Chemical Reactions

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Chemical Reactions

• A chemical reaction is a process in which one or
  more substances change to make one or more new
  substances.
• The chemical and physical properties of the
  substance will change.

• Signs of a chemical reaction
• Precipitate solid substance that is formed in a
  solution.
• Light
• Heat change
• Electricity
• Color change
• Gas formation

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Chemical Formulas

• The names of many covalent compounds use prefixes
  to express the number of atoms of each element.

mono- 1 Hexa- 6
di- 2 hepta- 7
tri- 3 octa- 8
tetra- 4 nona- 9
penta- 5 deca- 10
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Chemical Formulas 2

• In an ionic compound the name of metallic element
  is written first and ide is added to the
  nonmetal.

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Chemical Equations

• A chemical equation is a representation of a
  chemical reaction in which the reactants and
  products are expressed as formulas.
• Reactant Reactant ? Product
• Reactants are the substances that undergo the
  change.
• Products are the new substances formed.
• C O2 ? CO2
• Reactants ? Product

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2 3 5

• Antoine Lavoiser determined that the mass of the
  products is always equal to the mass of the
  reactants.
• The law of conservation of mass states that mass
  is neither created nor destroyed in a chemical
  reaction.
• In order to show mass is
• conserved an chemical
• equation must be balanced.

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Balanced Equations

• Equations need to be balanced.
• Matter can not be created nor destroyed. This
  means there needs to be the same number of atoms
  on each side of the equation.
• The equation below is balanced
• S8 12 O2?8 SO3
• Subscript The small numbers to the lower right
  of chemical symbols. Subscripts represent the
  number of atoms of each element in the molecule
• Coefficients - The large numbers in front of
  chemical formulas. Coefficients represent the
  number of molecules of the substance in the
  reaction

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How Many Atoms?

• Using coefficients and subscripts to count atoms
  in equations
• number of atoms coefficient X subscript
• Example How many of each type of atom are
  represented by 2Al2(SO4)3
• of Al atoms 2 X 2 4
• of S atoms 2 X 1 X 3 6
• of O atoms 2 X 4 X 3 24

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Types of Reactions

• A synthesis reaction is a reaction in which two
  or more substances form a single substance.
  (Simple ? Complex)
• A B ? AB

• A decomposition reaction is when a compound
  breaks down into two or more simpler substances.
  (Complex ? Simple)
• AB ? A B

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Types of Reactions II

• Single Displacement reaction in which one element
  takes the place of another element in a compound.
  (One person breaks up a couple and goes out with
  one of them. A BC? AC B

• Double Displacement reaction is one in which two
  different compounds exchange positive ions and
  form two new compounds.
• AB CD ? AD CB

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Energy Changes

• Chemical energy is the energy stored in the
  chemical bonds of a substance.
• Breaking a chemical bond requires energy.
• During a chemical reaction, energy is either
  gained or released.

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Conservation of Energy

• The law of conservation of energy states neither
  mass or energy can be created or destroyed in a
  chemical reaction.
• Energy can be transferred from one object to
  another.

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In/Out

• Endothermic Reaction More energy is required to
  break the bonds in the reactants than is released
  by the formation of the products.
• Endo Enters
• Examples
• Alka-seltzer
• Ice packets
• Camera

• Exothermic Reaction the energy released as the
  products form is greater than the energy required
  to break the bonds.
• Exo Exit
• Examples
• Fire
• Digestion
• Hand warmers

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Endothermic Exothermic
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Reaction Rate

• Reaction rates tell you how fast a reaction is
  going.
• Temperature Increasing the temperature
  generally increases the reaction rate.
• Stirring Increases the exposure of the reactant
  increases the reaction rate.

• Surface Area Increasing surface area increases
  the reaction rate.
• Concentration Increasing concentration
  increases the reaction rate.
• Catalyst A catalyst is a substance that affects
  the reaction rate with out being used up in the
  reaction.

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Get Started

• Many reactions occur at room temperature, but
  some need a little help to get started.
• You may need to add heat (Lighting a candle) or
  pressure (Diesel engine) to start the reaction.

• Activation Energy Energy needed to start a
  reaction. This is represented by an activation
  curve on a graph.

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Reaction Rate Catalyst

• You can speed up a reaction (rxn) rate by
• Increasing Temperature
• Increase Surface Area
• Increase Concentration
• Using a catalyst
• A catalyst is a substance that speeds up a
  reaction without being part of the reaction.

• A catalyst works by decreasing the amount of
  activation energy required for the reaction to
  occur.
• Remember, reactions require activation energy, a
  catalyst simply lowers the amount of energy.

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Catalyst Continued

• A catalyst is neither a reactant or a product in
  the reaction. When you finish the reaction all
  the catalyst is still there none was used or
  changed in the reaction. It is shown above the
  yield sign.

Without Catalyst
With Catalyst
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Inhibit

• An inhibitor is a substance that slows down or
  stopping a reaction may sometimes be useful.

• Examples
• Penicillin
• Preservatives
• Fungicide