	Acidic	Neutral	Basic
[H₃O⁺]	More than 10^-7	10^-7	Less than 10^-7
[OH^-]	Less than 10^-7	10^-7	More than 10^-7
pH	Less than 10⁷	10⁷	More than 10⁷
pOH	More than 10⁷	10⁷	More than 10⁷