Doc Brown's Advanced A Level Chemistry Revision Notes

Theoretical–Physical Advanced Level Chemistry – Equilibria – Chemical Equilibrium Revision Notes PART 5.1

5.1 Acid–Base Theory – Lewis & Bronsted–Lowry Theories

This page explains the Lewis theory of acids (electron pair acceptors) & bases (electron pair donors) and the Bronsted–Lowry theory of acids (proton donors) and bases (proton acceptors). The terms conjugate acid, conjugate base and conjugate base are also explained via fully described acid–base reactions.

Chemical Equilibrium Notes Parts 5 & 6 Index

 (I shouldn't use it, but M = old fashioned shorthand for mol dm–3 !)



5.1 Acid–base theory – Lewis and Bronsted–Lowry

  • Basic ideas on acids, bases and their reactions, pH scale, using indicators and simple acid–base theory are described on the GCSE notes pages and are essential reading before tackling parts 5 and 6 of these more advanced notes, and much of it is not repeated here.

    • These notes in Parts 5. and 6. involve a major upgrade from GCSE grade 9-1 notes on Acids, bases, salts, pH and neutralization, though these notes do describe the basic ideas on pH, examples of solution pH's, indicators and the reactions of acids with metals, soluble/insoluble oxides, hydroxides, carbonates, hydrogencarbonates and aqueous ammonia, salt preparations and introduction to pH titration curves, all this you should know.

  • Lewis acids and bases and the Bronsted–Lowry theory of acids and bases

    • Lewis acid–base electron pair theory

      • A base is an electron pair donor and an acid is an electron pair acceptor.

      • e.g. a non B–L, but a Lewis acid–base interaction is boron trifluoride (Lewis–acid, electron pair acceptor) reacting with ammonia (Lewis–base, electron pair donor).

        • F3B + :NH3 ==> F3B–NH3

        • Note: In organic chemistry mechanisms, nucleophiles are Lewis bases and electrophiles are Lewis acids and they may fit into the Bronsted–Lowry definition too e.g. protonation of alcohols and alkenes via acid.

        • In Transition Metal chemistry, ligands like water, can donate a pair of non–bonding electrons (lone pair) into a vacant orbital of a central metal ion and so dative covalent (co–ordinate) bonds hold a complex together.

        • The central metal ion with vacant bonding orbitals can act as a Lewis acid by accepting an electron pair to form a dative covalent bond.

        • Ligands act as Lewis bases by electron pair donation to form the metal–ligand co–ordinate bond.

    • 5.1.1: An acid is a proton donor and a base is a proton acceptor – Bronsted–Lowry acid–base theory

      • Bronsted–Lowry acids and bases are a 'sub–set' of the general Lewis acid–base theory, namely acids are electron pair acceptors and bases are electron pair donors.

      • All bases X:, will have a lone pair of non–bonding electrons that will except the electron deficient proton H+ to form a covalent X–H bond.

        • In general, a Lewis acid – Lewis base interaction involves the formation of a single dative covalent/co–ordinated bond where the bonding pair of electrons is donated by the base to the electron pair accepting acid.

        • The Bronsted–Lowry theory concentrates on proton donation and acceptance.

      • The oxonium ion, H3O+(aq) (or more simply, the aqueous hydrogen ion, H+) is formed by any acidic substance in water.

        • Increase in H+ concentration decreases the pH of a solution. (see section 5.2)

      • The hydroxide ion, OH(aq), is formed by any soluble base forming an alkaline solution.

        • Increase in OH concentration increases the pH of a solution. (see section 5.2)

      • Incidentally water is a neutral oxide because its pH is 7, logistically the oxonium/hydrated proton ion concentration equals the hydroxide ion concentration ...

        •  [H3O+(aq)] = [OH(aq)] via the tiny fraction of water molecules undergoing dissociation or self–ionisation because of the reaction

          • 2H2O(l) (c) doc b [H3O+(aq)] + [OH(aq)]

          • water <=> oxonium/hydrogen ion + hydroxide ion

        • BUT, in this reaction, water acts as both acid and base i.e. one water molecule (acid) donates a proton to another water molecule which becomes an oxonium ion (hydrated proton) and another water molecule (base) simultaneously accepts a proton!

        • Therefore water is an amphoteric oxide i.e. it reacts as both a proton acceptor and a proton donator.

          • e.g. water acting as a base – proton acceptor with a stronger acid like the hydrogen chloride gas

            •  HCl(g) + H2O(l) ==> H3O+(aq) + Cl(aq)

            • This is how hydrochloric acid is formed which you write simply as HCl.

          • e.g. water acting as an acid – proton donor with a weak BUT stronger base like ammonia gas

            • NH3(aq) + H2O(l) (c) doc b NH4+(aq) + OH(aq)

            • This is why ammonium solution is alkaline – sometimes wrongly called 'ammonium hydroxide' instead of aqueous ammonia.

        • More details on these reactions are given in subsequent sections on this web page.

    • 5.1.2: Examples of soluble substances giving aqueous solution acid–base interactions

      • 5.1.2a: Conc. sulphuric acid: H2SO4(l) + 2H2O(l) ==> 2H3O+(aq) + SO42–(aq)

        • Sulphuric acid, H2SO4, is the acidic proton donor and H2O is the proton accepting base.

        • Note the products are also acids and bases:

          • H3O+ is the conjugate acid of the base H2O

          • SO42– is the conjugate base of the acid H2SO4

          • The conjugate acid and original base or the conjugate base and the original acid are known as a conjugate pair and are related by proton transfer.

      • 5.1.2b: Hydrogen chloride gas: HCl(g) + H2O(l) ==> H3O+(aq) + Cl(aq)

        • HCl is the acid and Cl is the conjugate base.

        • H2O is the base and H3O+ is the conjugate acid.

        • The resulting solution is called hydrochloric acid.

      • 5.1.2c: Ammonia: NH3(aq) + H2O(l) (c) doc b NH4+(aq) + OH(aq)

        • Ammonia is the base and the ammonium ion, NH4+, is its conjugate acid,

        • and water is the acid and the hydroxide ion is its conjugate base.

      • 5.1.2d: The hydrogen carbonate ion, HCO3, can act as an acid with a base or act as a base with an acid, such behaviour is described as amphoteric.

        • HCO3 + H3O+(aq) ==> 2H2O(l) + CO2(aq)

          • HCO3 acting as a base, accepting a proton from an acid.

        • HCO3 + OH(aq) ==> H2O(l) + CO32–(aq)

          • HCO3 acting as an acid, donating a proton to the hydroxide ion base.

      • 5.1.2e: Since any soluble base gives hydroxide ions in aqueous and any soluble acid gives oxonium/hydrogen ions, they combine to form water. The  ionic equation for these neutralisations is:

        • H3O+(aq) + OH(aq) ==> 2H2O(l)

        • or more simply: H+(aq) + OH(aq) ==> H2O(l)

        • More reactions of H3O+/H+ are given in 5.1.4

    • 5.1.3: Acids can be described as monobasic, dibasic or tribasic etc. depending on the maximum number of protons that are available for transfer in an acid–base reaction. The terms mono/di/triprotic are used to mean the same thing, the term then applies to the maximum number of protons the final conjugate base can accept.

      • monobasic acids e.g.

        • hydrochloric HCl, nitric HNO3, ethanoic CH3COOH (the alkyl H's are not acidic),

        • giving the salts e.g. NaCl, NaNO3 and CH3COONa with sodium hydroxide respectively.

      • dibasic acids e.g. with sodium hydroxide

        • sulphuric H2SO4 ==> NaHSO4 ==> Na2SO4

        • ethanedioic (COOH)2 ==> HOOC–COONa ==> NaOOC–COONa or (COONa)2

        • and the three isomeric benzene–x,y–dicarboxylic acids (x,y = 1,1 1,2 and 1,3) C6H4(COOH)2,

      • tribasic acids e.g.

        • boric acid H3BO3, phosphoric(V) H3PO4,

        • citric acid (c) doc b, the middle–left hydrogen of the HO–C (alcohol) is not acidic in water,

        • the trisodium salt is formed with excess sodium hydroxide via the monosodium and disodium salts.

    • 5.1.4: Examples of water insoluble bases giving acid–base neutralization reactions.

      • 5.1.4a: Water insoluble copper(II) oxide dissolving in dil. sulphuric acid.

        • CuO(s) + H2SO4(aq) ==> CuSO4(aq) + H2O(l)

        • but omitting any non–changing/reacting 'spectator' ions the actual ionic reaction is

        • CuO(s) + 2H3O+(aq) ==> Cu2+(aq) + 3H2O(l)

          • or more simply: CuO(s) + 2H+(aq) ==> Cu2+(aq) + H2O(l)

          • where CuO is the insoluble base and H3O+ is the acid. Effectively, the oxide ion, O2–, acting as a base, gains two protons to form water.

        • Incidentally, a group 6, connection (O and S), copper(II) sulphide reacts with acids to form the copper(II) salt and hydrogen sulphide gas (hydrogen sulphide, rotten egg smell and very harmful, not just to our aesthetics!).

        • e.g. Copper(II) sulphide will dissolve in dil. hydrochloric acid to form a solution of copper(II) chloride and release hydrogen sulphide.

        • CuS(s) + 2HCl(aq) ==> CuCl2(aq) + H2S(g)

          • CuS(s) + 2H+(aq) ==> Cu2+(aq) + H2S(g)

          • where CuS is the insoluble base and HCl/H+/H3O+ is the acid. Effectively, the sulphide ion, S2–, acting as a base, gains two protons to form hydrogen sulfide.

      • 5.1.4b: Water insoluble calcium carbonate dissolving in hydrochloric acid.

        • CaCO3(s) + 2HCl(aq) ==> CaCl2(aq) + H2O(l) + CO2(g)

        • full ionic equation: CaCO3(s) + 2H3O+(aq) ==> Ca2+(aq) + 3H2O(l) + CO2(g)

        • or more simply: CaCO3(s) + 2H+(aq) ==> Ca2+(aq) + H2O(l) + CO2(g)

        • where CaCO3 is the insoluble base and HCl/H3O+ is the acid. Again, effectively, the carbonate ion, CO32–, acting as a base, gains two protons to form water and carbon dioxide.

    • 5.1.5: Examples of two solids reacting together in an acid–base reaction.

      • 5.1.5a: When solid ammonium salts are heated with solid calcium hydroxide (slaked lime) ammonia gas is produced.

        • 2NH4Cl(s) + Ca(OH)2(s) ==> CaCl2(s) + 2H2O(l) + 2NH3(g)

        • ionically: NH4+(s) + OH(s) ==> H2O(l) + NH3(g)

        • The ammonium ion is the acid and the hydroxide ion the base.

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