Friday, March 25, 2016

Acid-Base Equilibria and Beginner Teachers

Cathy is a Year 12 student in a Brisbane school (capital city of Queensland, Australia). Last week she had a chemistry exam, the topic was equilibrium which included acids, bases and acid-base titrations. I met her on her way to school on the morning of this exam. When I asked her if she felt confident about her exam, I was horrified by her response,
"Sort of. I've got a new teacher this year and she's not any better than the one I had last year. We did an experiment, she said it took too long so we didn't do any more."
Seasoned teachers are used to:

  • student claims that their teacher is "no good" (especially if the student is performing poorly)
  • student exaggeration (only 1 experiment in a whole term of equilibrium, surely not!)
"Oh come on", I said in my best 'you're kidding me' voice, "you studied equilibrium for a whole term and only did one experiment?"
"Yeah", she confirmed, "we watched a video on titration though".
"Didn't you do a titration experiment?"
"Nah. She said we didn't have time."

Even allowing for the possibility of student exaggeration, the thought that you would play a video showing someone else performing a titration rather than giving your own students the opportunity to carry out even a simple titration, is, quite frankly, appalling. 

From Cathy's description of the teacher I assume this is the teacher's first year out teaching (beginner teacher). Reflecting on my own first year of teaching (a long, long time ago), I remember struggling to meet the requirements of the chemistry syllabus in the time-frame allowed, and, I also remember that while teaching the techniques of titration was time consuming, the students learned more in a few practical sessions than they learned during the whole of the preceding theoretical lessons, and once they have mastered the techniques they can be put to use in real-world problems.

A typical sequence that is often taught for (monoprotic) acid-base equilibria assumes prior knowledge of solutions, concentration (molarity) and equilibrium concepts and calculations (including self-dissociation of water, Kw ):
  1. What are the properties of acids and bases?
  2. How do we define an acid and a base?
  3. What is meant by the terms "strong acid"  and "strong base"?
  4. How do we measure the strength of an acid or a base (pH scale)?
  5. What happens when you add an (Arrhenius) acid to a (Arrhenius) base (neutralisation)?
  6. How much (Arrhenius) acid do we need to add to a known amount of (Arrhenius) base in order to neutralise it (acidic, basic, neutral solutions)?
  7. Discussion of titration techniques, including preparation of a standard solution.
  8. Performing a strong acid - strong base titration.
  9. Using the results of the experiment to calculate the concentration of the unknown acid or base.
  10. Perform calculations for each 1 mL addition of strong acid to strong base in the titration experiment and graph the results (strong acid - strong base titration curve)
  11. Discussion of weak acids (Ka).
  12. Discussion of other acid-base reactions (proton transfer reactions) and other titration curves
  13. What indicator should you use for a particular acid-base reaction? (optional, how does an indicator work)
  14. Titration of weak acid - strong base (such as determination of acetic acid in vinegar)
If you see your students 4 or 5 times a week, this teaching program for acid-base equilibria will take about 4 weeks using a traditional, structured approach. If you have the luxury of being able to time your practical work so that it occurs in the correct sequence, and take time to link the practical work to the theoretical concepts, your students have a good chance of understanding and being able apply the concepts to unfamiliar problems.
If you don't do any experimental work, you could probably bowl it over in 2 to 3 of weeks, and be faced with a lot of bored students wondering why they ever took a course in chemistry.
If you take a student-centered constructivist approach (for example, start with the questions  like "what gives vinegar its tangy taste?",  "if acids are corrosive and burn skin, how come you can drink vinegar?",  "how can you measure the strength of an acid?", "how can we determine which brand of vinegar has the greatest concentration of acetic acid?"), be prepared to add another week (unless you give the students a lot of reading/research for homework). The benefits, however, are enormous. Your students are more likely to be engaged with the content and "on task", they will have to be able to justify decisions they make in order to design and perform experiments thereby linking concepts and practical work, and because they "invest" in the whole learning process they are more likely to be apply the understanding and knowledge gained to other problems.

So, if you are new to teaching acid-base equilibria, here a few suggestions:
  1. Even if you firmly believe that constuctivist approaches to teaching are the most effective way to teach chemistry, be prepared to spend your first year of teaching chemistry taking a more traditional approach, using guided questioning to lead students towards the experiment(s) you need them to do (syllabus requirements) while still giving them "ownership" of the experiment and its results. Keep a list of the misconceptions you come across when you teach, this will help you be better prepared for next year. As you feel more confident in your ability to meet the syllabus requirements within the time you have, and you have a better feel for the misconceptions you will meet, you can start "loosening your hold" and give more time to truly constructivist approaches.
  2. Let the students do as much practical work as possible (students not only need to be exposed the practical techniques of chemistry, they need to do the experiments in order to fully appreciate the significance of what you are trying to teach them). You also need to devote time to discussing the results of their experiments with them, and reinforcing the concepts, calculations, techniques etc involved. 
  3. Spend time discussing the self-dissociation of water (that is, it is a lesson in its own right, not just a passing reference before you discuss acid-dissociation). Students will have been exposed to an "acids and bases" topic sometime between Years 7 and 10, but even so, many of them may still think that an acid has a pH less than 7, a base has a pH greater than 7, and that a neutral substance has a pH of 7. Believe me, it can be an uphill struggle to separate the two concepts of "acid, base, neutral" from the concept of "pH" in a student's mind (and if you don't believe me, think about the number of times you have seen/heard advertisements for products which talk about the "neutral pH" of skin/hair etc). If the students do not have a good grasp of the self-dissociation of water then they will not understand the pH of  aqueous solutions. (And a word of caution, just because a student can calculate the pH of an aqueous solution of base at 25oC, it doesn't mean they understand the relevance of pH + pOH = 14, or [H+][OH-] = 10-14, and if you want to test this statement, ask you students to calculate the pH of 0.001 M NaOH(aq) at 50oC, or ask them to find the pH of 0.01 moles of HCl(g) dissolved in 1 L of ethanol and see what happens, because the chances are they will simply do a pH + pOH = 14 calculation without even thinking about it!)
  4. Spend time making the distinction between "strong acids", "weak acids", "dilute aqueous solutions of acids" and "concentrated aqueous solutions of acids" (similarly for bases) because once again, you are likely to have an uphill struggle to separate the two concepts "strength of an acid/base" and "concentration of an acid/base". Remember, they have already been exposed to statements such as, "I need a cup of strong coffee", or, "this cordial is a bit strong" which, in chemical terms should be "I need a cup of concentrated aqueous solution of coffee (or cordial as the case may be)". On the other hand, they have also been exposed to ads which say things like "concentrated laundry detergent" which is a slightly more appropriate use of the technical term "concentrated" (although I do remember one example that used "concentrated laundry liquid" which introduces the other problem of the loose usage of the word "liquid" instead of "solution"). One way to do this is to give each pair of students a bottle of acetic acid labelled with its concentration, and have them measure its pH with a pH meter. Also provide them with volumetric flask of HCl(aq) of known concentration (say 0.1 M) and have them measure its pH, then have them perform sequential 1:10 dilutions and measure the pH at each stage say they can see that pH is dependent on the concentration of the strong acid and that you can reach a point at which the pH, and therefore the concentration, of a strong acid is the same, and even greater than, the concentration of an aqueous solution of weak acid. When you tabulate the class results and ask them for an explanation be prepared for many of them to believe you somehow "tricked them", it can take time for them to break the strength/concentration misconception and replace it with a more appropriate separation of the two concepts. If the students do this activity themselves, it will easily take a lesson, if you do it as a demonstration it will take about 10 minutes, BUT, it is better for the students to do it themselves partly because it reduces the instances of "there must be a trick in this" thinking, but mostly because they can see the pH change with the concentration and they are going to have to justify that all the way to the point at which the pH of the strong acid is  greater than the pH of a weak acid.
  5. Related to point 2 is the common misconception students may have that when you add an acid to a base you end up with a neutral solution that has a pH of 7. Personally, I think the best way to deal with this is to let the students work it out for themselves before you attempt to explain it to them. For example. give each pair of students a bottle of methyl orange indicator (you will need a fair degree of tolerance in establishing the end-point so don't use a pH meter) and a conical flask and have them add a 10.00 mL aliquot of standardised 0.1 M NaOH to the flask and record what happens to the indicator colour. Have them calculate the moles of hydroxide ions in the flask, as well as calculate the pH of the solution (so they are convinced the indicator is giving a true reading). Then give each pair a 100 mL volumetric flask containing 0.1 M monoprotic acid (some will get a strong acid such as HCl(aq), some will get a weak acid such as acetic acid). . Have them calculate the volume of acid they will need to add to the NaOH(aq) to neutralise it. Have them add this volume (straight from the pipette to the flask), give it a swirl, and record the colour of the indicator. Tabulate the results on the board (yellow vs red). Ask them why some changed colour and some didn't (be prepared to let the students with weak acids try adding more acid, many will believe they made a mistake in the calculations or in adding the solutions), if the students do not come to the realisation that only students with strong acids got a colour change at neutralisation, you can use questions to help guide them. I have found this is a far more effective method than just "telling them" and it need only take 15 minutes if all the solutions and equipment are prepared before hand AND you don't expect them to write it up as a prac (a demonstration takes even less time, but may not be quite as effective, that is, some students will believe you have somehow "tricked" them).
  6. Have the students perform the calculations that will enable them to draw a strong monoprotic acid - strong base titration curve. If you have a class of 20 students, they only need to do one of the calculations each, you can tabulate the results and then they can graph the class results. There are a number of reasons for this, it reinforces the nature of the neutralisation reaction, stoichiometry, and of "limiting reagents" and "reactants in excess". It is also enables them to come to a greater understanding of the shape of the curve than if you just present it to them and discuss key points. Finally, if the students do not have a good grasp of why titration curves are the shapes they are, they will have a much harder time coming to terms with the nature of different indicators and why some indicators are more appropriate than others for particular titrations.
  7. Do use "real-world" examples. The acetic acid concentration of brands of vinegar is not hard to do, and empowers them (if you have mothers whinging that their daughter will now only let them buy brand X because its better value because it has a higher concentration of acetic acid than other brands, then pat yourself on the back for a job well done!) If you are in a position to be able to safely determine the concentration of sulfuric acid in a lead-acid battery, then this is also not hard to do (but check whether it can be done at your school). Similarly, you will find concentrated HCl(aq) available at you local hardware store (for cleaning bricks) or pool suppliers (for addition to pools) and, if your safety guidelines allow, you can determine the concentrations of these.If you are prepared to take your students through back titrations (indirect titrations) then a wealth of new "real-world" opportunities is open to you.
  8. Finally, do not deceive yourself. It is NEVER about what you "teach", it is ALWAYS about what the students "learn". YOU can make up time by giving the students notes you have prepared for them, making them read stuff for homework, making them watch a 30 minute video instead of doing a 2 or 3 day prac, then you can happily tick this off on your list of things to teach, BUT, you must also find out what the students have learned, because you may very well find out that you have been a bit hasty in ticking something off your list! 

No comments:

Post a Comment