Question

How is CN- a weak base if its the conjugate base of a weak acid...? I have a feeling my book is wrong.

Answer 1

CN- is a fairly strong base

Answer 2

The conjugate bases of most weak acids are weak bases. The only conjugates that are strong bases are those of extremely weak acids, like water (pKa = 14), for which OH- is the conjugate base.

What may be puzzling you is that "weak" spans a huge range of actual strength. One "weak" acid can easily be 8 to 10 orders of magnitude stronger than another. So you're thinking that the conjugate base of a weak acid should be strong, which is a natural instinct, but doesn't fit the jargon. The right way to think of this is to imagine an extra adjective that says *how* weak a weak acid or base is.

Then you could say something like "The conjugate base of a relatively weak weak acid is a relatively strong weak base, and the conjugate base of a relatively strong weak acid is a relatiively weak weak base."

Answer 3

They've got it written as CN- is weak and NH2- is strong I'm so very confused on how NH2- is considered strong

Answer 4

NH2- is the conjugate base of an extremely weak acid, NH3 (pKa = 35). Why are you surprised it's a strong base? On the otehr hand, CN- is the conjugate base of a moderately weak acid, HCN (pKa = 9.2), so it's a moderately weak base.

Answer 5

Yeah we havent done pKas yet. it says in the book "strong conjugate acids us a weak bronsted acid and a strong conjugate base is a weak bronsted base" I did not think that they would be expecting us to be comparing Ka and Kb values... god I hate this book they seem to always do pellet like this

Answer 6

I kind of see how that works but still Its just odd that they dont mention that at all.

Answer 7

Hopefully you've studied equilibrium. So you know about equiibrium constants. Very useful things! At equilibrium the composition of an aqueous solution of acid must satisfy this equation:\[\frac{[{\rm H}^+][{\rm A}^-]}{[{\rm HA}]} = K_a\]where [H+] is the molarity of H+, i.e. 10^-pH, [A-] is the molarity of the conjugate base, and [HA] is the molarity of the acid. The equilibrium constant K_a is characteristic of the acid. The "pKa" I keep mentioning is just -log_10 of Ka, e.g. pKa = 10 means K_a = 10^-10, and so forth.

Now it's easy to keep track of acid strength. A big K_a means the composition at equilibrium is tons of A- and H+, very little HA -- so you've got a strong acid, one that entirely dissociates. Take note that a big K_a equals a *small* pKa, because of the minus sign in the log. For example, the pKa of HCl is something like -7. Very strong acid.

A weak acid has a small K_a, so at equilibrium it's mostly not dissociated -- mostly still HA, with only a bit of H+ and A- formed. Small K_a equals large pKa. But keep in mind, as I said, there's a LOT of room between moderately strong weak acids, like HF (pKa = 3.2) or acetic acid (pKa = 4.8) and pretty weak weak acids, like HCn (pKa = 9) or phenol (pKa = 10), and water itself (pKa = 15), and exceptionally weak acids, like NH3 (pKa = 35).

And the additional magic is that the pKa of a weak acid and the pKb of a weak base (same idea, only the composition top row is [OH-][B+]) are related by pKw, -log_10 of K_w, the dissociation constant of water: pKb = pKw - pKa. So as pKa (acid strength) goes up, pKb (base strength) goes down.

Answer 8

Don't get too mad at the book. They're trying to throw you bite-size chunks, so you're not overwhelmed. But some books are better than others, certainly. If you give me the name of your book, I may be able to suggest some better ones for reference.