5) For the given sets of acid/base reactions below, (i) Draw...

5) For the given sets of acid/base reactions below, (i) Draw curved arrows to represent the flow of electrons in the reaction on the left (ii) Label the reactants on the left as either "Acid" or "Base" (iii) Decide which direction the equilibrium arrows will point in each reaction, based on the given $pK_a$ values Et (a) Et-N-Et + H-O-pKa = 17 (b) O-H + H-Br pKa = -8.7 (c) H-O-H + O=C(C)C-O-H pKa = 15.7 (d) Et-N-Et + H-O-pKa = 10 (e) H-N-H + H-C?C-H pKa = 25 (f) H-pKa = 50 + H-N-C(C)=O Et Et-N-Et + O=C(C)C-O-H pKa = 10 H H-O-C(C)C=O + Br-pKa = -2 H H-O-H + O=C(C)C-O-H pKa = 5 Et Et-N-Et + O=C(C)C-O-H pKa = 10 H H-N-H + H-C?C-H pKa = 38 H H-pKa = 10 + H-N-C(C)=O

Transcript

00:01 Okay, so this is about equilibrium when we're talking about pka.

00:02 So first, it's important to remember overall that a lower pka means a stronger acid.

00:10 That's really key here.

00:13 And with stronger acids, that means the equilibrium is going to move towards where those dissociate.

00:19 They tend to dissociate more.

00:21 They're stronger.

00:26 So if we have a battle between a low pka and a high pka, the low pka wins.

00:32 Okay, so if we have 1a here, we have ch3 and h3 plus oh, and then we have ch3 and h2, so the deprotonated version, and h2o.

00:49 The pka of ch3 and h2 is 10.

00:54 The pka of h2o is 16, which is the stronger acid.

00:59 It's the lower pka, which means that this is going to go in this direction, towards the weaker acid, which is less likely to dissociate, because lower pka, stronger acid, more likely to dissociate.

01:18 The second one.

01:20 We have acetylene deprotonated and water, and then we have acetylene and hydroxide.

01:32 So let's look at the pka's.

01:34 For water, we have 10 to the negative, the ka.

01:38 Okay, i didn't see that part.

01:41 Okay, so ka, we have 10 to the negative 16th for water, and ka, 10 to the negative 25th for acetylene.

01:54 Now remember that a pka is a negative log of the ka.

02:02 So with these, the negative log of 10 to the negative 16th is 16.

02:09 Remember, logs are a way to get rid of exponents in a sense.

02:12 So this is a negative log of 10 to the negative 25th.

02:19 So we have the exponent there.

02:20 It's going to be 25.

02:22 That's the pka.

02:25 So now we have the pka's...

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