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Bronsted, Lowry, , , , , INTRODUCTION defined according, er times, acids and , ben acids were those, to their — nt a For ad vorraded he eas, a were substances, which had bitter - wera, neutralised the acids. A little later, It hehe po ieee, Fdentify acids and bases with the help of litmus p: per, tumed blue litmus into red and the base tumed red It oa, blue. Thus, different definitions were proposed for act ‘ ‘, Fases as time passed by. Every theory or definition of acid an, base had some limitations or drawbacks. To overcome. the, limitation of a theory, a new theory was proposed which was, broader than the earlier one. In this chapter, we shall review, some theories of acid and base., , Inearli, , , , , , 7.2. ARRHENIUS CONCEPT OF ACIDS, AND BASES, , Svante Arrhenius, as a part of his theory of ionisation, proposed a, theory of acids and bases. According to Arrhenius theory:, - An acid is a substance that dissociates in aqueous jon to give, , drogen ions. Thus, HC1, HNO,, H,SO. CH,CO, s ++ tINO;, H,SO,, OH and HCN, acids because they liberate H* ions when dissolved im woe _ “i, , ACL ==, , Hi+cr, — HNO, ==, , “HSo, ==, , Ab; @.substance that disco; :, hydroxyl ions. Thus, NaH aseciates ma, when dissolved in water.
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acios AND BASES 265, dronium Ton (H 30°), , ‘According to Arrhenius, an acid releases H' ions in aqueous medium. However, it has been, found that H¢ ion cannot exist independently., , . % a . eh, In fact, H” ion is unique among tons. It has no electron. Its radius is equal to that of nucleus, , jo 8 cm) The concentration of charge in such a small volume makes the attraction between water, , ( og SoTeORY : — Dear, and H’ ions éxiremely large. H' ion accepts a pair of electrons from water molecules (H —O— HJ to, orm hydrated p n or hydronium ion. . — " a, , ~ Ca, , Hy’, , H+ H-O-H — H,0* or (Hydronium ion), a H, , Dissociation of hydrocloric acid in aqueous solution may be represented as:, ~ HCI+H,O == H,0* + Cr, , Limitations of Arrhenius Theory to explain acid-base concept, , 1. Limited scope. Arrhenius concept is limites medium only. It fails to explain the, behaviour of acids and bases in non-aqueous solvents like ammonia, si dioxide, alcohol, etc., , 2, Proton in aqueous solutions. According to Arrhenius concept acids are those which furnish, H ions in water. H” -ion is nothing but a proton, which is not capable of independent existence. AS, afeady discussed it may exist as hydronium Toh, i.e. HO"., , _¥” Acids and bases without H* and OH ions. There are a number of substances (like CO,, $05, NO;, SO; etc.) which do not contain Hrion, but behave as acids. Similarly, compounds like, a0, MgO, anhydrous NH3, Na,CO, ete. which do not contain OHT ions, behave as bases., , A. Neutralisation without H¢ and OH ions. Neutralization between HCI(g) and NH; (g) does, not involve H* ions and OH ionse.g. :, , —— NH; (g) + HCl(g) > NH, Cl(s), “CaO (s)+ SO3(g) > CaSO;(s), , 73. BRONSTED-LOWRY CONCEPT OF ACID AND BASE, , JN. Bronsted and J.M. Lowry indenendently and simultaneously (1923) put forward a more, , general concept for acids and bases. According to this concept :, , (i) Acid is a substance (molecule or ion) which has a tendency to donate a proton to any, , substance, an, (ii) A base is a substance (molecule or ion) which has a tendency to accept a proton from any., , other substance., In other words, an acid_is.g proton donor and base is a proton acceptor. In fact, acid-base, , reaction involves transference of proton. That is why, this concept is also called proton transfer, theory of acids and bases. In accordance with this broader definition, an acid or base may bemmotecutar— ~, orionieg, ’, , Molecular acids . HF, HC1, HNO, H,SO,, CH;COOH, HS, H;PO,, H,0, etc., , _ Tonic acids. :_H,O*, NHy» HSO,, HSO;, HPO? etc.
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Molecular bases :, RNH,, NH;, H,O, R,NH etc., Tonic bases: * ,, , OH, S*, CO}, Cl, Br, NO3 , ete., , It is important to note that no single, substance is an acid or a base, A single, , substance cannot donate.a proton unless and, until some other substance which accepts the, Froth yal proses present. This proton transfer, , may or may not involve any medium, ¢.g., J.N. Bronsted, , the reaction does not involve any medium. —, Here, HC1 has denoted a proton to NH3. Hence, HC1 is an acid with respect to NH3, which is a base, , NH. HC NH; C1(), ne) + lp > 4 Ss, , 8 tas, ons!, , am some more acid-base reactions., , NH, + H,O0 == NH, + OH, , Base Acid, , CH,COOH + NH; == CH,COO + NH;, Acid Base, , HCl + H,O == H,O* + Cr, , Acid Base, , , , JM. Lovdy, , , , H,SO,+H,0 == H,0* + HSOg, , . Acid Base, careful study of the above equations reveals that water is an acid w.r.t. NH; whereas it isabo*, w.r.t. HC] or HSO,. Such substances like H,O, HCO; , HSO, etc. which can act as acids (prow?, , donor) as well as bases (proton acceptor) are called amphoteric or amphiprotic substances., , : + wil] have!, Conjugate Acid-Base pairs. When an acid loses a proton, the residual part of it will hast, , tendency to accept a proton. Thus, it will ehave as a base. Such pairs of substances whichdiffet ee, encency to accept a proto: ean rae, eee ‘ ral exam, , one another by a proton are known as conjugate acid-base pairs. Consider a gene!, a cece tenets ‘, , acid :, , Bronsted Lowry Theory, , ple oe, , WHA == _f, Acid Proton Conjugate base, Consider the reaction between HC1 and H,O., -HCI+H,O == H,O*+Cr, Acid Base Acid Base, HCI donates a proton to water in the forward reaction., fi HC1 is an acid dnd H,O-is a base., In the backward reaction, H,O* ion donates its proton to Cl ion., : H,O* is an Acid and, Cl is a base., , It is clear that HC1 loses a proton and forms Cl- ion (which is a base). HCl and, called conjugate acid-base pair., , cr ions #, , HCL == Ht + cl, Acid, , —_—, , , , , , , , Conjugate base
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similarly, H,O (base) accepts a proton and forms H,, gate base-acid pair., H,O+H* == H,0*, Oa, Base Conjugate acid a ;, Inthe above example, consider HC] as acid-1 and CI as base-1. Similarly, consider H,O as, , pase-2 and H;O* as acid-2. The overall reaction between HC1 and H,O, can, hence, be written, , as:, , O* (acid). Here, H,O and H,O* are also, , con), , ~Ha + HO = — 4H,0* + Cr, ~ io, Acid-1_— Base-2 Acid-2 Base-1, , Some other examples of acid base reactions and conjugate acid- base pairs are given in the, following table 7.1. ;, , , , , , , , , , = Table: 7.1. Conjugate acid-base pairs., Acid-1 Base-2 Acid-2 Base-1, H,SO, + | H,O == H,0* + HSOj, H,0 + | NH, aaa NH; + OHCH,COOH | + | H,0 = .} Hot} 4+ CH,COO| ek, | Hs + | HO = H,0* + HS| A _ wiry, | H,0 + | HCO; ~S H,CO,} + H,0, | #30 + | ClO; = HCIO,} + H,0, L_H,P0, + | HO = H,0t + H,PO;, , , , , , , , , , , , , , , , , , All Bronsted bases may not be Arrhenius bases., , According to Arrhenius, a base must furnish OH ions in aqueous solution. Acco; di, = : Tding to, , xample carbonate ion, co?, According to Bronsted theory, CO3” ion is a base as it acce;, Pts a proton to fe, ee form ~, _ ee HCO;, , ene,, , , , , , , , , Bronsjeq i, “nsted theory, a base must accept a proton. Consider for e, , ‘on,, , = CO} +H* + HCO;, , a According to Arrhenius concept, CO is not a base as it does not furnj, “Won. Hence, all Arrhenius acids are Bronsted acids but all Bronsted ba ish OH i in ag, SES are 1 Ueous, ot, , a ‘i val 4, 4 LUX-FLOOD THEORY OF ACIDS AND p ASES enius bases, , rag Concept was proposed by Lux and later extended by Flo, 0, , on in terms of n_and is mainly se lood, It Visuay:, ‘ting wt camel a aid a bend bibs Olt oxi ce as the ac, i wee which danates the oxide fan. 4a other words, cepts the oxi hi em, oxide ion-donor. For example, in the reaction Acid is gy Ox] ide ion and Peratures, CaO + S10) — Og, ide lOmaccenjn: 88 is g, Buse Acid aSio,, Or and, , an
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Sui : action., le bi ide i acid. imilarly, in the re:, i ide is a base and silicon dioxide is an acid. S, calcium oxide is Y;, , << ppeo+SO, —> PbSO,, PbO is a base while SO, is an acid., , «, , This concept, has only a limited scope., , 7.5. LEWIS CONCEPT OF ACID AND BASE, , According to Lewis, an acid is defined as any molecule, on or, radical that can accept one or more electron pairs, that is, an acid is a ', electron Pair acceptor. In other words, a substance which has vacan, orbitals to accommodate one or itiore" pairs of electrons can act as a Lewis, acid. A base is defined as a species that can donaie one or more electron, Pairs, i.e., a base is an electron pair donor. Consequently any substance, , ~~with unshared Pair of electrons can behave as a Lewis base. Acid base or, neutralisation reactions may, therefore, be termed as donor-;, , , , , , , , , , , , , , , , , , , , acceptor, Teactions or coordinate covalent bond formation reactions as shown, below:, S. No. Acceptor Donor Donor-acceptor compound, (cia) j (Base) Neutralization Product, 1. BF, NH, Baa, 2. SnC1, 2R,0, (R,0), Snci, 3. Ht NH, NE, 4. FeCl, oe ec1,5. so, Q?$0,2Different kinds of Lewis acids Oe a, , These can be classified into the following two catego:, , (} All the cations are Lewis acids becay an Rories:, cations of alkali and alkaline earth metals behave acai ave tende,, tendency to accept electrons. Smaller t, , srs, Large!, t ative ney to accept electron Pit jess!, cro} Cations like q+ 7 WY Weaker Lewis-acid due to i, Lewis acids due to their higher tendenc : mh, , tro, ¥ W0 acce “~ _ &tc., are likely to act as st?, ahi Pte ©Ctrons al, , charged cations have a higher tendency to acce nd f, , a given metal. For example, Fe** behaves 8 @ stronger lativ, , igh, , Pt clecty, ‘Orm stronger bonds. so, , Ber L <-£0 the lower charged Csi, , ~ Sn*" is a stronger Lewis acid than Sn?*. Examples of som, | acid as compared ig. Fer Sane, below: ME cations acting as Lewis acids, , i a, S. No. Lewis Acid Lea, , , , , , , , fl, 1 Ag, Cu2* . ae, 2 aor : ', 3 4 H,0, Ni2*, 4, , , , , , 4cCy
