Chapter 16 11

Strong Acids• The strongest common acids are HCl, HBr, HI, HNO3,

HClO3, HClO4, and H2SO4

• Strong electrolytes• Ionize completely in solution:

HNO3(aq) + H2O(l) H3O+(aq) + NO3-(aq)

• Remember, H+ and H3O+ are used interchangeably

16.5: Strong Acids and Bases

Chapter 16 21

• In solutions the strong acid is usually the only source of H+

• If the molarity of the acid is less than 10-6 M then the autoionization of water needs to be taken into account

• Therefore, the pH of the solution is the initial molarity of the acid

Chapter 16 31

Strong Bases• Most soluble ionic hydroxides are strong bases

• Group 1 hydroxides, the heavy group 2 metal hydroxides• Strong electrolytes, dissociate completely

• The pOH (and hence pH) of a strong base is given by the initial molarity of the base

• (stoichiometry!)

• Bases do not have to contain the OH- ion:O2-(aq) + H2O(l) 2OH-(aq)

H-(aq) + H2O(l) H2(g) + OH-(aq)N3

-(aq) + H2O(l) NH3(aq) + 3OH-(aq)Bases are proton acceptors!

Chapter 16 41

• Weak acids are only partially ionized in solution• There is a mixture of ions and unionized acid in solution

• Therefore, weak acids are in equilibrium:

16.6: Weak Acids

HA(aq) + H2O(l) H3O+(aq) + A-(aq)

HA(aq) H+(aq) + A-(aq)]HA[

]A][OH[ -3

aK

]HA[]A][H[ -

aK

Chapter 16 51

• Ka is the acid dissociation constant

• The larger the Ka the stronger the acid• more ions are present at equilibrium

relative to unionized molecules

• If Ka >> 1, then the acid is completely ionized and the acid is a strong acid

Text, P. 628

Chapter 16 61

Calculating Ka from pH

• The pH gives the equilibrium concentration of H+

Chapter 16 71

• Using Ka, the concentration of H+ (and hence the pH) can be calculated

– Write the balanced chemical equation (equilibrium)– Write the equilibrium expression. Find the value for Ka

– Set up the ICE table– assume that the change in concentration of H+ is x

– Substitute into the Ka expression and solve, change to pH if necessary• Use the quadratic equation OR • Assume x is very small compared to the initial concentration and drop it from the

equilibrium concentration expression (check S.F. and x)• If x is more than 5% of the initial value, it is better to use the quadratic formula

Chapter 16 81

• Remember: weak acids are partially ionized

• [H+](aq) is only a fraction of the concentration of the acid solution

• Physical properties of the acid solution reflect this:• Poor conductors of current• React slowly with metals

Chapter 16 91

Using Ka to Calculate pH• Percent ionization is another method to assess acid

strength• For the reaction

HA(aq) + H2O(l) H3O+(aq) + A-(aq)

100]HA[]OH[

ionization %0

3 eqm

Chapter 16 101

• Percent ionization relates the equilibrium H+ concentration, [H+]eqm, to the initial HA concentration, [HA]0

• The higher percent ionization, the stronger the acid

• Percent ionization of a weak acid decreases as the molarity of the solution increases• For acetic acid, a 0.15 M solution is 1.0 % ionized whereas a 0.05 M solution is 2.0 %

ionized • For dilution, remember LeChâtelier:

• The reaction will shift in the direction of the larger # of particles• Counters the effect of the decreasing concentration of the particles (more

molecules are ionized)

HA(aq) + H2O(l) H3O+(aq) + A-(aq)

Text, P. 633

Chapter 16 121

Polyprotic Acids• Polyprotic acids have more than one ionizable proton• The protons are removed in steps:

• It is always easier to remove the first proton in a polyprotic acid than the second• Therefore, Ka1 > Ka2 > Ka3 etc.

• As long as successive Ka values differ by a factor of 103 or more, pH can be determined by considering only Ka1

H2SO3(aq) H+(aq) + HSO3-(aq) Ka1 = 1.7 x 10-2

HSO3-(aq) H+(aq) + SO3

2-(aq) Ka2 = 6.4 x 10-8

Chapter 16 131

Text, P. 635