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Author: Subject: Stupid chemistry exercises in school
MeshPL
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[*] posted on 7-9-2017 at 06:33
Stupid chemistry exercises in school


Sometimes chemistry exercises in schools are plain dumb. They are made for and by people who know little about chemistry.

For example:

Given the disocciation constants of ammonia and hypochlorous acid given below estimate the pH of ammonium hypochlorite solution...

This is stupid because ammonium hypochlorite would be extremely unstable and if ammonia and hypochlorites exist in solution together, chloramine is formed which may be dangerous and is something that probably should not be omitted.



Another one is:

There are metals in test tubes numbered from I to V. (In test tube IV there is sodium, there's also Cu, Ag, Mg, Zn)

Fill in the following sentences:

In test tube ___ evolution of gas is observed upon the addition of cold water.
[...]
Here the sentence could be true, but addition of water to sodium would rather cause the test tube to explode and simple evolution of gas could not be easily observed.

Do you know any examples of simmilarly stupid exercises?


[Edited on 7-9-2017 by MeshPL]
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DraconicAcid
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[*] posted on 7-9-2017 at 07:03


I don't care if the solution is unstable, I just want to know if the student can figure out the pH based on what we just learned. I also know very well that iron(III) iodide isn't going to exist; I still want my students to be able to figure out what its formula is. And if they reply FeI2, I know they just googled up the formula for iron iodide instead of trying to apply what they've learned.



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MeshPL
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[*] posted on 7-9-2017 at 08:54


That's what I hate about how chemistry is often taught: the tasks, exercises and theories are simplified and taken away from reality whether it makes sense or not, or whether the student can comprehend it or not. Sometimes the simplification is OK because it omits some complicated concept like using concentrations instead of ionic activities.

What especially gets me is treating bromine water as equivalent to bromine in inert solvent. Bromine water as far as I know reacts with alkenes to form bromohydrins and not dibromoalkanes and does not react with toluene or benzene (but it does so with phenol or aniline and even some polyalkyl benzenes or anisol derivatives) with or without catalyst.

And I've seen a few exercises which rely on synthesis of such dibromides with bromine water or reaction of toluene with bromine water.

Is it that hard to teach about Br2 in DCM?
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mayko
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[*] posted on 7-9-2017 at 09:25


You might like Bill Beatty's page of pedagogical science misconceptions; there's also a chapter in Surely You're Joking where Feynman rants at length about bad textbooks.


http://amasci.com/miscon/miscon.html




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TheNerdyFarmer
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[*] posted on 7-9-2017 at 13:31


I agree with DraconicAcid on this one. I'm no teacher but I get where they are coming from. Whether it does or does not exist it doesn't matter. The same rules apply anyway. All that the teachers are there to do is show to how to do it "If" it was real.
BTW: Sodium metal does bubble hydrogen when it is put in water. The part that makes it explode is when the reaction heats up so much that the hydrogen ignites and thus explodes. So, theoretically if you could manage to keep the whole reaction cool enough, it would just simply bubble vigorously. :)
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DraconicAcid
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[*] posted on 7-9-2017 at 14:10


Whether or not sodium will bubble in water or explode depends on how big the piece of sodium is....



Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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j_sum1
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[*] posted on 7-9-2017 at 18:06


I am going to chime in as an educator here. It is a bit of an art. One thing is that we need to differentiate between a question that might be used for teaching or illustrating a principle and a question that might be posed as an assessment instrument. The two are not the same. One difference is that one question opens up avenues for discussion and the other doesn't.

A peculiarity of chemistry is that there seem always to be exceptions to the rule. An ionic compound is formed between a metallic element and a non-metallic element. Except when it is not. Oxygen in a compound has an oxidation state of -2. (But that is not always true.) The octet rule -- Yaay! (But let's never mention PCl5 or any boron compounds.) Etc. The goal of a teacher is to reinforce these broad general principles without getting bogged down in extraneous detail that can lead to confusion.

As much as possible I always try to give realistic examples and avoid situations that are simply impractical. But I am not going to allow rigidity on this point to stand as an obstruction to student learning. And I may want to deliberately move off familiar ground for the purposes of assessing.

A recent example
Students are sitting an exam that involves some redox chemistry. They have a table of standard reduction potentials and associated half reactions provided to them. This is reasonable and it is an important tool for them to be able to use.
But as well as that I want to assess whether they can write their own half equations and balance redox reactions by themselves. For that I need to choose something unfamiliar that is not on the chart. I chose bromate to bromine as the reduction and sulfur to sulfate as the oxidation. I am fully aware that the situation is never going to be that simple, that there are numerous possible intermediates and side-reactions, that this reaction is unlikely to be that clean and I ignore the issue of what the apparatus might look like if one was to attempt this reaction. This is an exam and I can gloss over all of that stuff. I want to know if the student can balance the oxygens, hydrogens and charge, that they can combine two equations in the right ratio and interpret the situation as requiring an acidic environment. If, having done all of that they were to pull me up on some of the technicalities, I am delighted and will probably be quite liberal on the A grades.

This is in no way unique to chemistry. How often are physics students introduced to an idea where masses are considered points or surfaces are considered frictionless? How realistic was the train in that mathematics problem? And who is really going to be interested in or persuaded by that grade 9 English essay?

In summary, teaching and assessing have different goals. Simplification and/or generalisation are part of teaching -- all teachers do this when needed. An example to illustrate or asses a particular principle does not have to be practical in every possible sense.
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[*] posted on 7-9-2017 at 19:12


Try to remember that school isn't specifically for you. It's for everybody.

You have a lot more knowledge than your fellow students (obviously), but the classes have to be effective for everyone, even that kid in the back row who's really struggling.

With the right approach, he could catch on and perhaps find that he has a real talent for this stuff. Then he could take off and wind up being a real hotshot too. I remember reading that Newton was a pretty lazy kid and a slow learner who blossomed about half way through his schooling.

So an advanced kid like you gets stuck in a class designed for everybody and isn't ideal for your particular situation.
That's life. Just give the teachers the answers they want for now and you'll get to the more detailed stuff eventually.

But what you're building now is a theoretical foundation for your later studies, so use some of that time you're probably saving by not having to study for school to build a really massive foundation. That way you can build a skyscraper of knowledge on it later. Or even if you just wind up building a cottage, it'll be a cottage with a damn good foundation.

If you're not already doing it, look into getting some really good books on basic theory to study. Lab work and practical chemistry are fine and will serve you well, but a really detailed knowledge of the underlying theory will pay off again and again as your studies get more advanced,

Have you read Pauling's THE NATURE OF THE CHEMICAL BOND? It's a great introduction to some of the more arcane facts about chemical bonding and what was know about it from quantum mechanics back then.

Ronald Breslow is pretty good too. His ORGANIC REACTION MECHANISMS is a good read. I like the fact that he discusses alternate explanations of bonding theory and their limitations.

I'm sure various members here can make even better recommendations. There's probably some great stuff in the library right on this site. And old chemistry books can be dirt cheap if they're not reference works.












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diddi
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[*] posted on 7-9-2017 at 19:30


also from teaching point of view (but 'ex' unlike j_sum who has not seen the light) i found that for the rare instance of the student who had an extended knowledge of the subject that it was useful to have other tasks i could assign them. you might be able to ask for something a little more challenging to study whilst the rest are doing introductory exercises.



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