Stoichiometry

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Stoichiometry
(pronounced stoy-key-OM-uh-tree)

• derived from the Greek words stoicheion
  ("element") and metron ("measure").
• The mass relationships among reactants and
  products in a chemical reaction
• If we know the quantities of the starting
  materials (reactants) in a reaction, how much
  product will be formed?
• How much starting material must be used to obtain
  a specific amount of product?.

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A recipe for 20 cupcakes
1 cake mix, 2 cups of milk, and 3 eggs
1 mix 2 cups of milk 3 eggs 20 cupcakes
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"How many cupcakes can you bake for me?"
On looking in your cupboard, you find 2 cake
mixes, 4 cups of milk, and 3 eggs
1 cake mix, 2 cups of milk, and 3 eggs
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How much can we make ?
1 mix 2 cups of milk 3 eggs 20 cupcakes You
have 2 cake mixes, 4 cups of milk, 3 eggs
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Work in Dozens
1 mix 2 cups of milk 3 eggs 20
cupcakes Multiply both sides by 12 1 doz. Cake
mixes 2 doz. Cups of milk 3 doz. Eggs 20
doz. cupcakes
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Review

• Conservation of Mass
• In chemical reactions the quantity of matter does
  not change
• Balanced Chemical Equations
• same number and kind of atoms on both sides
• Chemical equations can be scaled
• as long as both sides are scaled the same
• We can work in units of moles and molar ratios

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4.1 Writing Chemical Equations
Balanced Equations
Describes a chemical reaction in which the
amounts of all elements and of electrical charge
are conserved
N2(g) 3 H2(g) ? 2 NH3(g)
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4.1 Writing Chemical Equations
Balanced Equations
N2(g) 3 H2(g) ? 2 NH3(g)
All balanced chemical equations have the
following features

• The reactants appear on the left, and the
  products appear on the right. The arrow joining
  them indicates the direction of reaction.

2. Stoichiometric coefficients, integers, precede
the formula of each substance. When no number
appears, the stoichiometric coefficient is 1.
3. The stoichiometric coefficients in a chemical
equation are the smallest integers that give a
balanced equation.
4. Charge is conserved.
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4.1 Writing Chemical Equations
Balanced Equations
The numbers of atoms of each element is conserved
in any chemical reaction.
N2(g) 3 H2(g) ? 2 NH3(g)
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6 steps to solving Stoichiometry problems

• Define the objective
• Write down the balanced chemical equation

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4.1 Writing Chemical Equations
Balancing Equations
To determine if an equation is balanced, make a
list of the elements of atoms on each side
C3H8 O2 ? CO2 H2O
3 C 8 H 2 O ? 1 C 2 H 3 O
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4.1 Writing Chemical Equations
Balancing Equations
To begin balancing the equation, start with an
element that appears in only one substance on
each side.
C3H8 O2 ? CO2 H2O
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3 C 8 H 2 O ? 3 C 8 H 10 O
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4.1 Writing Chemical Equations
Balancing Equations
Finish the balancing.
C3H8 O2 ? 3 CO2 4 H2O
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3 C 8 H 10 O ? 3 C 8 H 10 O
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Balancing Chemical Equations

• Begin with the compound with the most atoms or
  kinds of atoms
• start with one atom of that compound
• Balance atoms that appear once on each side first
• Balance atoms that appear more than once next
• Balance free elements next
• Scale all numbers so only integers remain

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Balancing equations example
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Box 4-1 Pg 143 1800s 1 acre
25 bushels of corn 1980s 1 acre 110 bushels of
corn
Courtesy Cargill, Inc.
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Box 4-1 Pg 143
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April 19, 1995. Alfred P. Murrah Building is
bombed. The bombing was caused solely by a single
truck bomb consisting of 4,800 pounds of ammonium
nitrate transported to the location in a Ryder
truck which was parked outside the building.
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Example 4.1 Pg 138
To get the oxygen balanced on the product side,
we must use ½ O2 !
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Example 4-1
We get rid of the ½ by multiplying the equation
by two.
2 NH4NO3 2
N2 4 H2O O2 4 N 8 H 6 O
4 N 8 H 6 O
Pg 138
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Practice in small groups4.1.1, 4.1.2