Stoichiometry “The Mathematics of Chemical Reactions” By: Ms. Buroker
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- Slide 2
- Stoichiometry The Mathematics of Chemical Reactions By: Ms.
Buroker
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- What youll Learn . You should be able to. 1.) write mole ratios
from balanced chemical equations. 2.) calculate the number of moles
and the mass of a reactant or product when given the number of
moles or the mass of another reactant or product. 3.) identify the
limiting reactant in a chemical reaction. 4.) determine the percent
yield of a chemical reaction.
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- Okay, so what in the world is stoichiometry? Technically,
stoichiometry is the study of quantitative relationships between
amounts of reactants used and products formed by a chemical
reaction. Stoichiometry allows a chemist or scientist to know how
much of an element or reactant to use, how much product is expected
to come out of the reaction, and how to express different amounts
of a substance in many units of measurement.
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- A Little Bit of Review Before We Get Started So let's say I ask
you to hand me 5 moles of Li. You look at me and wonder how you're
supposed to measure out 5 moles. Before completely giving up you
remember that you can use a simple conversion equation to find a
simpler measuring unit to give you 5 moles of Li.
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- Chocolate Chip Cookies!! 1 cup butter 1/2 cup white sugar 1 cup
packed brown sugar 1 teaspoon vanilla extract 2 eggs 2 1/2 cups
all-purpose flour 1 teaspoon baking soda 1 teaspoon salt 2 cups
semisweet chocolate chips Makes 3 dozen How many eggs are needed to
make 3 dozen cookies? How much butter is needed for the amount of
chocolate chips used? How many eggs would we need to make 9 dozen
cookies? How much brown sugar would I need if I had 1 cups white
sugar?
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- Cookies and ChemistryHuh!?!? Just like chocolate chip cookies
have recipes, chemists have recipes as well Instead of calling them
recipes, we call them reaction equations Furthermore, instead of
using cups and teaspoons, we use moles Lastly, instead of eggs,
butter, sugar, etc. we use chemical compounds as ingredients
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- Chemistry Recepies Looking at a reaction tells us how much of
something you need to react with something else to get a product
(like the cookie recipe) Be sure you have a balanced reaction
before you start! Example: 2 Na + Cl 2 2 NaCl This reaction tells
us that by mixing 2 moles of sodium with 1 mole of chlorine we will
get 2 moles of sodium chloride What if we wanted 4 moles of NaCl?
10 moles? 50 moles?
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- Mole Ratios Within a chemical equation, coefficients not only
represent the individual number of atoms or molecules but also the
number of moles! Example: Fe (s) + O 2(g) Fe 2 O 3(s) 4 32 What we
can then say is, 4 moles of Fe react with 3 moles of O 2 to produce
2 moles of Fe 2 O 3
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- Mole Ratio: a ratio between the number of moles of any two
substances in a balanced chemical equation. Note: Think of the mole
ratio as your bridge between reactants, between products, and
between reactants and products
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- Determine . All possible mole ratios for the following balanced
chemical equations: 4Al (s) + 3O 2(g) 2Al 2 O 3(s) 2HgO (s) 2Hg (l)
+ O 2(g) 4 mol Al/ 3 mol O 2 3mol O 2 / 2mol Al 2 O 3 4 mol Al/ 2
mol Al 2 O 3 2 mol Al 2 O 3 / 4 mol Al 3 mol O 2 / 4 mol Al 2 mol
Al 2 O 3 / 3 mol O 2 2 mol HgO/ 2 mol Hg2 mol Hg/ 1 mol O 2 2 mol
HgO/ 1 mol O 2 1 mol O 2 / 2 mol HgO 2 mol Hg/ 2 mol HgO1 mol O 2 /
2 mol Hg
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- Practice Write the balanced reaction for hydrogen gas reacting
with oxygen gas. 2 H 2 + O 2 2 H 2 O How many moles of reactants
are needed? What if we wanted 4 moles of water? What if we had 3
moles of oxygen, how much hydrogen would we need to react and how
much water would we get? What if we had 50 moles of hydrogen, how
much oxygen would we need and how much water produced?
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- Mole- to- Mole Conversions What happens when I want to go from
moles of one substance to moles of another substance? Or If I know
how much I have of one thing and I want to use that information to
make predictions about something else in the reaction? Moles of
Known X Moles of unknown Moles of known = Moles of unknown Mole
Ratio
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- Mole- to- Mole Conversions Lets look at the following reaction
involving the reaction of sodium metal with chlorine gas to produce
sodium chloride: 2Na (s) + Cl 2(g) 2NaCl (s) How many moles of
sodium chloride will be produced if you react 2.6 moles of chlorine
gas with an excess (more than you need) of sodium metal? Remember
the formula!! Moles of known x mole ratio = moles of unknown 2.6
mol Cl 2 x 2 mol NaCl = 5.2 mol NaCl 1mol Cl 2
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- Lets Try Another One! One disadvantage of burning propane (C 3
H 8 ) is that carbon dioxide (CO 2 ) is one of the products. The
released CO 2 increases the growing concentration of CO 2 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? C 3 H
8(g) + 5O 2(g) 3CO 2(g) + 4H 2 O (g) 10.0 mol C 3 H 8 x 3 mol CO 2
= 30.0 mol CO 2 1 mol C 3 H 8
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- Mole- to- Mass Conversion Example: How many grams of chlorine
are required to react completely with 5.00 moles of sodium to
produce sodium chloride? 2 Na + Cl 2 2 NaCl Most of the time in
chemistry, the amounts are given in grams instead of moles We still
go through moles and use the mole ratio, but now we also use molar
mass to get to grams ?5 moles 5 mol Na x 1 mol Cl 2 x 70.9 g Cl 2 =
177.25g Cl 2 2 mol Na 1 mol Cl 2
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- What if I Wanted to go in Reverse? Mass- to- Moles We can also
start with mass and convert to moles of product or another reactant
We use molar mass and the mole ratio to get to moles of the
compound of interest Calculate the number of moles of ethane (C 2 H
6 ) needed to produce 10.0 g of water 2 C 2 H 6 + 7 O 2 4 CO 2 + 6
H 2 0 10.0 g H 2 O 1 mol H 2 O 2 mol C 2 H 6 18.0 g H 2 O 6 mol H 2
0 = 0.185 mol C 2 H 6
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- Lets Practice! Calculate how many moles of oxygen are required
to make 10.0 g of aluminum oxide. 4Al (s) + 3O 2(g) 2Al 2 O 3(s) ?
10.0g 10.0g Al 2 O 3 X 1 mol Al 2 O 3 101.96g Al 2 O 3 X 3 mol O 2
2 mol Al 2 O 3 = 0.147 mol O 2
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- Mass- to- Mass Conversions Most often we are given a starting
mass and want to find out the mass of a product we will get (called
theoretical yield) or how much of another reactant we need to
completely react with it (no leftover ingredients!) Now we must go
from grams to moles, mole ratio, and back to grams of compound we
are interested in
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- Mass-Mass Conversion Ex. Calculate how many grams of ammonia
are produced when you react 2.00g of nitrogen with excess hydrogen.
N 2 + 3 H 2 2 NH 3 2.00g N 2 1 mol N 2 2 mol NH 3 17.06g NH 3
28.02g N 2 1 mol N 2 1 mol NH 3 = 2.4 g NH 3
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- How many grams of calcium nitride are produced when 2.00 g of
calcium reacts with an excess of nitrogen? 3Ca (s) + N 2(g) Ca 3 N
2(s) ? 2.00g 2.00g Ca 1 mol Ca 1 40.08g Ca X = 0.0500 mol Ca MOLE
RATIO!!! 0.0500mol Ca x 1 mol Ca 3 N 2 3 mol Ca = 0.0167 mol Ca 3 N
2 X 148.26g 1 mol = 2.48 g Ca 3 N 2
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- Stoichiometry with Gases According to gas laws, when gases are
at STP (standard temperature and pressure), 1 mole of a gas will
occupy 22.4L. 1.) Standard Temperature is 25C or 273K 2.) Standard
Pressure is 1atm 3.) This allows us to take volumes of gases that
are at STP and turn it into moles.
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- Lets take a look at an example! If I had the following
reaction: How many Liters of ammonia could I make (at STP) if I
began with 25g of N 2 gas? N 2(g) + 3H 2(g) 2NH 3(g)
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- Why do reactions stop? The answer is limiting reagents! If you
put 10 boys and 6 girls in a room and asked them to pair up: one
boy to one girl, who will be left without a partner? The boys there
will be four boys left with no partner. The girls are the limiting
factor, in other words, they control how many pairs will be
formed.
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- Limiting Reactant: Cookies 1 cup butter 1/2 cup white sugar 1
cup packed brown sugar 1 teaspoon vanilla extract 2 eggs 2 1/2 cups
all-purpose flour 1 teaspoon baking soda 1 teaspoon salt 2 cups
semisweet chocolate chips Makes 3 dozen If we had the specified
amount of all ingredients listed, could we make 4 dozen cookies?
What if we had 6 eggs and twice as much of everything else, could
we make 9 dozen cookies? What if we only had one egg, could we make
3 dozen cookies?
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- Limiting Reactant Most of the time in chemistry we have more of
one reactant than we need to completely use up other reactant. That
reactant is said to be in excess (there is too much). The other
reactant limits how much product we get. Once it runs out, the
reaction s. This is called the limiting reactant.
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- How do I Find The Limiting Reagent? Convert from grams of both
reactants into grams of product whichever one makes the least
amount is your limiting reagent.
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- Finding Excess Reagent Step 1: Take the limiting reagent and
convert from grams of the limiting reagent into grams of the excess
reagent. Step 2: Subtract grams of excess reagent actually used
from how much was available.
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- Percent Yield Percent yield is often found to give one a
picture of the accuracy of their work. Remember!!! Stoichiometry
gives you the theoretical yield, or what you should get in a
perfect world! Your actual yield is what you actually get back from
the experiment. Percent yield= (actual yield/theoretical yield) x
100