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Chemical Reactions Section 1: Observing Chemical Changes How can changes in matter be described?

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Title: Chemical Reactions Section 1: Observing Chemical Changes How can changes in matter be described?


1
Chemical Reactions Section 1 Observing Chemical
Changes How can changes in matter be described?
  • In terms of two kinds of properties- physical
    properties and chemical properties
  • Changes in matter can be described in terms of
    physical or chemical changes

2
What is a physical property?
  • A characteristic of a substance that can be
    observed without changing the substance into
    another substance
  • Examples ice melting, color, hardness, texture,
    shine, etc.

3
What is a chemical property?
  • A characteristic of a substance that describes
    its ability to change into other substances
  • Examples burning magnesium, rusting
  • Substances that undergo chemical changes
    reactants
  • The new substance formed - product

4
What is the difference between a physical and a
chemical reaction?
  • A change in matter that produces one or more new
    substances is a chemical change, or chemical
    reaction
  • Physical change examples braiding your hair,
    squashing a marshmallow
  • Chemical change examples burning of gasoline,
    burning a marshmallow

5
How can you tell when a chemical reaction occurs?
  • Chemical changes occur when bonds break and new
    bonds form
  • Chemical reactions involve two main kinds of
    changes that you can observe-formation of new
    substances and changes in energy

6
What are some changes in properties that indicate
a chemical change?
  • Precipitate a solid that forms from a solution
    during a chemical reaction
  • Color change may indicate a chemical reaction
  • Gas production bubbles

7
How do you categorize changes in energy?
  • Endothermic A reaction in which energy is
    absorbed
  • More energy is required to break the bond of the
    reactants than is released by the formation of
    the new products
  • Examples baking soda and vinegar gets cooler
    when combined
  • Exothermic A reaction that releases energy in
    the form of heat
  • The energy released as the products form is
    greater than the energy required to break the
    bonds of the reactants
  • Examples burning of airplane fuel

8
  • endothermic
  • exothermic

9
Section 2 Describing chemical reactions What
information does a chemical equation contain?
  • Chemical equations use chemical formulas and
    other symbols instead of words to summarize a
    reaction
  • Reactants substances you have at the beginning
  • Products new substances produced when the
    reaction is complete

10
What does the principle of conservation of mass
state?
  • That in a chemical reaction, the total mass of
    the reactants must equal the total mass of the
    products
  • Matter is neither created nor destroyed

11
What does open or closed system mean?
  • Open system matter can enter from or escape to
    the surroundings
  • Closed system matter is not allowed to enter or
    leave

12
What must a balanced chemical equation show?
  • The same number of each type of atom on both
    sides of the equation
  • Represents the conservation of mass
  • Coefficients a number placed in front of a
    chemical formula in an equation that tells you
    how many atoms or molecules of a reactant or
    product take place in the reaction

13
How do you balance chemical equations?
  • Count the of atoms of each element in the
    reactants products
  • ID element that is not equal on both sides
  • Add coefficient to the front of the formula that
    will make the on both sides for that element

14
Sample Problems
Reactants Products
1 Zn 1 Zn
1 H 2 H
1 Br 2 Br
  • Sample Problem
  • Zn HBr ? H2 ZnBr2
  • Multiply HBr x 2 to correct (balance)
  • Zn 2HBr ? H2 ZnBr2

Reactants Products
1 Zn 1 Zn
2 H 2 H
2 Br 2 Br
15
Try these!!
2
2
  • Na Cl2 ? NaCl
  • Ca Cl2 ? CaCl2
  • H2O ? H2 O2
  • N2 H2 ? NH3
  • Al2O3 ? Al O2
  • P4 O2 ? P4O6
  • Fe H2O ? Fe3O4 H2

2
2
3
2
4
3
2
3
3
4
4
16
What are the three categories of chemical
reactions?
  • Synthesis
  • Decomposition
  • Replacement

17
What is a synthesis reaction?
  • When two or more substances combine to make a
    more complex substance
  • A B ? AB
  • 2H2 O2 ?2H2O
  • Example hydrogen and oxygen to make water

18
What is a Decomposition reaction?
  • Breaking down compounds into simpler products
  • AB ? A B
  • 2H2O2 ? 2H2O O2
  • Example Hydrogen peroxide decomposes into water
    and oxygen gas

19
What is a replacement reaction?
  • When one element replaces another in a compound,
    or when two elements in different compounds trade
    places
  • Example copper metal obtained by heating copper
    oxide with carbon
  • Single (one element replaces another)
  • AB C ? AC B
  • Zn 2HCl ? ZnCl2 H2
  • or double (elements appear to trade places with
    another compound)
  • AB CD ? AC BD
  • NaCl AgF ? NaF AgCl

20
Section 3 Controlling Chemical Reactions How is
activation energy related to chemical reactions?
  • The minimum amount of energy needed to start a
    chemical reaction
  • All chemical reactions need a certain amount of
    activation energy to get started

21
What factors affect the rate of a chemical
reaction?
  • Surface area the greater the surface area that
    faster the reaction (ex. Chewing)
  • Temperature raising the temperature causes
    particles to move faster and therefore have more
    energy they also come into contact more often
    lowering temperature slows things down
  • Concentration amount of substance in a given
    volume increased concentration-increased
    reaction
  • Catalysts increases the rate of a reaction by
    decreasing the energy needed to start
  • Enzymes biological catalysts
  • Inhibitors material used to decrease the rate
    of reaction (ex. preservatives in food)
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