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I wish I had a smaller pK a. My names C 2 H 6 O and Im a weak acid you know … (very!) Weak Acids dont dissociate very much in solution. (a.k.a. ethanol)

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Presentation on theme: "I wish I had a smaller pK a. My names C 2 H 6 O and Im a weak acid you know … (very!) Weak Acids dont dissociate very much in solution. (a.k.a. ethanol)"— Presentation transcript:

1 I wish I had a smaller pK a

2 My names C 2 H 6 O and Im a weak acid you know … (very!) Weak Acids dont dissociate very much in solution. (a.k.a. ethanol)

3 I cant donate H + to a Brønsted base Dont be so tight! That vinegar didnt mind N A Brønsted-Lowry base (like ammonia) acts as a proton acceptor.

4 An ions stabilised, if charge is delocalised When a proton leaves acetic acid, the negative charge on its conjugate base is spread over 3 atoms. This cant 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

5 HA [H + ] + [A - ] Protons leave with ease and so there is a decrease in pK aaaaaaaaaaaaaaaa If the conjugate base is stable, the equilibrium shifts right, increasing K a and reducing pK a. pK a = -log 10 K a -log 10 ( x ) K a = [H + ] [A - ] [HA]

6 And I wish I had a smaller pK a Cos I just cant seem to give my proton awaaaayy The anion formed from dissociation is very unstable because the charge is localised on one atom (the oxygen)

7 I wish I had a smaller pK a I wish I was a stronger acid Strong acids have low pK a s. The pK a of ethanol is about 16, the pK a of acetic acid is about 5 so its 100,000,000,000 times stronger than ethanol but even acetic acid is still a weak acid!

8 Through a - bonded framework … bond between carbon atoms bond between C and H atoms bond between carbon atoms bond between C and H atoms bond between carbon atoms

9 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

10 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

11 And where theres conjugation theres 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

12 So I wish I had a smaller pK a Cos I just cant seem to give my proton awaaaayy The anion formed from dissociation is very unstable because the charge is localised on one atom (the oxygen)

13 I wish I had a smaller pK a I wish I was a stronger acid Strong acids have low pK a s. The pK a of ethanol is about 16, the pK a of acetic acid is about 5 so its 100,000,000,000 times stronger than ethanol but even acetic acid is still a weak acid!

14 My names C 2 H 6 O Im not as strong as H 2 O Even water is better at dissociating – it has a pK a of 15.7

15 I cant donate H + to a Brønsted base Not YOU again A Brønsted-Lowry base (like ammonia) acts as a proton acceptor. N

16 But when systems can be found to move electrons around pK a of 11 Then their K a is large because their negative charge is spread (hooray!) Large K a means small pK a

17 And I wish I had a smaller pK a Cos I just cant seem to give my proton awaaaayy The anion formed from dissociation is very unstable because the charge is localised on one atom (the oxygen)

18 I wish I had a smaller pK a I wish I was a stronger acid Strong acids have low pK a s. The pK a of ethanol is about 16, the pK a of acetic acid is about 5 so its 100,000,000,000 times stronger than ethanol but even acetic acid is still a weak acid!

19 openclipart.org Words: Aimee Hartnell Music: Wizzard


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