Conjugate acid-base Pairs - Acids and bases, Chemistry Video Lecture

in this video we're going to be talking
about conjugate acid-base pairs we're
going to introduce the idea of a
conjugate acid-base pair using an
example reaction the example reaction is
between hydrogen fluoride or HF and
water so hydrogen fluoride is a weak
acid and when you put in water it will
dissociate partially so some of the HF
will dissociate and you'll get fluoride
minus ions and then that dissociated H+
ion
so this dissociated H+ ion will get
donated to our water so water then
becomes h3o plus or hydronium and so
this process is in dynamic equilibrium
because it can go forward and it can go
backward and eventually those two rates
are equal and they're both happening at
the same time so in this reaction we
have a couple things going on and we're
going to think about it in terms of
hydrogen ions being exchanged so if we
just look at the hydrofluoric acid and
we look in the forward direction our HF
is becoming f- and it's doing that by
donating or losing so I'll put a minus
for losing a proton so our HF loses a
proton that forms our F minus or
fluoride ion and then we can look at
that same process happening in the
backwards reaction so if we look at the
backwards reaction which is also
happening the fluoride ion can pick up
or accept a proton from somewhere so we
can pick up an H+ so I'll have a plus h
plus here and so when fluoride accepts a
proton we reform our HF so we can see
that HF nf- have this special
relationship where you can form one or
the other by losing or gaining a proton
and we can see a
our relationship between water and
hydronium so water here we said water is
accepting a proton from each F so we see
that water will gain a proton and that
will give us hydronium in the reverse
reaction hydronium can lose a proton to
reform water so minus h plus so again we
have these two species water and
hydronium that are related to each other
by having or not having one H+ so in
chemistry we call these species that are
related in this way conjugate acid-base
pairs so the official definition or my
official definition of a conjugate
acid-base pair is when you have two
species that are related to each other
let's see two species that are related
to each other
related by one H+ in this case we have h
F and F minus that are related to each
other by that one H+ and so H F and F
minus our conjugate acid-base pair we
also have water and hydronium which are
also related by that one H+ so water and
h3o plus are also a conjugate acid-base
pair you can probably tell from the name
but whenever you have a conjugate
acid-base pair one thing in the pair
will be an acid and the other thing will
always be a base the definition of which
one is the acid and which on is the base
comes from the bronsted-lowry definition
of acids and bases so the bronsted-lowry
definition says anything that can donate
an H+ so anything that will give away an
H+ is an asset so we can see that in
this case our hydrofluoric acid is
acting as the acid in the conjugate
acid-base pair and that means that
fluoride has to be acting as the base
and that makes sense because the
bronsted-lowry definition of a base is
something that will accept an H+
and that's exactly what it does in the
reverse reaction your f-minus will pick
up an H+ and go back to your acid so we
can also get water and h3o plus
so here water is gaining a proton or
accepting it so water is acting as a
base and then the reverse reaction h3o
plus is donating a proton so h3o plus is
acting as an acid the relationship
between conjugate acid-base pairs we can
write a little bit more generally so if
we represent any generic acid as H a so
this is our acid we said that a acid is
something that donates a proton so it'll
lose the proton and when it does that it
will form the conjugate base which is
represented by a minus in the reverse
reaction our base a minus can gain a
proton and remake our acid or conjugate
acid so whenever you have two species
that have basically the same formula
which we abbreviated here as a - except
for one has an H+ and one doesn't then
you know you have a conjugate acid-base
pair so let's look at some more examples
of conjugate acid-base pairs we saw
above HF or hydrofluoric acid its
conjugate base is f- so here HF is our
acid and when it loses that proton we
are left with f- we saw in the same
reaction that water can act as a base so
if water is our a - if that water
accepts a proton it forms the conjugate
acid h3o plus so the example we've gone
through so far HF is for a weak acid but
we can also talk about the conjugate
base of a strong acid like hydrochloric
acid HCl is a strong acid so that means
it completely dissociates so it gives
away all of its protons and when it does
that we're left with the conjugate base
alright
so even though chloride isn't
particularly basic it's still the
conjugate base of HCl and last but not
least we're going to go through two
examples where it looks like we might
have a conjugate acid-base pair but we
actually don't so one example is what
about the relationship between h3o plus
and Oh H - if we think of our acid up
here being h3o plus if we lose one
proton we saw that it's conjugate bases
water if water loses another proton we
get Oh H - so the difference between
these two species here is two protons
instead of one proton so these two
hydronium and hydroxide are not a
conjugate acid-base pair because they
differ by two protons instead of one and
then the last example we'll look at is
we said that fluoride is a conjugate
base of HF so what about the
relationship between sodium fluoride and
fluoride and so these two are also not a
conjugate base pair because if we take
our fluoride ion and it accepts a proton
we don't get sodium fluoride they are
related by a sodium ion so by definition
that these two are not a conjugate
acid-base pair so in this video we
learned that a conjugate acid-base pair
is when you have two species and they
have the same formula except one has an
extra proton so the acid has an extra
proton which it can lose to form the
base