CHEMICAL%20REACTIONS

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CHEMICAL REACTIONS

• Balancing, States of Matter, and Writing

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Evidence for A Chemical Reaction

• Five Signs of a Chemical Change
• Gas Given Off (Bubbles)
• Color Change (BIGGIE!!!!!!)
• Heat is Produced or Absorbed
• Odor Change
• Precipitate Forms (Solid is Made)

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Chemical Equations

• Chemical Equations are used to represent chemical
  reactions.
• Reactants ? Products
• 127. Law of Conservation of Matter (Mass) States
  Matter cannot be created or destroyed during a
  chemical reaction.
• We must balance chemical reactions (add
  coefficients) to comply with the law.

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Chemical Equations (Cont)

• Example
• CH4 O2 ? CO2 H2O
• Methane Oxygen ? Carbon Water
• Dioxide
• Reactants Products
• 1 C 1 C
• 4 H 2 H
• 2 O 3 O

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Balancing

• Chemical reactions change the groupings of the
  atoms, but DO NOT create or destroy atoms, so
  there must be the same number of each type of
  atom on each side of the arrow. We must use
  coefficients to Balance a Chemical Equation

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Balancing Chemical Equations

1. Determine the correct formula for each reactant
   and product by supplying the subscripts to the
   formulas.
2. Once you write the correct formula, DO NOT change
   the subscripts.
3. Add coefficients in front of balanced formulas to
   give the same number and kind of atoms on both
   sides.
4. Use the lowest possible ratio of coefficients.

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Example

• CH4 2 O2 ? CO2 2 H2O
• If I place a 2 in front of the O2 and the H2O,
  then I end up with
• Reactants Products
• 1 C 1 C
• 4 H 4 H
• 4 O 4 O

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You Try this One

• C2H4 O2 ? CO2 H2O
• 2 C - 1
• 4 H - 2
• 2 O - 3

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• C2H4 3O2 ? 2CO2 2H2O
• 2 C - 2
• 4 H - 4
• 6 O - 6

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Another Example

• C3H6 O2 ? CO2 H2O
• 3 C - 1
• 6 H - 2
• 4 O 3

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• 2 C3H6 9 O2 ? 6 CO2 6 H2O
• 6 C - 6
• 12 H - 12
• 18 O 18

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You Try This One

• What would the coefficients be for this reaction
• PCl5 H2O ? H3PO4 HCl

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Physical States

• Symbols in parentheses after each formula
• Solid (s)
• Liquid (l)
• Gas (g)
• Aqueous (aq)
• Aqueous means dissolved in water.
• Example
• 2 H2O2 (aq) ? 2 H2O (l) O2 (g)

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Words to watch for

• States of matter This is a symbol that is used
  to identify the state of matter for each
  substance in a reaction.
• Gas, Liquid, Solid, Aqueous
• Gas (g)
• Liquid (l)
• Solid (s) may also be called a Precipitant (ppt)
  or a crystal (c)
• Aqueous (aq) means dissolved in water.