FAQ's Chem 141, Fall 2010
•>> Sept. 1, 2010
ques: When checking my homework I find a new problem: what does "acidic" and "basic" mean?
What does it mean if you say the solution is acidic or basic?
ans: "acidic" has several meanings:
1. if used of a solution, it means that, as a consequence of an acid having been present, there is
an elevated concentration of H3O+ beyond what would be present pure water.
2. if used of a chemical substance or species, it means that that substance or species tends to
react with bases so as to push or force a proton onto that base.
3. It also has meanings that go beyond this, some drawing from the Bronsted definition, the one
we are working with now, and some drawing from the Lewis definition, with which we have
not yet worked.
The meaning of "basic" parallels this, but inverted. Does this help?
•>> Sept. 1, 2010
ques: I have some questions about the conjugate bases and acids table. Do we just recite the table or
is there a rule behind it? If there is, can you tell me something about it? Thank you so much!
ans: Your question is not clear to me, but here are some thoughts for you to consider:
a. I will not likely ask you to recite the table, but rather, I am trying to give you the tools to learn
and remember where things are on the table, so that you can answer questions such as those on
problem set 3--which of two species is the stronger acid?, which of two species is the stronger
base?, will a solution of this or that species be acidic or basic?, that sort of thing.
b. There is not so much a "rule" behind the table as much as there is a compilation of lots and lots of
individual data points. Tables of acid and base strengths can be very, very extensive, (for
example, see:
http://evans.harvard.edu/pdf/evans_pKa_table.pdf )
or fairly compact, (see, for example:
http://www.saskschools.ca/curr_content/chem30/modules/module6/acid.html )
c. I am trying to provide you with a sense of where simpler acids (and their conjugate bases) fall on
such charts or tables, even before I try to give you a sense of why a given acid or base is where it
is. Look at the latter table, and see if, from our discussions, you do not recognize many of the
species on the table, and see if you could not have predicted that they would have been about
where they are.
d. In your lab this week, you are to do some titrations. The process of doing this work *should*
give you more insight into this concept area.
•>> Aug. 31, 2010
ques: I can't fully understand the handout on your website and I have some questions here. First is
that I find the definition of acid and basic is different from what I have learned in high school
and I think I need some help to figure it out. Second is that I don't really understand the
Goldilocks diagram because I am confused about why HI and HBr are stronger than HCl. Third
is that I can't fully understand how to compare the stronger base such as NH3 and PH3. This
problem is from the homework and I have no idea what to do about it. So I think maybe I haven't
understand the core thing.
ans: For the moment, here are a few points to consider:
1. there are three fundamental definitions in use for acid base concepts--this one (Bronsted)
focuses on the proton, and it is one of the two BIG concepts. The second big one (Lewis)
focuses on the connection that holds a proton to a base. The third, usually used only in a
cursory, simplified sense, focuses on what water forms in reaction with acid or base,
respectively.
2. There *IS* a table of observed acid strengths, and it *DOES* relate to a correlated table of
observed strengths of the conjugate bases of these acids. What I am attempting to do here is
to provide you with a way to [generate--lay out--remember in a qualitative and near
quantitative way] what that table looks like, using a few memorized anchor points and using
logic to generate the other points. With the table in place, you can then answer questions like
those on the problem set.
3. there are reasons that these acid strengths are as they are, but those are complicated reasons
and so I am showing you the pattern first; later in the course we will work a bit on the
"whys" of all this, though in the end, covering all the details is beyond this one course.. I
have found that, in most cases, this works well to lock this information (the pattern) into the
minds of my students.
======
So now, to the particular points you raised. Here is the memory tool I tried to give you:
4a. I told you where HF and HCl are on the table. These locations are among the few points you
have to remember to generate the table.
4b. Separately, I showed you the electronegativity values and trends, and in that, we noticed a
pattern in which these values change from F at one end to I at the other, and the changes
were large from F to Cl and small from Cl to Br and from Br to I, but all in the same
direction.
4c. Now, I told you that while it is not electronegativity values that dominate the trend, it
remains that the order of acid strengths parallels the order of EN's, IN THE SENSE THAT if
HF is lower and HCl higher in acid strength, then it would follow that HBr is even higher
than HCl and HI even higher than that, BUT only a little higher in each case.
4d. Finally, each acid has a conjugate base, whose strength, as a base, correlates (as I showed
you) inversely with that acid's strength as an acid--that is, stronger acids have weaker
conjugate bases.
======
5a. I say a number of important things each day, so that flagging each as important would be
distracting, but one very important thing I said was that, in our approach, we focus almost
exclusively on acid strength, and we use the principle (see 4d, above) of inverse acid/conj
base strengths to deduce base strengths.
5b. Thus, the key to working with the comparison of base strengths of NH3 and PH3, is to
convert them to acids and then consider what those acid strengths would be. Can you do
that?
ques: Kind of, but I am still wondering how to define the strength of the acid. I mean is there any
property can show the strength of an acid?
ans: The thing that shows acid strength is: how far to the right does the standard acid reaction go?
We have not talked about equilibrium constants yet, but maybe you know something of equil
constants already. If we knew the equilibrium constants for the std acid rxn for each acid, then
the stronger acids would be those for which the eq. constant of that reaction was larger--i.e., the
acids for which that reaction was driven further and further to the right.
It is not quite that simple, but that would get us close to where we would need to be.
In the meantime, I am still interested in the definitions of acid and base that you were used to-but that seemed so different from what I am using. Can you tell me what were those definitions?
ques: We actually don't have the concept of base and we defined acid as the substance which can
give H+ in solution or at melting status. And we used to define strong acid as an acid that can
transform all their H atom into ions. I feel it is really different with what we are learning so
maybe they are just different things?
ans: I need to ponder this a little. In the meantime, I am confident I am showing you an accurate,
useable, and reliable version of acid-base concepts, and I am confident that with what I am
showing you, you will be well-prepared to think about and work with acid-base aspects of
chemistry.