professor Dave here, let's discuss acid-base equilibria
when a strong acid or base reacts with water we can say that it completely ionizes, that
every molecule of acid will transfer its acidic proton to a water molecule. if
every molecule reacts to completion this makes things neat and tidy but when weak
acids and bases react they only partially ionize and they establish an
equilibrium between the ionized and unionzed form and that means we have
to do some math. Ka is the acid ionization constant. this is derived just
like we got Kc for generic chemical equilibria. note that we don't include
water as it is a pure liquid. in this way Ka is a measure of how thoroughly an
acid-base reaction proceeds in the forward direction. a larger Ka means more
products and a stronger acid a smaller Ka means more reactants and a weaker
acid so an acid with a larger Ka will be more acidic and therefore associated
with a lower pH. we can determine Ka experimentally. let's say we want to know
the Ka for a weak acid like phenol. we can set up an icebox just like the kind we
already know. we can use the pH to calculate hydronium concentration
and therefore also the concentration of the conjugate base since they are the
same
as it happens the change in initial concentration of the acid is so small
that it's negligible and we treat things as though none got used up
plug these values into the expression and solve for K
from this information we can also calculate the degree of ionization which
is the ratio of ionized to unionized acid molecules in solution or the
percent ionization which is the same thing but expressed as a percentage. this
is a pretty weak acid considering so few molecules transferred a proton to a
water molecule
sometimes like in the previous example the acid is so weak that we pretend the
concentration of the acid doesn't change but if it's a little stronger than that
we can't make that simplifying assumption and the math gets trickier
but still essentially the same. we can make ice boxes for weak bases as well, or
multiple ice boxes for polyprotic acids, one per ionization, just more math
a buffer is a solution that has the ability to resist changes in pH
when limited amounts of acid or base are added to it
buffers are made by combining a weak acid or base with its conjugate. in such
a solution because both the acid and conjugate base are present they are able
to neutralize small amounts of any acid or base that are added to the solution
many biological fluids are buffer solutions because they must maintain a
steady pH to avoid loss of critical biological function. the bloodstream for
example maintains a pH of around 7.4 otherwise its capacity to carry oxygen
around the body would be lost
buffer solutions of a particular pH can be prepared using the Henderson-Hasselbalch
equation. we know about pH and pKa is similar in that if pH is the negative
log of the hydronium concentration, pKa is the negative log of the Ka for that
acid. what this shows us is that in order to create a buffer solution of a
desired pH we must use an acid conjugate-base pair where the Ka is approximately
equal to the desired hydronium concentration or rather where the pKa is
equal to the pH. let's check comprehension
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