Solving Weak Acid Equilibrium Problems (continued)
Define change at equilibrium (as “x”).
Write equilibrium concentrations in terms of x.
Substitute equilibrium concentrations into equilibrium expression.
Solve for x the “easy way.”
Verify assumptions using 5% rule.
Calculate [H+] and pH.
It is useful to specify the amount of weak acid that has dissociated in achieving equilibrium in an aqueous solution. The percent dissociation is defined as follows:
For a given weak acid, the percent dissociation increases as the acid becomes more dilute.
Figure 14.10 The Effect of Dilution on the Percent Dissociation and (H+) of a Weak Acid Solution
Arrhenius concept: A base is a substance that produces OH- ions in aqueous solution.
Bronsted-Lowry concept: A base is a proton acceptor.
“Strong” and “weak” are used in the same sense for bases as for acids.
strong = complete dissociation (hydroxide ion supplied to solution)
NaOH(s) Na+(aq) + OH(aq)
weak = very little dissociation (or reaction with water)
H3CNH2(aq) + H2O(l) H3CNH3+(aq) + OH(aq)
H3CNH2 molecule accepts a proton and thus functions as a base. Water is the acid in this reaction. Methyl amine contains no hydroxide ion, it still increases the concentration of hydroxide ion to yield a basic solution.
. . . can furnish more than one proton (H+) to the solution. A polyprotic acid always dissociates in a stepwise manner, one proton at a time.
For a typical weak polyprotic acid,
Ka1 > Ka2 > Ka3
When a substance is dissolved in water, it produces an acidic solution if it can donate protons and produces a basic solution if it can accept protons.
Two factors for acidity in binary compounds:
Bond Polarity (high is good)
Bond Strength (low is good)
Figure 14.11 The Effect of the Number of Attached Oxygens on the O-H Bond in a Series of of Chlorine Oxyacids