Does pH=pKa at the equivalence point? I'm getting confused now as I reread some of this stuff bc they are stating that from henderson hasselbach pH=pKa halfway to the equivalence point. Maybe I'm not fully understanding what the equivalance point is. Isn't the equivalance point occur when the [conj base] = [conj acid] , if thats the case why doesn't the pH always equal 7? I know that when titrating a week acid with a strong base the equivalance point will be basic. If someone could just explain the main points to know about titrations I'd greatly appreciate it, thanks in advance!
No, pH does not equal pKa at the equivalence point.
pKa is the pH at which [acid]=[conj base]
Equivalence point is the point at which all of the analyte is consumed by the titrant.
Soooooooo, let's take an example:
sodium carbonate, Na2CO3. We're going to titrate it against HCl.
Sodium carbonate has 2 pKa values, 6.35 and 10.33. There are two pKa values because, as you add HCl, you first put on one proton, and then a second one.
So, say I make a solution of sodium carbonate, and stick a pH meter in it, and say I read something like 11.2.
Ok, as I add HCl, the pH will start to drop. When I get to a pH of 10.33, the first pKa, I am at the point where [CO3 2-]=[HCO3-]. You'll also notice that the pH changes most slowly at this point, because the solution is buffered best at its pKa.
Anyway, we keep adding HCl. Eventually I will have run out of CO3 (2-). This is called the equivalence point, and will occur when I've added as many moles of HCl as I have of sodium carbonate. At this point, pH changes rather rapidly since the solution is unbuffered. You'll also notice that the amount of HCl you used to get to this equivalence point is twice as much as it took to get to your first pKa value. That makes sense because the pKa value was where the concentrations of the acid and base form are equal, right? So if we start with 2 moles of CO3 (2-), it'll take 1 mole of HCl to make 1 mole of HCO3- and 1 mole of CO3 (2-). Similarly, if I add another mole of HCl, I'll convert the rest of the CO3 (2-) to HCO3-, so I'll have 2 moles of HCO3-.
If I add another mole of HCl, I'll have converted half of my HCO3- to H2CO3. Thus, I'll have equal concentrations of the 2 species, which is my next pKa. If I read the pH, it should be 6.35 at this point.
If I add a final mole of HCl, I'll have converted all of my HCO3- to H2CO3, and thus I'll be at another equivalence point. After this, the pH of the solution will drop extremely rapidly since you're basically adding HCl to an unbuffered solution.
Hope that helps. pKas are halfway between starting points and equivalence points.
