# ch12

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```					                        Chapter 12: Stoichiometry
“To Measure the Elements”

   Chemical reactions stop when one of the reactants is used up.
   Stoichiometry: study of quantitative relationships between amounts of reactants
used and products formed by a chemical reaction
o Based on the law of conservation of mass
o Chemical bonds in reactants break and new bonds are formed to produce
products
o BUT the amount of matter present at the end of the reaction is the same as
was present at the beginning.
o We use stoichiometry to
 See how much product we will get from known amounts of
reactants.
 See how much reactants we will need to form a known amount of
product.
 See how much of one reactant will be needed to react completely
with a known amount of another reactant.
 Etc.

   Reading a chemical equation:
o The coefficients can mean several things: particles or moles
4Fe + 3O2  2Fe2O3
o   4 atoms Fe     +       3 molecules O2        2 formula units Fe2O3
o   4 moles Fe     +       3 moles O2            2 moles Fe2O3
o   4(55.85g)      +       3(32.00g)             2(159.70g)
o   319.4g reactants      319.4g products

   The combustion of butene (C3H8) provides energy for heating homes, cooking
food, and soldering metal parts. Interpret the equation for the combustion of
butene in terms of particles, moles, and mass. Show that the law of conservation
of mass is observed.
   Barium hydroxide reacts with sulfuric acid to create water and barium sulfate.
Write a balanced equation. Interpret the equation in terms of particles, moles,
and mass. Show that the law of conservation of mass is observed.

   Practice: p.356 a-e

   Mole Ratios: a ratio between the number of moles of any two substances in a
balanced chemical equation

2Al + 3Br2  2AlBr3
2mol Al       2mol Al           3mol Br2     3mol Br2      2mol AlBr3     2mol AlBr3
2mol AlBr3    3mol Br2          2mol AlBr3   2mol Al       2mol Al        3mol Br2

2KClO3  2KCl + 3O2

   Practice p.357 2 (a-c)
   Using Stoichiometry
o Start by writing the balanced chemical equation.
o Then identify the substance that you know and the substance that you need
to determine.
o The mole ratio can be used to convert between the different substances.

   One disadvantage of burning propane (C3H8) is that carbon dioxide is one of the products. The
released carbon dioxide increases the growing concentration of carbon dioxide in the atmosphere.
How many moles of carbon dioxide are produced when 10.0 moles of propane are burned in
excess oxygen in a gas grill?

   Practice p.359 9-10

   Taking mass into account:
o Determine the mass of sodium chloride produced when 1.25 moles of
chlorine gas reacts vigorously with sodium.

o Ammonium nitrate, an important fertilizer, produces dinitrogen monoxide
gas and water when it decomposes. Determine the mass of water
produced from the decomposition of 25.0g of solid ammonium nitrate.

o Practice p.362 13-14
   Limiting Reactant

o Rarely are reactants present in exact ratios. Generally one or more
reactants are in excess and the reaction proceeds until all of one of the
reactant is used up.
o The amount of product depends on the reactant that is limited.
o Limiting reactant: limits the extent of the reaction and, thereby,
determines the amount of product. [It runs out first.]
o Excess reactant: the leftover reactant. [You still have plenty at the end,
along with the product.]
o http://www.chemcollective.org/stoich/limiting-reagents.php [If you need
extra help at home.]
o LR problems
S8 + 4Cl2  4S2Cl2
If 200.0 g of sulfur react with 100.0g of chlorine, what mass of disulfur
dichloride is produced?
 First, you must determine which reactant is limiting because the
reaction will stop producing product when the LR is used up.
 So, you determine the amount of product that will be created by
EACH amount of the reactants.
 The reactant that creates the LOWEST amount of product is the
LR.
 Once you figure out the LR, you can determine the amount of
product formed.
 Finally, you will need to see how much of the excess reactant was
actually used and/or left over. [Take the final amount of product
and use stoichiometry to figure out how much was used.]

   Practice p.368 20-21
   Percent Yield
o Most reactions never succeed in producing the predicted amount of
product … aqueous solutions evaporate, molecules stick to the side of the
container, etc.
o Percent yield is a way to determine how well the lab was performed.

Actual Yield (from an experiment)                  x100%
Theoretical Yield (from stoichiometry)

o   When potassium chromate (K2CrO4) is added to a solution containing 0.500g silver (I)
nitrate, solid silver (I) chromate and aqueous potassium nitrate are formed. Determine
the theoretical yield of silver (I) chromate. If 0.455g of silver (I) chromate is obtained,
calculate the percent yield.

o Practice p.372 27-28

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