Stoichiometry - What??

Now I doubt that many people would start out their blog by talking about stoichiometry, but since that is what inspired the blog in the first place it seems like a good place to start.

Actually I'm pretty sure that 95% of people have no idea what stoichiometry is. They have never even heard the word, or maybe vaguely remember a teacher talking about it once but have since put it out of their mind and moved on to more interesting (and less complicated) things.

So I asked the Internet "What is stoichiometry?" and Wikipedia told me "Stoichiometry is the calculation of quantitative relationships of the reactants and products in a balanced chemical reaction."

Well that clears things up, I'm glad we all understand Stoichiometry now.



...What? You don't actually have any idea what most of those words mean, much less what the sentence means all put together? Hmm...

I guess the first thing you need to know is that stoichiometry is part of chemistry and without some basic background knowledge it won't ever make sense.

• You need to know what elements and compounds are.
  
• You need to know that in a chemical reaction the atoms rearrange into different combinations but that you always end up with the same number of each type of atom that you started wit.
• You need to know that those chemical reactions can be represented on paper by chemical equations use letters, numbers and symbols.
• And you need to understand what a balanced chemical equation is and how to get one.

Now if there are any of those things you don't know already then you are going to have to back up, figure that out and come back. Stoichiometry is confusing enough when you do have all the pieces to start with, it's just silly to try if you don't.

So let's look at that definition again...

"Stoichiometry is the calculation..." So that tells us that we are doing some kind of calculation, which means math. We are starting with some information and then we will be calculating some other, new information.

"...of quantitative..." Well quantitative means numbers, or measurable. That means we are dealing with exact numbers, we can't just say things are getting a little bit bigger or a lot smaller.

"...relationships of the reactants and products..." That's a big chunk all at once but it actually doesn't make sense cut into smaller pieces (trust me). In the chemical reactions we talked about earlier (remember, the stuff you are supposed to know already?) the things we start out with are the reactants and the stuff we have after the reaction happens are the products. We are talking about the relationships between the relationship between the two, how they are related or connected. And remember we are talking quantitative, so this will be a numerical relationship.

"...in a balanced chemical reaction." And those relationships only happen when the equation is balanced. You have to know how many of each substance you are going to have before you can find any of the more complicated relationships.


Practically speaking, stoichiometry is using the information from a balanced chemical equations to make predictions about the amounts of the substances in relationship to each other. It is what lets scientists figure out how much salicylic acid and acetic anhydride to mix together to get 1 kilogram of aspirin. It lets companies know how much material to start with to get the most product with the least waste. It is also what lets scientists figure out a lot of information about new materials.



It also turns out you can learn to do stoichiometry problems without actually understanding what on Earth it all means, and that's what most people do. But that is for another post.


-HeyScienceTeacher