Transcript Buffer solutions Buffer solutions A single drop of dilute HCl is added to water. The water is stirred… and the final solution has a pH of about 2. We repeat the.

Buffer solutions
Buffer solutions
A single drop of
dilute HCl is
added to water.
The water is
stirred…
and the final
solution has
a pH of
about 2.
We repeat the process using a drop of sodium hydroxide
solution.
This solution has
a pH of about 12.
We have seen that a single drop of acid or base
added to water can change the pH by 5 pH units –
that’s a huge change in H3O+ concentration.
Buffer solutions are solutions which resist changes
in pH caused by addition of small amounts of acid
or base.
We can make buffer solutions by mixing a weak
acid (or base) with its conjugate base (or acid).
Ethanoic acid is added to a
solution of sodium ethanoate
containing a few drops of
universal indicator solution.
The final mixture has
a pH of about 3.
A large squirt of
NaOH is added.
No change in pH
is noticed.
We add more
NaOH…
… and more NaOH.
Still little change
in pH.
Remember that one drop of NaOH was enough to change
the pH of water by 5 units, yet here we have added
several mL of NaOH and not changed the pH significantly.
Adding significant amounts of HCl does not
change the pH significantly either.
The CH3COOH/CH3COO- mixture absorbs excess OH- or
H3O+ as follows:
CH3COOH(aq) + OH-(aq)
CH3COO-(aq) + H2O(l)
CH3COO-(aq) + H3O+(aq)
CH3COOH(aq) + H2O(l)
You’ll be asked to write equations like these in the exam.
We can turn ammonium chloride solution into a buffer
by adding ammonia solution:
An ammonia/ammonium buffer has a pH of about
9.5.
The pH is not changed
by addition of
moderate amounts of
acid…
… or base.
Can you write the equations to show what happens when
acid or base is added to an ammonia/ammonium buffer?
NH3(aq) + H3O+(aq)
NH4+(aq) + OH-(aq)
NH4+(aq) + H2O(l)
NH3(aq) + H2O(l)
In these examples we took salt solutions and added the
appropriate acid or base to make the buffers.
Buffer solutions can also be made starting from just the
acid (or base) and adding sufficient NaOH (or HCl) to
react with half the acid (or base), resulting in a mixture
of unreacted acid (or base) and its salt.
Buffer solutions containing equal amounts (moles) of the
acid and its conjugate base are the most effective at
absorbing moderate amounts of acid or base added.
When the amounts of acid and conjugate base are exactly
equal the pH of the solution is equal to the pKa of the
acid.