I wish I had a smaller pKa
My name’s C2H6O and I’m a
weak acid you know …
(very!)
Weak Acids don’t dissociate very much in solution.
(a.k.a. ethanol)
I can’t donate H+ to a Brønsted
base
Don’t be so tight! That
vinegar didn’t mind
N
A Brønsted-Lowry base (like ammonia) acts as a proton
acceptor.
An ion’s stabilised, if
charge is delocalised
When a proton leaves acetic acid, the negative charge on its conjugate base is spread over 3 atoms. This can’t happen with
ethanol.
The ethanoate anion is stabilised by distributing its negative charge over more than one atom. It could be represented by the diagram on the right.
Negative charge distributed over 3 atoms
HA [H+] + [A-]
Protons leave with ease and so there is a decrease in
pKaaaaaaaaaaaaaaaa
If the conjugate base is stable, the equilibrium shifts right, increasing Ka and reducing pKa.
ppKKaa = -log = -log1010KKaa
-log10(x)
Ka = [H+] [A-]
[HA]
And I wish I had a smaller
pKa Cos I just can’t seem to
give my proton
awaaaayy
The anion formed from dissociation is very unstable because the charge is localised on one atom (the oxygen)
I wish I had a smaller pKa
I wish I was a stronger acid
Strong acids have low pKas. The pKa of ethanol is about 16, the pKa of acetic acid is about 5 so it’s 100,000,000,000 times stronger than
ethanol but even acetic acid is still a weak acid!
Through a -bonded
framework …
bond between carbon atoms
bond between C and H atoms
bond between carbon atoms
bond between carbon atoms
bond between C and H atoms
bond between carbon atoms
Mesomeric effects work when double
and single bonds
alternate there
This is sometimes known as a mesomeric effect
The ethanoate anion is stabilised by distributing its negative charge over more than one atom. It could be represented by the diagram on the right.
Negative charge distributed over 3 atoms
So canonical structures, form when resonance
occurs
These are two different ways of drawing the same structure we call them resonance structures or sometimes canonical structures
And where there’s
conjugation there’s
delocalisation of lone pairs
Conjugation is another way of saying “alternating double and single bonds” – the electrons are evenly spread across all the bonds in an
extended orbital
So I wish I had a smaller
pKaCos I just
can’t seem to give my proton
awaaaayy
The anion formed from dissociation is very unstable because the charge is localised on one atom (the oxygen)
I wish I had a smaller pKa
I wish I was a stronger acid
Strong acids have low pKas. The pKa of ethanol is about 16, the pKa of acetic acid is about 5 so it’s 100,000,000,000 times stronger than
ethanol but even acetic acid is still a weak acid!
My name’s C2H6O I’m not as
strong as H2O
Even water is better at dissociating – it has a pKa of
15.7
I can’t donate H+ to a Brønsted
baseNot YOU
again
A Brønsted-Lowry base (like ammonia) acts as a proton
acceptor.
N
But when systems can be found to
move electrons around
pKa of 11
Then their Ka is large because their negative
charge is spread
(hooray!)Large Ka means small pKa
And I wish I had a smaller
pKaCos I just
can’t seem to give my proton
awaaaayy
The anion formed from dissociation is very unstable because the charge is localised on one atom (the oxygen)
I wish I had a smaller pKa
I wish I was a stronger acid
Strong acids have low pKas. The pKa of ethanol is about 16, the pKa of acetic acid is about 5 so it’s 100,000,000,000 times stronger than
ethanol but even acetic acid is still a weak acid!
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Words: Aimee HartnellMusic: Wizzard