CONCEPTS OF ACIDS AND BASES - PUNJAB-CURRICULUM-AND-TEXTBOOK-BOARD-PCTB / 10TH-CLASS / CHEMISTRY / CHEMISTRY-10TH-CLASS-NBF-2024 / ACID-BASES-AND-SALTS / CONCEPTS-OF-ACIDS-AND-BASES

10.1 Concepts of Acids and Bases

Acids and bases are recognized by their characteristic properties, such as:

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Acids	Bases
1. Acids have sour taste. For example, unripe citrus fruits or lemon juice.	1. Bases have bitter taste and feel slippery, for example, soap is slippery to touch.
2. They turn blue litmus red.	2. They turn red litmus blue.
3. They are corrosive in concentrated form.	3. They are non-corrosive except concentrated forms of NaOH and KOH.
4. Their aqueous solutions conduct electric current	4. Their aqueous solutions conduct electric current.

10.1.1 Arrhenius Concept of Acids and Bases

According to Arrhenius concept (1787):

Acid is a substance which dissociates in aqueous solution to give hydrogen ions. In general, the ionization of acids take place as follows.

H A_{(a q)} \overset{w a t e r}{\to} H_{(a q)}^{+} + A_{(a q)}^{-}

For example, substances such as $H C l$ ${H N O}_{3}$ ${C H}_{3}$ $C O O H$ $H C N$ etc., are acids because they ionize in aqueous solutions to provide $H^{+}$ ions.

H C l_{(a q)} \overset{w a t e r}{\to} H_{(a q)}^{+} + C l_{(a q)}^{-}

\mathrm{HNO_{3(aq)}}\xrightarrow{\mathrm{water}}\mathrm{H^{+}_{(aq)}}+\mathrm{NO_{3}^{-}_{(aq)}}

C H_{3} C O O H_{(a q)} \overset{w a t e r}{\to} C H_{3} C O O_{(a q)}^{-} + H_{(a q)}^{+}

On the other hand, base is a substance which dissociates in aqueous solution to give hydroxide ions The general ionization of bases take place as follows;

{M O H}_{(a q)} \overset{w a t e r}{\to} M_{(a q)}^{+} + {O H}_{(a q)}^{-}

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Figure 10.1: Ionization of bases in water.

Diagram image pending upload by Admin.

The substances such as NaOH, KOH, $N H_{4} O H$ , $C a (O H)_{2}$ etc. are bases because these compounds ionize in aqueous solutions to provide ${O H}^{-}$ ions

N a O H_{(a q)} \overset{w a t e r}{\to} N a_{(a q)}^{+} + O H_{(a q)}^{-}

K O H_{(a q)} \overset{w a t e r}{\to} K_{(a q)}^{+} + O H_{(a q)}^{-}

<span class="katex-error" title="ParseError: KaTeX parse error: Double subscript at position 65: …}\mathrm{NH_4^+_̲{(aq)}}+\mathrm…" style="color:#cc0000">\mathrm{NH_4OH_{(aq)}}\xrightarrow{\mathrm{water}}\mathrm{NH_4^+_{(aq)}}+\mathrm{OH^-_{(aq)}}

Thus, according to Arrhenius Concept:

Acids give $H^{+}$ ions in water, bases give OH ions in water.

Examples of some important acids and bases are given in Table 10.1.

📊 Data TableScroll ↔

Table 10.2 Acids and Bases
Acids	Bases
Hydrochloric acid, HCl	Sodium hydroxide, NaOH
Nitric acid, HNO3	Potassium hydroxide, KOH
Sulphuric acid, H2SO4	Calcium hydroxide, Ca(OH)2
Phosphoric acid, H3PO4	Aluminium hydroxide, Al(OH)3

Limitations of Arrhenius Concept

Although this concept has limited scope yet, it led to the development of more general theories of acid- base behaviour.

10.1.2 Bronsted-Lowry Concept

In 1923, the Danish chemist Bronsted and the English chemist Lowry independently presented their theories of acids and bases on the basis of proton- transfer. According to this concept:

An acid is a substance (molecule or ion) that can donate a proton $(H^{+})$ to another substance. A base is a substance that can accept a proton $(H^{+})$ from another substance. For example, HCl acts as an acid while ${N H}_{3}$ acts as a base:

<span class="katex-error" title="ParseError: KaTeX parse error: Double subscript at position 35: … + \mathrm{NH_3_̲{(aq)}}\rightle…" style="color:#cc0000">\mathrm{HCl_{(aq)}} + \mathrm{NH_3_{(aq)}}\rightleftharpoons \mathrm{NH_4^+_{(aq)}} + \mathrm{Cl^-_{(aq)}}

It is a reversible reaction. In the forward reaction, HCl is an acid as it donates a proton, whereas $\mathsf{H}_{2}\mathsf{O}$ is a base as it accepts a proton. In the reverse reaction, $\mathsf{Cl}^{- }$ ion is a base as it accepts a proton from acid $\mathsf{H}_{2}\mathsf{O}^{+}$ ion. $\mathsf{Cl}^{- }$ ion is called a conjugate base of acid HCl and $\mathsf{H}_{2}\mathsf{O}^{+}$ ion is called a conjugate acid of base $\mathsf{H}_{2}\mathsf{O}$ . It means every acid produces a conjugate base and every base produces a conjugate acid such that there is conjugate acid- base pair. Conjugate means joined together as a pair.

A conjugate acid is a specie formed by accepting a proton by a base. A conjugate base is a specie formed by donating a proton by an acid.

Thus, conjugate acid- base pair differs from one another only by a single proton. Similarly

C H_{3} C O O H_{(a q)} + H_{2} O_{(a q)} ⇌ C H_{3} C O O_{(a q)}^{-} + H_{3} O_{(a q)}^{+}

According to Bronsted- Lowry concept, an acid and a base always work together to transfer a proton. That means, a substance can act as an acid (proton donor) only when another substance simultaneously behaves as a base (proton acceptor). Hence, a substance can act as an acid as well as a base, depending upon the nature of the other substance. For example, $\mathsf{H}_{2}\mathsf{O}$ acts as a base when it reacts with HCl as stated above and as an acid when it reacts with ammonia such as:

<span class="katex-error" title="ParseError: KaTeX parse error: Double subscript at position 35: … + \mathrm{NH_3_̲{(aq)}}\rightle…" style="color:#cc0000">\mathrm{H_2O_{(l)}} + \mathrm{NH_3_{(aq)}}\rightleftharpoons \mathrm{NH_4^+_{(aq)}} + \mathrm{OH^-_{(aq)}}

Such a substance that can behave as an acid, as well as, a base is called amphoteric.

It has been observed that there are certain substances which behave as acids though they do not have the ability to donate a proton, e.g., $\mathsf{SO}_{3}$ . Similarly, CaO behaves as a base but it cannot accept a proton. These observations prove the limitations of Bronsted- Lowry concept of acids and bases.

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Figure 10.2: Bronsted-Lowry acid-base concept.

Diagram image pending upload by Admin.

All Arrhenius acids are Bronsted- Lowry acids, but except OH other Bronsted- Lowry bases are not Arrhenius bases

📊 Data TableScroll ↔

Table 10.3 Conjugate acid-base pairs of common species
Acid	Base	Conjugate acid
HNO3(aq)	H2O(l)	H3O+(aq)
H2SO4(aq)	H2O(l)	H3O+(aq)
HCN(aq)	H2O(l)	H3O+(aq)
CH3COOH(aq)	H2O(l)	H3O+(aq)
H2O(l)	NH3(aq)	NH4+(aq)
H2O(l)	CO32-(aq)	HCO3-(aq)
HCl(l)	HCO3-(aq)	H2CO3(aq)