Acid base theoriesArrhenius TheoryBrönsted – Lowry TheoryLewis acid – base Theory
Acid Base behaviourIn this unit when ever we discuss acids andbases we study the behaviour of compoundswhen dissolved in waterThe solvent is all the time Water
Arrhenius TheoryAcid - any substance that increases thehydronium (H3O+) ion concentration inneutral water. (or H+ ions for simplicity)Base - any substance that increases thehydroxide (OH-) ion concentration in neutralwater.The following pages shows some examplesof acids and bases
Strong Acids = Ka is very largePerchloric acid HClO4Hydroiodic acid HIHydrobromic acid HBrSulfuric acid H2SO4Hydrochloric acid HClNitric acid HNO3Memorise these examples
Weak Acid = Ka is very small at25oCSulfurous acid H2SO3 Ka = 1.2 x 10 -2Hydrogen sulphate ion HSO4- Ka = 1.0 x 10-2Phosphoric acid H3PO4 Ka = 7.1 x 10-4Citric acid H3C6H5O7 Ka = 7.1 x 10-4Nitrous acid HNO2 Ka = 7.1 x 10-4Hydrofluoric acid HF Ka = 6.8 x 10-4Formic Acid HCO2H Ka = 1.8 x 10-4
Weak Acids continued at 25oCAcetic acid HC2H3O3 Ka = 1.8 x 10-5Carbonic acid H2CO3 Ka = 4.5 x 10-7Hydrogen cyanide HCN Ka = 6.2 x 10-10Ammonium ion NH4+ Ka = 5.7 x 10-10Bicarbonate ion HCO3- Ka = 4.7 x 10-11Water H2O Ka = 1.8 x 10-16
Brönsted Lowry TheoryAn acid is a proton donorA base is a proto acceptorAn amphoteric substance is one that can actboth as an acid and a base.Amphoteric substances are proton acceptorsor donors eg. H2O, HCO3-, etc.
pKa and pKbThese expressions are analogous to theexpression for pH. We derived it to expressthe concentrations in a more convenientway using whole numbers.pKa = -log KapKb = -log KbSmaller value of pKa , stronger the acidSmaller value of pKb , stronger the base.
Ka and Kb for Conjugate acid –base pairFor a conjugate acid base pair this isapplicable.Ka x Kb = KwHA H+ + A-A- + H2O HA + OH-
Calculations involving KaCalculate the value of Ka for a specific acidfrom the [H+] or pH of a solution for whichwe also know the initial concentration of theacidCalculate the equilibrium concentrations ofH+ and A- from the initial concentration of aspecific weak acid and its Ka value
Calculations involving KbCalculate the value of Ka for a specific acidfrom the [OH-] or pOH of a solution for whichwe also know the initial concentration of thebase or from pH as pOH = 14 - pHCalculate the equilibrium concentrations of[H+ ]or [OH-]from the initial concentration ofa specific weak base and its Kb value (Orwhat is almost the same is calculating thepH or pOH from the values of Kb and initialconcentration of base)
Hydrolysis of IonsThere are many ionic salt’s that make theaqueous solution acidic or basic.The reaction of ionic salts with water iscalled “Hydrolysis of salts”Hydrolysis can be explained using BrönstedLowry theory.If the conjugate acid or base is strong thenthere will be hydrolysis.Weak acids and bases produce strong
Illustration of HydrolysisA- is a strong conjugate baseA- (aq)+ H2O = HA(aq) + OH-The dissociation constant for the hydrolysisin this case is Kb as one of the products is abase.
Hydrolysis of strong conjugateacidNH4+ + H2O = H3O+ + NH3NH4+ is a strong conjugate acid thereforeundergoes hydrolysis and the solutionbecomes more acidic or pH is lowered.Other positive metal ions that can undergohydrolysis is Fe3+, Al3+ etc.………………
Titration of weak acid Vs baseAcetic acid Vs NaOH
Polyprotic acidFor any diprotic acid the value of Ka1 > Ka2It is easier to pull a H+ from a neutral moleculethan an ionKa1 is usually 104 to 105 times larger than Ka2.Therefore the contribution of acidity due to thesecond or third proton is negligible and can beignored.
Calculating Ka from pH #1Formic acid HCHO2 is a monoprotic acid. In a0.100 M solution of formic acid, the pH is2.38 at 25oC. Calculate Ka for formic acid atthis temperature.Ans: Ka = 1.8x10-4
#1Determine the [H+] ion concentration frompHMake Ice table sand substitute theconcentration of H+ in the table andcomplete it to get the equilibriumconcentration of HA, H+ and A-Use the values to determine the Ka for theacid.
Calculating [H+], pH from Kafor weak acid #2The concentration of vinegar HC2H3O2 wasfound to be 0.75 M acetic acid, Calculate thevalue for the [H+] ion concentration and pHfor this acid solution if Ka for acetic acid is1.8 x 10-5
#2Create an ice table initial concentration is0.75 M for acetic acid. Change inconcentration is –x,x and x respectively forHA, H+ and A-Since Ka is very small 0.75 –x can be takenas 0.75 and solve for x.X is the concentration of H+ use this valueto calculate pH