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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 ... – PowerPoint PPT presentation

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Title: Chemical Reactions


1
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

2
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
3
Chemical Formulas 2
  • In an ionic compound the name of metallic element
    is written first and ide is added to the
    nonmetal.

4
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

5
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.

6
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

7
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

8
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

9
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

10
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.

11
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.

12
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

13
Endothermic Exothermic
14
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.

15
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.

16
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.

17
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
18
Inhibit
  • An inhibitor is a substance that slows down or
    stopping a reaction may sometimes be useful.
  • Examples
  • Penicillin
  • Preservatives
  • Fungicide
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