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