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Chemical Reactions

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Title: Chemical Reactions


1
Chapter 9
  • Chemical Reactions

2
9.1 Reactions Equations
  • Objectives
  • Recognize evidence of chemical change
  • Represent chemical reactions with equations
  • Balance chemical equations

3
What is a chemical reaction?
  • It is the process by which the atoms of one or
    more substances are rearranged to form new,
    different substances.
  • A chemical reaction is a chemical change!

4
The 4 signs
  • There are 4 kinds of observations that indicate a
    chemical reaction may have taken place.
  • A change in energy A
  • temperature change is often
  • used to determine if
  • energy was released
  • or absorbed. In addition,
  • release of light also indicates
  • an energy change.

5
The 4 signs
  • 2. A color change

6
The 4 signs
  • 3. Production of a gas may be observed as
  • bubbling or visible fumes. Detection of an
  • odor also indicates a gas was produced.

7
The 4 signs
  • 4. Formation of a precipitate (or appearance
  • of a solid) in a solution that was
  • previously clear.

8
Practice
  • Recall burning magnesium as part of a lab
    activity. What evidence was there that a
    chemical reaction occurred?

9
Review
  • Equations represent chemical reactions.
  • Reactants (on the
    left) are the starting
    substances.
  • Products (on the
    right) are the
    substances formed
    during the reaction.

10
Review
  • An arrow shows the way the reaction progresses
    It is found between the reactants and products
    and is usually read as yields.
  • signs are placed between each reactant and
    product They are read as and.

11
Review
  • Symbols are often used after each reactant and
    product to indicate their state.
  • (s) for solid - (g) for gas
  • (l) for liquid - (aq) for water
    solution

12
Review
  • Coefficients are
    whole numbers
    written in front
    of formulas in the
    equation. They
    indicate the
    number of atoms,
    molecules, or ions
    of that
    substance
    that are involved in the reaction.

13
Practice
  • Identify the reactants and products and, using
    words not formulas, interpret the following
    equations.
  • Mg (s) H2SO4 (aq) ? H2 (g) MgSO4 (aq)
  • C(s) 2ZnO (s) ? 2Zn (s) CO2 (g)

14
Types of Equations
  • Word Equation words are used to summarize the
    reaction.
  • Ex. iron chlorine iron (III)
    chloride
  • Ex. carbon sulfur carbon disulfide
  • Skeleton equation Chemical formulas are
    substituted for words.
  • Ex. Fe Cl2 FeCl3
  • Ex. C S CS2

15
Skeleton Equations
  • Skeleton equations are also called unbalanced
    equations because they do not obey the Law of
    Conservation of Mass.
  • Fe Cl2 FeCl3

  • 1 iron atom 2 chlorine atoms

  • 1 iron atom

  • 3 chlorine atoms

16
Practice Problems
  • Write skeleton equations for the following
  • - hydrogen bromine hydrogen bromide
  • - carbon monoxide oxygen carbon dioxide
  • - potassium chlorate potassium chloride

  • oxygen
  • 4 Manganese metal reacts with sulfuric acid to
    produce manganese (II) sulfate and hydrogen gas.
  • 5 Silver nitride is produced when silver reacts
    with atmospheric nitrogen.

17
Types of Equations
  • Balanced chemical equation This equation shows
    that the number of atoms of each reactant and
    each product is equal on both sides of the arrow.
  • 2Fe 3Cl2 2FeCl3
  • 2
    iron atoms
  • 2 iron atoms 6 chlorine atoms
    6 chlorine atoms

18
Balanced Chemical Equation
  • This is defined as a statement that uses chemical
    formulas to show the identities and relative
    amounts of the substances involved in a reaction.
  • Balanced chemical equations obey the Law of
    Conservation of Mass.

19
Steps to Balancing Equations
  • - Write a skeleton equation.
  • Problem Hydrogen and chlorine combine
  • to form hydrogen chloride
  • H2 Cl2 HCl
  • Recall there are 7 elements that
  • exist as diatomic molecules.

20
Steps to Balancing Equations
  • - Determine the number and kinds of atoms in the
    reactants.
  • - Determine the number and kinds of atoms in the
    products.
  • H2 Cl2 HCl
  • 2 H 1 H
  • 2 Cl 1 Cl

21
Steps to Balancing Equations
  • - Change the coefficients (never the subscripts)
    until the number of atoms of each element is
    equal on both sides of the equation.
  • H2 Cl2 2HCl
  • 2 H 2 1 H
  • 2 Cl 2 1 Cl

22
Steps to Balancing Equations
  • - Make sure the coefficients are in the lowest
    possible ratio. Reduce if necessary.
  • - Go back and add the atoms up again. Make sure
    the number of atoms of each element is equal on
    both sides of the arrow.
  • If you cannot balance the equation, go
    back and make sure your formulas are written
    correctly!!

23
Practice Problems
  • - Sodium hydroxide and calcium bromide react to
    produce solid calcium hydroxide and sodium
    bromide.
  • - Carbon disulfide reacts with oxygen gas to
    produce carbon dioxide and sulfur dioxide.
  • - Zinc and sulfuric acid react to produce
    hydrogen gas and zinc sulfate.
  • - Iron (III) chloride reacts with sodium
    hydroxide to produce iron (III) hydroxide and
    sodium chloride.
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