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Chemical Reactions and Stoichiometry :
Predicting Reactants and Products
There are two main parts to this unit. One has as its focus
writing
balanced chemical reactions. Given reactants, you should be able to
predict the kind of products you can expect to get. The other part is
making quantitative predictions about the amounts of reactants needed or
products produced. We'll call this study Stoichiometry.
First, you will need to know how to balance
chemical equations. As we'll say,
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"A balanced chemical equation is a
powerful tool.
With it we can be predictive, we might even say we rule."
Melbeth 1990
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The basic idea is "what goes in, must come
out!" But remember that the conservation principle relates to number
and kind of atoms and to mass and not to the number of moles. If you find
the need to practice, in addition to doing the assigned work and reading with in
the unit, you might check out the following website http://www.wfu.edu/%7Eylwong/balanceeq/balanceq.html
For additional sites see Internet and Other Resources
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Second, you will need to know how to make
predictions for reactions. You will need to know how to identify the type
of reaction and then to make predictions. Simply we are going to
categorize reactions into 5 different groups. We will define the types of
reactions as such...
| Synthesis: |
a reaction in which simpler
substances come together to form a
more complicated one (for instance, if you see two elements as reactants, they
will come together to form a compound.) |
| Decomposition: |
a reaction in which one compound breaks down to form
smaller pieces. |
| Single
Displacement: |
a reaction in which an element displaces one of the elements
in a
compound
to form a new compound and an element. |
| Double
Displacement: |
a reaction in which two compounds exchange partners. |
| Combustion: |
a reaction between a carbon-based fuel and
oxygen. |
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Follow
these steps in writing balanced chemical equations.
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1.
Identify the type of reaction by
classifying the reactants.
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a.
If two elements then it is a synthesis
reaction
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b.
If one compound, then it is a
decomposition reaction
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c.
If one compound and an element, then it
is a single displacement reaction
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d.
If two compounds, then it is a double
displacement reaction.
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e.
If one compound is a fuel (carbon
containing) and the other reactant is oxygen, then it is a combustion
reaction.
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2.
Once
the reaction is identified, then predict the products by knowing what
happens in that kind of reaction.
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a.
a.
If a synthesis reaction, put the elements together
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b.
If a decomposition, note the kind of
decomposition, and make the appropriate predictions. See http://www.chem.vt.edu/RVGS/ACT/notes/Types_of_Equations.html
for the different types of
decomposition reactions you are to know.
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c.
If a single displacement, then check
activity series, and kick out appropriate element to form a new compound
and an element
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d.
If double displacement, then switch
partners (be certain to have matched + ions with negative ions) Check solubility table to make predictions about the nature
of the products.
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e.
If combustion reaction, predict that
water and carbon dioxide will form.
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3.
Make
certain that you have written the compounds correctly, noting the
charges of the ions involved in the compounds.
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4.
Balance
the equation.
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Here is a web site that does a nice job of
picturing the different types of reactions. http://www.usoe.k12.ut.us/curr/science/sciber00/8th/matter/sciber/chemtype.htm
Check it out!
Given that you can write balanced chemical
equations, our last task will be to use them in different ways. We'll
practice looking at various practical applications in class. What
happens when you remove the tarnish? Where does the phrase "being in
the limelight" come from? How does soap scum form? ... and many other
questions like that. Another area of practical application will be for us
to use the balanced chemical equation as a recipe of sorts. This will
allow us to be able to predict how much of the reactants we will need and the
amount of products we can expect to produce. Key in our interpretation
will be our ability to interpret the MOLE RATIO properly.
In solving these stoichiometry (mass relationships
between reactants and/or products) problems, we usually carry out the following
steps.
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5
Typical Steps in carrying out Stoichiometry (Mass Relationship) Problems
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1.
Write a balanced equation
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2.
List above the chemical formula what you are given and place an x above
what you must find
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3.
Covert your given quantity into moles.
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4.
Using the mole ratio as is shown by the balanced equation, determine the
number of moles of the unknown compound that is needed (or will be
produced)
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5.
Convert the moles of the unknown into grams.
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For one way to solve limiting
and excess reactants problems ....
Here are two web sites that do a very nice job of
looking at stoichiometrical relationships. This site, http://www.cmi.k12.il.us/Champaign/buildings/centennial/projects/rowe/chem/stoich/
gives a good overall summary of the process. This site, http://ir.chem.cmu.edu/irproject/applets/stoich/
is a favorite when describing limiting and excess reactants.
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We can determine products and
reactants
And what happens with a change.
Most impressively how much is needed or produced
Need not be very strange.
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Melbeth
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