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Here's a video discussion of stoichiometry (10 min): Khan
Academy: Stoichiometry, on YouTube
Here's another video on limiting reactants (15 min): Khan
Academy: Stoichiometry: Limiting Reagents
Here's another video discussion of stoichiometry in general,
including the previous 2 topics (13 min): CrashCourse
Chemistry, Stoichiometry: Chemistry for Massive Creatures on YouTube
Stoichiometry
What is it?
Stoichiometry is a general term for
relationships between amounts of substances in chemical
reactions. It also describes calculations done to determine how much
of a substance will be used in a reaction, left over after a
reaction, produced by a reaction, etc.
How do you use it?
The calculations of theoretical yield in the previous section are simple stoichiometry
calculations. To do stoichiometry calculations, you'll need:
- A balanced chemical equation: you may need to write the equation
and balance it yourself, based on what you know
- Formula weights or molecular weights (abbreviated FW or MW) for
relevant compounds: you will often need to calculate these for
yourself using the periodic table and the formulas; just add up the
atomic weights according to the formula
- Usually, you'll need to figure out how many moles react; to do
this, you'll need to decide what is the limiting reactant, by comparing
the number of moles of each compound that are present and the
coefficients in the equation
- Finally, once you have all the parts you can do a dimensional
analysis-style unit conversion to find the answer. Make sure you
understand what each step does and that each step is multiplying by
1. You might also need to do some other addition or subtraction,
depending on what the question asks for, such as subtracting the
amount of a compound that reacts from the initial amount to find the
part left over
Stoichiometry and Reaction Concepts
Doing stoichiometry calculations isn't just a procedure to solve
a problem in a book. It is a way to describe things that actually
happen, and it is related to concepts and understanding of
chemistry.
Stoichiometry calculations are based on the conservation of mass
(see the Lavoisier page) and the idea
that particles like nuclei and electrons aren't created or destroyed
during reactions, just rearranged. These understandings allow us to
make these calculations.
Stoichiometry calculations are also related to the concept of
chemical equilibrium. Equilibrium means
a stable state in which opposing forces are balanced. For instance,
when you stand on one foot, to maintain your balance and not fall
over, if you move a little to one side, you will need to correct
that and move back a little in the other direction so you don't lose
your balance. In chemistry, equilibrium means a state in which 2
opposite processes are occuring, but at the same rate. For instance,
the reaction may go in the forward direction (to the right), and at
the same time some molecules of product are turning into reactants,
going in the reverse direction (to the left). In many chemical
reactions, both directions are possible; when they are happening at
the same rate, that is called dynamic
equilibrium, which means "moving equilibrium", because the
individual molecules are moving back and forth between "reactant"
and "product" (which are really just defined by how you write the
equation), but the total amounts of reactant and product aren't
changing.
When we do stoichiometry calculations, we assume that the
reaction will be complete, meaning that the limiting reactant will
react completely, so none is left, forming as much product as possible. However, this is only sometimes
what happens. There are several reason why this might not happen.
Possible reasons why actual yield is less than theoretical yield
- Rate describes how fast a chemical
reaction happens. Some chemical reactions are very slow, like the
ones that dissolve rock and change the shape of mountains; others
are very fast, or in between. In general, the rate of a reaction
will depend on the conditions, such as temperature. Right now, we
will mostly talk about reactions that happen pretty quickly, but if
a reaction doesn't finish before you measure it, it could be because
it's too slow. This is discussed more when you study chemical kinetics.
- The position of the equilibrium is another possible reason a
reaction might not produce as much product as you
calculate. For all reactions, equilibrium is reached before all the
reactant is used; it might use almost all the reactant, so that you
don't notice the tiny bit left over, or it might reach equilibrium
with more than half the reactant left. If equilibrium is reached
(if the reaction is fast enough), the predicted theoretical yield will
only be reached if the equilibrium is almost all product. Later, you'll learn ways to
predict where the equilibrium is. For now, just know that reactions
can go both directions, and that the equilibrium isn't always all
product.
- The last possible complication is side or
competing reactions. This means that some other reactions you
haven't thought of or learned about might be happening, that use up
some of the reactant or product. Because chemistry is really
complicated and very few materials are actually truly pure, almost
every real system or reaction will have some side
reactions. Sometimes they are very important and prevent the thing
you want to happen from happening; other times they don't matter at
all.
For your calculations, you don't usually need to really worry about
rate, equilibrium or side reactions, because we haven't learned
strategies for dealing with them yet. However, to keep your mental
models of chemical reactions matching real chemical systems, it's
important to know that real reactions are more complicated than we
make them seem in the first few weeks of chemistry.
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This work is licensed under a Creative Commons Attribution 4.0 International License.
Last modified: Tue Mar 4 16:46:57 KST 2014