Chapter 6 Acids and Bases Bronsted Lowry Model Bronsted Lowry acid a H ion donor Bronsted Lowry base a H ion acceptor Strong Acids Strong acid completely ionized in water HNO3 aq H2O l NO3 aq H3O aq H donor H acceptor Weak Acids Weak acid partially ionized in water HNO2 aq H2O l NO2 aq H3O aq H donor H acceptor Weak acid equilibrium between acid on left and ion on right HA aq H2O l A aq H3O aq Strong acids Ka 1 Weak acids Ka 1 Conjugate Acid Base Pairs Strong Bases Conjugate acid base pairs Differ from each other only by the presence or absence of a proton Strong Bases Oxides hydroxides of Group 1 2 metals Weak Bases Nitrogen containing compounds NH3 aq H2O l NH4 aq OH aq base conjugate acid Relative Strengths of Acids and Bases Leveling Effect H3O is the strongest H donor that can exist in water Strong acids all have the same strength in water are completely converted into H3O ions Water An Amphiprotic Substance Amphiprotic capable of acting as an acid or a base Water as a base HNO3 aq H2O l NO3 aq H3O aq H donor H acceptor Water as an acid NH3 aq H2O l NH4 aq OH aq H acceptor H donor Autoionization of Water The pH Scale pH log H pOH log OH Kw 1 x 10 14 H3O OH Log form of Kw log Kw log H OH 14 00 pH pOH Weak Acids HNO2 aq H2O l H3O aq NO2 aq To calculate pH of strong acid H Acid initial weak acid H calculated by solving equilibrium problem Ch 5 Sample Exercise pH Is Solution A with a pH of 9 58 more or less acidic than Solution B in which H 4 3 x 10 10 M 1 pH of Solution B pH log 4 3 x 10 10 9 37 9 37 2 H of Solution A 9 58 log H H 10 9 58 2 6 x 10 10 M Solution B is more acidic since it has a higher proton H ion concentration Sample Exercise More p functions Percent Ionization Degree of ionization quanity of substance t h an is ionized concentration of substance before ionization Percent ionization degree of ionization expressed as a percent ionization degreases as Acid 0 increases Sample Exercise Weak Acid Equilibria The pH of a 1 00 M solution of formic acid HCOOH a weak organic acid found in red ants and responsible for the sting of their bite is 1 88 a What is the percent ionization of 1 00 M HCOOH b What is the Ka value of the acid c What is the percent ionization of 0 0100 M HCOOH A the pH of the 1 00 M solution is 1 88 The corresponding H is H 10 1 88 1 32 x 10 2 M Inserting this value and the initial concentration of HCOOH into the percent ionization equation giv es B At equilibrium HCOO H 1 32 x 10 2 M and the equilibrium concentration of HCOOH is 1 00 1 32 x 10 2 M 0 99 M Inserting these values in the expression for Ka C To calculate H in 0 0100 M HCOOH we use an ICE table as in the equilibrium calculations in Chapter 5 to solve for H at equilibrium Since it is the unknown in the calculation we give it the symbol x Filling in the other cells in the ICE table Inserting the equilibrium terms into the Ka expression and using Ka from part b x Ka 0 0100 x 1 76 x 10 4 x and solving for x using the quadratic equation or an equation solver program x 1 24 x 10 3 The x value is equal to H at equilibrium Therefore the percent ionization of formic acid in a 0 0100 M solution is Weak Base Equilibria NH3 aq H2O l NH4 aq OH aq To calculate pH Obtain OH at equilibrium use Kw relationship pOH and the Calculation of pH for Bases To calculate pH of base solutions Obtain OH at equilibrium Kw H3O OH H3O Kw OH Or pOH log OH pKw 14 00 pOH pH Sample Exercise Weak Base Equilibria The concentration of NH3 in household ammonia ranges between 50 and 100 g L or from about 3 M to almost 6 M What is the pH of a 3 0 M solution of NH3 Taking the negative logarithm of OH to calculate pOH pOH log OH log 7 3 x 10 3 M 2 14 Then we subtract this value from 14 00 to obtain the pH pH 14 00 pOH 14 00 2 14 11 86 Polyprotic Acids Monoprotic Acids One ionizable H atom per molecule Polyprotic Acids More than one ionizable H atom molecule Diprotic acids two ionizable H atoms Triprotic acids three ionizable H atoms Diprotic Acids Sulfuric Acid H2SO4 aq H aq HSO4 aq Ka1 1 HSO4 aq H aq SO42 aq Ka2 1 2 x 10 2 Carbonic Acid H2CO3 aq H aq HCO3 aq Ka1 4 3 x 10 7 2 HCO3 aq H aq CO3 aq Ka2 4 7 x 10 11 Polyprotic Acids Ka1 Ka2 Ka3 More difficult to remove H ion positive charge from negatively charged anion pH of Polyprotic acid solutions Typically only first ionizable H atom affects pH Ka1 Calculations are identical to those involving the pH of a weak acid Factors Affecting Acid Strength Number of Oxygen Atoms in Anion The greater the number of oxygens the stronger the acid Structure Electronegativity Acid Base Properties of Salts Weak Acid HF aq H2O l F aq H3O aq acid conj base Salt NaF NaF s Na aq F aq F a base F aq H2O l HF aq OH aq base conj acid pH of Salt Solutions NaF aq 1 Write balanced equilibrium equation for basic anion or acidic cation F aq H2O l HF aq OH aq 2 Determine Ka or Kb for equilibrium 3 Solve equilibrium problem for H OH Relationship between Ka and Kb Sample Exercise pH of a Salt Solution