Subject: Some aspects about acids and bases

This was sent in response to questions from several students.

Remember the following:

        stronger acid => weaker conj base

        stronger base => weaker conj acid


You can see this by looking at the relationship for Ka and Kb for a 
conj acid/base pair:

                Ka * Kb = Kw

A strong acid has a large Ka so it's conj base will have a small Kb (and
be weaker).

In order to know what acids are weak acids and what bases are weak bases
you need to know which are considered strong (ionize or dissociate
completely).  The most common strong acids and bases (and a few other
less common strong bases) are listed in chapter 4, section 16.5, class notes,
homework solutions and some are discussed in this e-mail and follow-up
e-mail.  You MUST know these.  If you see any other acid or base than one
of these strong ones it will be a weak acid or base (unless I specifically
say otherwise in the problem).

The 7 common strong acids are:
HCl, HBr, HI, HNO3, HClO3, HClO4 and H2SO4 (1st proton only).

For a strong acid, such as HCl, the Ka is huge (so large in fact we
don't even list it and say the reaction goes to completion - only the
products, H+ and Cl- exist).  This means the conj base of HCl, the Cl-
anion, is an extremely weak base and in fact does not act as a base
in water (gives a neutral solution).

If we look at a weak acid, such as HF, it's Ka is much smaller and thus
the Kb for it's conj base will be larger (relatively speaking to something
like Cl-) and it's conj base will be a weak base.  The weaker the weak
acid the stronger it's conj base.  A very weak acid with a very small
Ka (let's say 1 x 10^-10) will have a realtively "strong" weak conj.
base (with a Kb of 1 x 10^-4).  So F- is a weak base and has a stronger
tendency to attract an H+ from H2O molecules and produce HF and OH- and
make the solution basic, whereas the Cl- ion won't do this (because it is
the conj base of a strong acid).

Remember what we mean by a weak acid or weak base.  They simply do not
ionize completely whereas a strong acid or base does.  That means for
a solution of 0.1 M HCl compared to 0.1 M HF the HCl solution will contain
more H+ and be more acidic.  The HF solution has less H+ and has some
molecular HF still remaining in solution while in the HCl solution no
molecules of HCl exist (it has all ionized to give only H+ and Cl-).

The same can be said for a weak base.  A strong base such as NaOH completely
dissociates or ionizes and none of the base exists in solution (it is all
converted to Na+ and OH- in this case).  Something like the H- anion
(from something like NaH) reacts completely with H2O to produce OH- so
0.1 M H- will produce 0.1 M OH- ions in solution.

Group 1A and 2A hydroxides are the most common strong bases.  Group
1A hydroxides are fairly easy to deal with since there's 1 OH- per
formula unit (as noted above).  Thus a 0.1 M NaOH solution is 0.1M in OH-.
Group 2A is a little trickier.  A 0.1 M Ca(OH)2 is 0.2 M in OH- (since
there are 2 OH- per formula unit).  There are a few other less common
strong bases, such as H-, O^2-, S^2-, NH2^-, NH^2-, N^3-.
CH3- (conj. bases of hydrocarbons), CH3O- (conj. bases of alcohols) that
you might encounter.

For the weak base NH3 it does not completely react with H2O. Some NH3
remains at equilibrium (actually it is mostly in the form of NH3 at
equilibrium).  So a solution of 0.1 M NH3 will not give a solution with
0.1 M OH- (it will be less, ~ 1.3 x 10^-3 M OH- and 0.0986 M NH3 remaining).
The conj acid of this weak base (NH4+) will act as a weak acid in water
and produce H+ (Ka is 5.6x10^-10) when added to water.

Thus the conj base of a weak acid will itself be a weak base and will act
as a base in water.  The weaker the acid the stronger the conj base and
vice versa.

The conj bases of one of the strong acids are so weak they will not behave as
bases to make the solution basic.  When in solution they do nothing and
give a neutral solution (Cl-, Br-, I-, NO3-, ClO3-, ClO4-) or acidic
(HSO4-).

The conj acid of a weak base will be a weak acid and act as an acid in
water.  The weaker the base the stronger its conj acid and vice versa.
The conj acids of strong bases are so weak they do not act as acids in 
water to make the solution acidic.  When in solution they do nothing
and give a neutral solution.  The group 1A and 2A cations (Na+, K+, Ca^2+,
Ba^2+, etc) can be thought of as being the conj acids of the strong hydroxide
bases of those groups (NaOH, KOH, Ca(OH)2, Ba(OH)2, etc.) and will not
act as acids.

Dr. Zellmer