Friday, March 30, 2007

Why (high school) Chemistry Rots









Why do I despise high school chemistry? Because it's described in the antiquated terms of early 20th century science.

I'm talking about you, Avogadro constant.

I was tutoring my son this week when I came to a realization - we don't need the Avogadro constant anymore.

It was useful before the existence of the atoms and molecules were established. But in 2007, we know about matter at the atomic and molecular scale, so we should stop replacing the concrete picture of subatomic particles, atoms and molecules with the (in)convenient ideas implied when we talk about moles of material.

The Avogadro constant is really just a bad approximation of one divided by the mass of the proton. It's a bad approximation because instead of using the actual mass of the proton to calculate the constant, the accepted value is one twelfth the mass of a carbon 12 atom. The error comes about for several reasons. For one thing, there are twelve particles in the nucleus of carbon 12, but only half are protons, the other half are neutrons (which are heavier than protons, leading to a small over estimate). There are also six electrons floating around a normal (that is, neutral) carbon 12 atom, leading to another error from over estimating.

All of these things stick together to form one atom, which leads to yet another error. The binding energy that holds atoms together reduces the mass of carbon 12 by a lot more than the added mass of heavier neutrons and the extra electrons.

Put all these things together, and one divided by Avogadro's number is nearly equal to the mass of a proton in grams. How annoying is that?

Why do I hate the Avogadro constant? Well kids, I have loads of reasons. But here's three.

-- If we use proton mass in chemistry calculations instead of the Avogadro constant, then you could lighten your load of constants that you need to know by one at least. Sure, I'll have to memorize or look up the mass of the proton now, but I have to look that up on occasion anyway. I hate memorizing stuff, so this is a big deal to me. As a bonus, you can forget about the ideal gas constant (R) too. We only had to make that one up to atone for inventing the Avogadro constant in the first place.

-- Instead of struggling to remember the abstraction of moles, we could just think of the actual constituents of molecules in balancing chemical equations, leading to a more clear understanding of what's going on in chemistry. For example, Wikipedia says 'A mole is much like "a dozen" in that both units can describe any set of elementary objects . . .' In other words, using the Avogadro constant in chemistry makes as much sense as going to Dunkin Donuts a and asking for a twelfth of a dozen donuts when you only want to buy one, or one and a twelfth dozen when you want 13 (which can also be written 1.0833333333 dozen donuts).

-- The ideal gas law would make a lot more sense. What we're really talking about in the ideal gas law is particles bouncing off of the walls of a container, so PV=nRT is really PV=NkT, where 'N' is the number of particles in the container, and 'n' is the number of 6.0221415x10^23 sized batches of particles in the container. 'N' is much more sensible than 'n', and takes a LOT less oxygen to describe.

My boss argued with the third point by essentially paraphrasing this Wikipedia entry about rationale behind moles -

Moles are useful in chemical calculations, because they enable the calculation of yields and other values when dealing with particles of different mass.

Number of particles is a more useful unit in chemistry than mass or weight, because reactions take place between atoms (for example, two hydrogen atoms and one oxygen atom make one molecule of water) that have very different weights (one oxygen atom weighs almost 16 times as much as a hydrogen atom). However, the raw numbers of atoms in a reaction are not convenient, because they are very large; for example, just one mL of water contains over 3×10^22 (or 30,000,000,000,000,000,000,000) molecules.


I don't get it, why would chemists be upset by 3×10^22 (or 30,000,000,000,000,000,000,000), when a number approximately equal to 6.022^23 (or 602,200,000,000,000,000,000,000) is not a problem? Just round to three significant digits and use exponential notation, for crying out load.

In fact, using moles actually forces us to talk about the number of a number of particles. I'd rather just talk about the number of particles.

Here's the bottom line: all you chemists using the Avogadro constant, hitch up your belts and move along from the science of the early 1900's to the science of the late 1910's and beyond by ditching that crazy constant.

Then, perhaps, students can start learning about chemistry as it happens for real.

6 comments:

  1. You could've told me this a decade ago when I was still in high school.. Sheesh :P

    ReplyDelete
  2. Could you have told me this three decades ago when i was in high school?

    Wait a minute. I did good in Chemistry. I held the school record on some standardized exam for nearly two weeks!

    I could have taught you how to compute trig functions to ten digits as mental arithmetic.

    ReplyDelete
  3. I wish I'd known it when I was in high school. As it was, high school chemistry class was so annoying that I majored in physics in part because it ensured that I wouldn't have to take any college chemistry.

    The engineers and biologists weren't so lucky. ;)

    -Buzz

    ReplyDelete
  4. I'm a chemist, and firstly i'd like to say you make a good point, and in principle i do agree with you. But i dont see how it would be any easier to abandon this constant, despite the inaccuracies youve pointed out.

    Using the example of 1ml of water is relatively easy, but what happens when i have 7.6ml of 2.5 Molar N-Butyl lithium? believe me it would take a lot longer to work out a balenced equation based on absolute molecular amounts.

    Still one day we'll be able to do away with it, hopefully

    ReplyDelete
  5. Thanks for the comment Anonymous.

    I'm glad you didn't take too much offense at the post. (I was in a cantankerous mood when I wrote it, I guess).

    I used to write for the journal Analytical Chemistry, and I enjoy the field as a whole. It's only the Avaogadro constant that bugs me.

    Your example is a good counter point to my rant.

    However, I imagine, once we stop using the constant, that instead of talking about 2.5 molar solutions, we would think in terms of the ratio of the solute molecules to the solvent molecules.

    If I need to mix up some reactants, and I know how many molecules of X I need for a balanced reaction with molecule Y, then I would use the mass per molecule to weigh out the appropriate ratios.

    It seems simpler to me, but I'm more in tune with physics than chemistry.

    Do you think we will ever do away with the Avogadro constant? It's been a staple of chemistry for more than a century now.

    Best,

    Buzz

    ReplyDelete
  6. As sympathetic as I am to your argument, you clearly have no experience as a practical chemist. Moles, including moles/L and all the derived units, are much simpler to deal with than having stray powers of 10^23 everywhere. It's like asking particle physicists to measure masses in kg. It just isn't a practical unit for that scale.

    Your proton mass proposal is actually pretty silly, as the 1/proton mass is just as arbitrary as anything else. Using 10^24 instead would make more sense, as it would be in the same ball park, but eliminate the pesky exponents.

    The truth, though, is that it is only ratios that matter, and if everything is in moles, N_A rarely shows up in practice. Of course, physicists prefer to criticize other sciences without ever actually having mastered them, so they're oblivious about this, and are prone to post tirades when they are forced to absorb simple concepts that have been used for centuries ... :)

    ReplyDelete