IB chemistry – acids and bases, the yin and yang of the chemical universe

Jacqueline Koay
20th August 2018 at 09:00

Subject Genius, Jacqueline Koay, IB chemistry – acids and bases, the yin and yang of the chemical universe

One of the most difficult chapters in International Baccalaureate (IB)/A-level chemistry is the one on acids and bases. It challenges students by hitting them from three angles: mathematically, synoptically and conceptually.

Mathematics

Especially for students who are weak in mathematics, I would advocate opening this section with a few minutes of playing with the calculator, namely, getting to grips with logarithms and anti-logs. Why use logs for acids and bases? (And indeed, why use logs at all?)

Because, at the risk of oversimplification, this whole chapter is about how many H+ ions you have, and thus, we are dealing with lots of zeros. By using the log scale, chemists can communicate effectively (and with less possibility of making an error) just how many of these ions we have and/or are dealing with. The elegant name we give this shorthand is of course pH, short for "power of H+".

Table 1: The relationship between H+ and pH:

Concentration of H+ (mol/dm3)

pH

1

1 x 100

0

0.1

1 x 10-1

1

0.01

1 x 10-2

2

0.001

1 x 10-3

3

0.0001

1 x 10-4

4

0.00001

1 x 10-5

5

0.000001

1 x 10-6

6

0.0000001

1 x 10-7

7

0.00000001

1 x 10-8

8

0.000000001

1 x 10-9

9

0.0000000001

1 x 10-10

10

0.00000000001

1 x 10-11

11

0.000000000001

1 x 10-12

12

0.0000000000001

1 x 10-13

13

0.00000000000001

1 x 10-14

14

 

At the end of this little trip into mathematics, students should be comfortable converting between logs and anti-logs.

Notes:

  1. I have deliberately focused only on the concentration of H+ in the table above, as this is a mathematical illustration.
  2. It might be prudent to point out to students that the log scale in use is to the base 10, not e.

Synoptic

I often think this chapter should be taught last, just before the options, when the whole syllabus has been tackled. This is because the questions that test the understanding of acids and bases are often problems that require up to four steps to solve.

It requires familiarity in these topics especially:

  1. Stoichiometry
  2. Reactions, e.g. concept of neutralisation
  3. Equilibrium
  4. Solutions


Reactions involving acids and bases are not merely confined to test tubes and litmus papers, but they happen dynamically every second of the day in the “real” world and inside living beings. It is the yin and yang of the chemical universe.

Concepts

There are so many of these “ethereal” concepts (and their corresponding equations) that, unless outlined clearly, will flummox those entering the acids and bases realm for the first, second or even third time. Thus, it is worthwhile pointing out that this whole chapter is all about the H+ ion and its life-partner, OH-. These two main characters are rather like Romeo and Juliet: they desperately want to meet and stick together for life, but unfortunately other players conspire to keep them apart and they are often imprisoned (or unsuitably married) to other spouses who are not their natural complements. The simplest example is the H+ in HCl would break its matrimonial bond when in proximity to its beloved OH-, who more often than not is married to another, most notably Na+. Crazy as it may be, but this analogy is a good point for students to return to and take a breather anytime they get lost in the labyrinth of Kw, pKa, pOH, etc.

There is no escape from formulae and calculations in any IB chemistry topic, and none more so than this one. Indeed, there are many formulae that a student has to remember for this topic to successfully navigate through the long questions. Having meticulously gone through quite a few of the exam questions in preparation for teaching this topic, I am convinced that the questions are designed to test students’ dexterity and confidence in choosing the right formulae to use and switching between the various derivations, in other words, fluency in the language of acids and bases.

My main focus would be to affix in students’ minds the vocabulary of this language, whose main “words” are: pH, pOH, [H+], [OH-], HA, A-, B, pKa, pKb, Ka, Kb, Kw and, crucially, their origins (derivations). I would expect students to be able to give me the definitions at the drop of a hat. These form the main concepts of this topic.

A good starting point for the above is to identify the “mother equation” when it comes to acids and bases. In my opinion, these are the two:

  1. H20 = [H+] + [OH-], from which progenies such as HA = [H+] + [A-] are born;
  2. pH = -log [H+], for this is after all the heart of this chapter. (Note: the most common mistake is not converting pOH to pH in exams, losing marks unnecessarily.)


This leads on to the derivation of subsequent formulae that are needed for this topic, and the values, eg [H+] [OH-] = 10-14

An interesting exercise for students (one that will surely give them clarity in the topic) is to create a universe based on the definitions and the relationships between the above, namely in seeing how [H+] [OH-] = 10-14 translates into observations that we see in test tubes.  There is nothing more sublimely beautiful than when we could relate numbers and abstract concepts into the brilliant colours in the real world, and this chapter (alongside the one on transition elements) lends itself very well to delivering this experience.

Jacqueline Koay teaches physics and chemistry as a subject specialist.

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