Lecture Outline Acid-base Equilibria

Name ______________________________________ ——————————————————————————————— 1 There was an exam on Friday, and we all probably took the weekend to decompress. So we're not really into this right now. Eh? True ___ Way true ___ I read Ch 4 over the weekend! ___ So this seems like a good time to get some feedback on these pre-lecture exercises (so yeah, this one will definitely be turned in). Please be honest — I won't hold your responses against you. In fact I'm going to start by cutting these at the dashed line — the name part gets you 10 points; the rest will be anonymous. 2 How helpful have the exercises been in encouraging you to prepare for class? (circle) very somewhat moderately not very not at all 3 How much time do you normally spend on each one? 4 How much reading in the textbook have you had to do to complete the exercises? a lot some a modest amount not much none at all 5 If the pre-lecture assignments had no points attached to them, would you still do them? absolutely probably maybe probably not never 6 Should we continue with these in Chem 22 next semester? yes, keep them as they are yes, but modify the system dump 'em 7 How could the pre-lecture assignments be modified to make them more useful? Thanks for your help! Lecture outline Acid-base equilibria — First a bit of review and some important terminology: + In an acid-base reaction, H is transferred from acid to base. That's all that ever happens in an acid-base rxn. Don't make it more complicated. (Unless stated otherwise, "acid" and "base" mean Bronsted-Lowry acid and base, not Lewis acid and base — we'll get to Lewis acids (also called electrophiles) and Lewis bases (also called nucleophiles) later.) + The acid is said to be deprotonated by (loses H to) the base + The base is protonated by (gains H from) the acid Here's the procedure for handling acid-base-equilibria in excruciating detail... (a) Write the equilibrium. Always draw structures clearly — in particular, we want to leave no + doubt whether compounds are ionic or covalent, and which atoms give up and accept the H . – Use curved arrows, and keep track of where those e pairs end up. (b) Label the acid and the base on each side of the equation. (c) Label the acids with their pKas, and decide which is the stronger and which is the weaker acid. (Do not under any circumstances label the bases with pKas! Bases do not have pKas. That a stands for acid. We could attach pKbs to the bases, but pKbs give us no new information and we don't want to mess around with a whole different set of numbers!) (d) Label the stronger and weaker bases. These labels are based entirely on the acid labels, so the weaker acid and base must be on the same side of the equation, and the stronger acid and base must be on the same side. So, yes, these labels are redundant; we're adding them just for emphasis. (Q: Can I assume that "must" means usually, but there are exceptions and you'll probably find one of those for the next exam? A: That's not "must", it's must, as in MUST! Absolutely, every time, without exception! Again, base strength is directly tied to the strength of the conjugate acid, so an exception to this statement would be physically impossible.) (e) Decide which way the equilibrium lies. Let's start with a simple one... NaH and CH3OH. How about CH3CO2K and water.... If we have the pKas, we can easily determine the equilibrium constant for the reaction, Keq, and from that, we can calculate — or just estimate — the ΔG°. pKas —> Keq For the general acid-base reaction, A–H + B A + B–H We can express the overall rxn equilibrium constant in terms of the Kas, express these in terms of pKas, then grind through a bit of algebra... Keq = [A– ] [HB] [HA] [B– ] = Ka for HA Ka for HB = 10 –pKa for HA 10 –pKa for HB = 10 pKa for HB – pKa for HA Keq —> ΔG° Recall: ΔGº = –RT lnKeq Where ΔG° is the change in free energy –1 –1 R = 1.987 cal/molK (i.e., 1.987 cal mol K ) Plug 'n' chug time: calculate ΔG° for the following values of Keq: 1, 10, 100, 1000, ... do you see a pattern? Yes you do. What is it? Now let's put that to use... What are the equilibrium constants for the two reactions above? And based on the Keqs, estimate the ΔG°s. And now, check that all the signs make sense. Of course, all of this depends on our knowing the pKas. There are extensive tables of pKa values (even more extensive than the one in your text), but we will find it extremely useful to learn the pKas for a few key compounds that we can use to make predictions for many others. Important pKas — acid pKa H3 O conj. base + RCO2H NH4 + H2 O H2 NH3 CH4 The key idea that allows us to extend this list is: similar structures —> similar pKas Similar in this context means no difference in any of the key factors that affect acidity and basicity. For example, let's estimate the pKas of the following compounds — think about their tendencies + to give up an H as compared to the ones in the list above. OH NH2 O OH OH OH These are a little harder — we can't predict the pKas, but we should be able to decide whether they're higher or lower than similar compounds in the list above. (If we need the actual values, we could try looking them up. You may not find these exact structures, but you could probably find some similar structures. Of course, you'd need to understand what needs to be similar to lead to a meaningful comparison...) OH CH3 Cl 3C PH3 SH COOH Which of the following compounds are strong enough bases to completely deprotonate water? NaH NH2 S O N O O On the next page is a handy template that you can place the 7 key compounds on, then add others as you learn them. A couple more that you may want to add are NaHCO3 as a base (baking soda; what's the conjugate acid?), phenol, and acetylene, which will be useful soon. Where would you place sulfuric acid, nitric acid, and HCl on the chart? pKa acid conjugate base 0 stronger acids weaker bases 10 30 40 50 stronger bases weaker acids 20