Wrong Question #1

Exam season is here in Britain, and students are bashing facts into brains in the hope that some will stick.   Here’s a revision question that really annoyed me. (Question from GCSE OCR 21st Century Chemistry, The Revision Guide, ISBN 9781847620095, 2007, p 86.)  It is poorly designed question and the sample answers are a bit muddled; but I don’t know whether the fault is in the revision guide or the GCSE exam.

Let’s analyze it.

Q: Give eight points you should consider when deciding whether a process is sustainable.

A:  Here is their answer, with their comments in green.    The question refers to a process of making ethanol by fermentation, starting from sugar, using yeast.

  1. Will the raw materials run out?  (Sugar beet and yeast grow quickly so won’t run out.)
  2. How good is the atom economy? — The waste CO2 produced means it has a low atom economy.  And because the enzyme is killed off by the ethanol produced, the reaction is even less efficient.
  3. What do I do with my waste products? — The waste CO2 can be released without any processing.
  4. What are the energy costs? — Energy is needed to keep the reaction at its optimum temperature.
  5. Will it damage the environment? — Carbon dioxide is a greenhouse gas so adds to global warming.
  6. What are the health and safety risks? —The chemicals and processes do not have any specific dangers.
  7. Are there any benefits or risks to society? —Making ethanol doesn’t impact society (drinking it does).
  8. Is it profitable? —This depends on what the ethanol is used for, e.g. drinking or fuel.

First of all, why exactly eight points?   Some of these categories overlap quite a bit.  For instance points 7 and 8 overlap: profits can be a benefit to society, and opportunities for large profits can change society.  Points 3, 1 and 5 overlap quite a bit.  Four and 1 overlap if your energy comes from non-renewable sources.  So, there isn’t anything special about eight points; you could cover all the ideas with four points.  The important issue here is that these categories are not “real”: they are not imposed by nature.  Rather, they were arbitrarily carved out by whomever wrote the test.

A better question would be “List the important points…” to avoid imposing a silly, arbitrary number of them.  The problem with asking for exactly eight is that it forces people, even people who deeply understands the issue to memorize the list.  Such people have better things to do. Some student who really knows what is going on, or even more so a professional chemist, would have just as much trouble with this question as someone who is scraping by.

Think of it.   A real chemist probably can make a list of 20 things to think about, each of which is a list of its own.  He or she has to somehow figure out how to lump all the ideas together to get down to eight.  Moreover, he/she has to get it down to the right eight categories and use the right keywords.  (These GCSE exams are scored by looking for important keywords.)  To get all the the right categories and use all the keywords that the examiners are looking for would force our chemist to memorize the answer, a horrible waste of time for anyone who already understands the problem.

[Objection 1: it makes you learn arbitrary things: the points.]

[Objection 2: knowing the subject does not help you.  You still need to memorize the answer, no matter how much you know.]

Then, the answers aren’t really self-consistent.   In point 3, we are told that the CO2 is safe, that it can be released without processing.  In point 5, we are told that it is a pollutant because it causes global warming.

[Objection 3: the answers are a bit confused.]

There’s also a bit of a wrong answer in there.  The optimal temperature for fermenting sugar into alcohol is pretty near room temperature, so the energy cost of keeping the temperature correct (answer to point 4) is pretty darn small.  It seems, from reading a few papers, that other energy costs are much bigger.  For instance, it seems like the energy required to stir the fermentation vats to keep the yeast cells suspended in solution is bigger.   And, certainly, the energy cost of  preparing the raw materials is relatively huge:  sugar doesn’t grow on trees you know, and even if it did, there would still be an energy cost for harvesting it.  So, the answer is spotlighting a real but unimportant cost.

[Objection 4: one answer isn’t quite real.]

Beyond that, it’s not too bad.   I learned a couple of things from writing this essay.   I thought that “atom economy” was a silly idea.  After all, if you want to make ethanol from sugar, you really have no choice but to produce CO2.  Count the atoms of the reactants, count the atoms of the product, and the different must be a waste product.  Right?   Basic conservation of atoms, I thought.

That is true enough: there is a certain minimum amount of waste involved in any kind of chemical reaction, simply because you cannot create or destroy atoms.  And the CO2 production here is a bad example because it is irreducible.  But, I searched around and read a few papers (most notably this paper by B. M. Trost) and it became obvious that in a lot of organic synthesis reactions, the atom economy could be much, much worse than the minimum you can compute by counting the input and output atoms.  The process of making a lot of complex molecules involves many stages of gluing on one bit and pulling off another group of atoms.  The wastage can be bigger than the final product.

And, then, after a bit of reading scientific papers on beer making, I realized that there is a lot of lossage in fermentation.   Dead yeast.  About 10% of all the sugar used in fermentation goes into growing new yeast cells to replace the damage done by the alcohol.  This is really part of the atom economy, too, and it is a loss beyond the irreducible minimum for the reaction.

So, I suppose I learned something from this question.   But even though atom economy is important, the CO2 production is irreducible.   The poor atom economy of the reaction isn’t due to CO2 production but to yeast production.

[Objection 5: an interesting answer, but again, not pointing at the real problem.]

So, I learned something, but even afterwards, I probably wouldn’t get any credit for it, because I have better things to do than memorize all the eight points.  A lot of British education seems that way.  Not awful, but with the flavour of a make-work project, forcing people to memorize useless clutter.