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  • Stoichiometry is a kind of calculation in chemistry. It’s sort of like bookkeeping.

  • Stoichiometry lets you figure out how much of a substance is used up or produced in a

  • chemical reaction. You need two things to do stoichiometry:

  • 1. A balanced chemical equation (balanced is absolutely essential here)

  • and 2. Some measured values (the amount of some of the chemicals involved in the reaction).

  • You can use these 2 things to solve for whatever is unknown - the mass or volume of

  • one of the other chemicals in this balanced chemical equation. It’s really common to

  • measure the amount of one reactant, in order to figure out how much product you expect

  • to get. Or, given how much of one reactant you have, you might want to figure out how

  • much of a second reactant to add so you don’t have any left over.

  • Stoichiometry works because of the Law of Conservation of Mass. Chemical reactions

  • don’t create or destroy matter, so say you have a certain number of atoms of oxygen at

  • the start of a reaction, you will have the same number of atoms of oxygen at the end

  • of the reaction. You can recombine the atoms with other atoms, but you can’t destroy

  • them, and you can’t change the atoms into some other kind of atom.

  • The balanced chemical equation tells you how the atoms combine in very definite ratios.

  • It’s like a recipe. Let’s say you want to make grilled cheese sandwiches. You take

  • 2 slices of bread and one slice of cheese to make one grilled cheese sandwich.

  • Another way you could say this is for every 2 pieces of bread, you need 1 piece of cheese

  • to make 1 grilled cheese sandwich. Believe it or not, this is just like a balanced

  • chemical equation. When you see the equation:

  • N2 + 3H2 → 2NH3

  • It’s the same as saying it takes 2 atoms of Nitrogen + 6 atoms of Hydrogen to make

  • 2 molecules of NH3. The equation also says 1 molecule of N2 + 3 molecules of H2 produces

  • 2 molecules of NH3. Another way to read the equation is 1 MOLE of N2 + 3 MOLES of H2 yields

  • 2 MOLES of NH3.

  • Let’s look at our grilled cheese sandwich recipe again. What if I asked: If you start

  • with 6 pieces of bread, and assume you have excess cheese (more cheese than youll need),

  • how many grilled cheese sandwiches can you make? It’s tempting to just solve this in

  • your head, but let’s solve it like we would a stoichiometry problem. We can use the recipe

  • to find conversion factors. Remember conversion factors are ratios that equal 1.

  • For every 2 pieces of bread, you need 1 piece of cheese. So you can write that as

  • a conversion factor: 2 bread/ 1 cheese = 1.

  • Similarly, for every 2 pieces of bread, you get 1 grilled cheese sandwich. That conversion

  • factor looks like this: 2 bread/ 1 grilled cheese sandwich = 1

  • And for every 1 piece of cheese, you get 1 grilled cheese sandwich. The conversion factor

  • is: 1 cheese/ 1 grilled cheese sandwich = 1

  • Back to our problem: Write down what you know, and what you want to get at the end,

  • and leave a blank space for a conversion factor: 6 pieces of bread gives us…. some number

  • of grilled cheese sandwiches. We want a conversion factor that has pieces

  • of bread and # of grilled cheese sandwiches in it.

  • Well use the conversion factor 2 bread/ 1 grilled cheese sandwich = 1. But careful

  • - we want our units to cancel. So in this case, well invert our conversion factor.

  • That’s the great thing about conversion factors - they equal 1, so you can flip them,

  • and theyll still equal 1. That’s why we can use our conversion factor in either

  • orientation.

  • So we have 6 pieces of bread times (1 grilled cheese sandwich/ 2 pieces of bread)

  • = some number of grilled cheese sandwiches. pieces of bread cancels...

  • 6/2 = 3 grilled cheese sandwiches.

  • I’m sure you could have done this problem in your head, but this is how youll want

  • to write out all your stoichiometry problems. Always write out your work. That way, youll

  • never accidentally divide instead of multiplying with a conversion factor - if you write it

  • out you can see when your units cancel properly. And, if youre in a class, you might get

  • partial credit for showing your work. It’s just a good habit.

  • {{Example 1}} Now let’s try a stoichiometry problem

  • using a balanced chemical equation. The balanced chemical equation will be just like our recipe.

  • N2 + 3H2 → 2NH3 If you start with 10 molecules of N2, and

  • you have an excess of H2, how many molecules of NH3 will you make?

  • It’s a good idea to check that your equation is balanced. Count up the number

  • of atoms on each side. On the left, we have 2 N and 6 H. And on the right we have

  • 2 N and 6 H. Okay, it’s balanced. If your equation isn’t balanced, you have to balance it before

  • you can do anything else, or stoichiometry won’t work. It would be like a bad recipe,

  • like trying to make grilled cheese sandwiches and starting with one piece of bread and expecting

  • 2 pieces of bread to magically show up in your finished sandwich. You can’t create

  • bread and cheese out of nothing - and the same goes for your chemical reactions.

  • Remember were going to use conversion factors. Start with what you know on the left,

  • and what you want to wind up with on the right. 10 N2 times some conversion factor = some

  • number of molecules of NH3 on the right.

  • We get our conversion factors from our balanced chemical equation. The equation tells

  • us for every 1 molecule of N2 you use 3 molecules of H2 to get 2 molecules of NH3. For this

  • problem, we need a conversion factor with molecules of N2 and molecules of NH3. We can

  • write it like this: 1 molecule of N2 over 2 molecules of NH3 = 1.

  • Let’s put that into our problem. Remember we need to make sure the units cancel. We

  • want to cancel N2 and wind up with NH3, so were going to flip our conversion factor.

  • 10 N2 times 2NH3 / 1 N2 = the number of molecules of NH3

  • N2 cancels. 10 times 2 = 20 molecules of NH3.

  • In this stoichiometry problem, we went from molecules to molecules. It turns out,

  • it’s the same kind of conversion as going from moles to moles. We could have asked - if

  • you start with 10 moles of N2, how many moles of NH3 will you wind up with - and you’d

  • get 20 moles of NH3. The Balanced chemical equation tells you the MOLE RATIOS of all

  • the reactants and products in the chemical reaction.

  • It gets a little more complicated when you start talking about mass - going from

  • grams to grams - but the general idea is the same. It will take 3 steps: grams to moles,

  • then moles to moles, and finally, moles to grams.

  • Well tackle these types of stoichiometry problems in PART II.

Stoichiometry is a kind of calculation in chemistry. It’s sort of like bookkeeping.

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B1 cheese grilled cheese conversion grilled n2 bread

Chemistry: Intro to Stoichiometry with Grilled Cheese Sandwiches | Homework Tutor

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    林宜悉 posted on 2020/03/06
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