Examining and Quantifying Chemical Reactions

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Examining and Quantifying Chemical Reactions

• Day 10

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Chemical Equations

• Chemical change involves a reorganization of the
  atoms in one or more substances.

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The Hindenburg Reaction

• Reactants are on left, products to the right.
• Arrow indicates the change

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A chemical equation

• Notice that the formula for the molecules is
  unchanged.
• To indicate that more than one molecule is
  required, use coefficients (shown in blue).
• The equation is balanced.
• 2 H2 O2 ? 2 H2O

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The Rules

• Change only the coefficients to balance the
  chemical equation.
• Balance one element at a time.
• Continue to balance the other elements in the
  same manner.
• Adjust coefficients to the lowest whole number
  common multiple.
• Check your answers.

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Example

• FeI2 Cl2 ? FeCl3 I2

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Balancing Combustion Reactions

• Balance carbon first
• Hydrogen second
• Oxygen last
• Multiply by 2 if you need to.
• ___ C3H8 ___ O2 ? ___ CO2 ___ H2O

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More Balancing and Things to Avoid

• ___ C3H8 ___ O2 ? ___ CO2 ___ H2O
• Coefficients should always have the lowest whole
  number ratio.
• Never Change the subscripts. 8 CO2 is not the
  same as C8O16.
• ___ CH4 ___ O2 ? ___ CO2 ___ H2O

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A Pictorial Representation
more help
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Quantifying Chemical Reactions
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The mole (6.02 x 1023)

• A macroscopic version of the molecule defined so
  we can use the periodic table for the macroscopic
  and microscopic world.
• Defined as the number of atoms in exactly 12.0 g
  of carbon-12 isotope.
• This is number is called Avogadros Number after
  the Italian physicist Amedeo Avogadro
  (1776-1856).

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How Big is a Mole
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Formula Weight

• The sum of the atomic weights of all the atoms in
  the molecular formula, whether ionic or
  molecular.
• Expressed in amu/molecule or grams/mole.
• Also called molecular weight, molar mass

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Example

• What is the molecular weight of Urea, (NH2)2CO (a
  molecular substance)?
• Solution
• 2 x N 2 x 14.0067 28.0134
• 1 x C 1 x 12.0111 12.0111
• 4 x H 4 x 1.00794 4.03176
• 1 x O 1 x 15.9994 15.9994
• TOTAL 60.0556 amu/molecule
• OR
• 60.0556 g/mole

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Two major conversion factors

• Avagadros number inter-converts the microscopic
  and macroscopic worlds
• formula weight inter-converts grams and moles.

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A silly problem

• How many molecules are in one drop (0.0500 g) of
  water? (4.25 b)

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Weight relationships in reactions

• Stoichiometry - Study of the mass relationship
  between reactants and product in chemical
  reactions.
• Always start with a balanced chemical equation!
• Convert grams of A to moles of A. (Use FW)
• Convert moles of A to moles of B. (Use Eq.)
• Convert moles of B to grams of B. (Use FW)

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Stoichiometry
Grams of other reactant required OR Grams product
formed
Grams of one reactant
X
Step 1. Use the Formula Weight
Step 3. Use the Formula Weight
Moles of other reactant required OR Moles product
formed
Moles of reactant
Step 2. Use the Balanced Equation
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How many grams of CO2 are produced from the
combustion of 12 grams of CH4?
CH4 2 O2 ? CO2 2 H2O
bal eq
moles
A three step process
fw
fw
grams
12.0 g
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How much Al is required to react with 55.0 g of
Fe2O3? (Thermite)
Fe2O3 (s) 2 Al (s) ? Al2O3 (s) 2 Fe (l)
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Describing phases

• (s) solid
• (l) liquid
• (g) gas
• (aq) aqueous (dissolved in water)

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Types of reactions
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Reactions in Water (Aqueous)

• In water, when ionic compounds dissolve, they
  dissociate into their anions and cations.
• It is the polarity of water that allows them to
  do this.
• Some compounds are soluble and others are not.
• Water is very stable

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Ionic compounds in water