; Science 9_ Unit B – Matter and Chemical Change
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Science 9_ Unit B – Matter and Chemical Change

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									Science 9: Unit B – Matter
 and Chemical Change
    Topic 7: Writing Chemical
            Equations
              Background
n   All compounds are created, changed, or
    broken apart through chemical reactions.
n   Signs that a chemical change have occurred
    include a color change, bubbling, a
    PRECIPITATE (solid) forming in a liquid, and
    heat and light being produced or absorbed.
n   In a chemical reaction, the BONDS between
    atoms are broken and new bonds are formed
    so that atoms are rearranged to form new
    compounds.
n   Eg. hydrogen + oxygen = water
n   2H2 + O2 → 2H2O
n   or: H-H + H-H + O-O = H-O-H + H-O-H
        Reactants and Products
n   In chemical reactions, the chemicals that
    undergo the reaction are called the reactants.
    These are your original materials. In the above
    example hydrogen and oxygen are your
    reactants.
n   The chemicals that come out of a chemical
    reaction are called the products. In the above
    example, two water molecules are the product.
n   Note that when we write down a chemical
    reaction, we are writing down the lowest RATIO
    of molecules that will react together. In reality,
    billions of molecules are reacting with each
    other.
    Signs of a Chemical Reaction
n   Products are new chemicals with different
    chemical properties than the reactants
    that make them. Water is a liquid at room
    temperature; hydrogen and oxygen are
    gases at room temperature.
n   There are signs that indicate a chemical
    reaction has taken place: color change,
    bubbles forming, heat absorbed or given
    off, and a new material formed all point
    towards a chemical reaction.
     Writing Chemical Reactions
n   To describe what occurs during a chemical
    reaction, scientists use chemical
    equations.
n   Chemical equations involve the following
    parts:
 The Parts of a Chemical Reaction
1. The reactants on the left side, using the
    symbols of elements for each atom. These
    symbols are the same ones from the periodic
    table. For each molecule indicate in brackets
    behind it whether it is a solid (s), liquid (l), or
    gas (g). An ‘+’ is used to separate the different
    reactants.
2. An arrow pointing to the right separating the
    reactants from the products.
3. The products on the right side of the equation
    are labeled in the same manner as the
    reactants.
n   Eg.     CH4(g) + O2(g) → CO2(g) + H2O(l)
          Exothermic and Endothermic
                  Reactions
n   Exothermic reactions RELEASE energy when reacting.
n   Reactants → Products + Energy
n   Endothermic reactions ABSORB energy from the surrounding
    environment.
n   Reactants + Energy → Products
n   An exothermic example is the starting of a barbecue involves
    the igniting of propane and oxygen. This releases a large
    amount of energy.
n   C3H8(l) + 5O2(g) → 3CO2(g) + 4H2O(g) + Energy
n   An endothermic reaction example is the splitting up of water
    into hydrogen and oxygen molecules using an electric current.
n   2H2O(l) + Energy → 2H2(g) + O2(g)
        An Important Note about
      Endothermic and Exothermic
              Reactions
n   An important note is that if a reaction is
    exothermic then the products are MORE
    stable than the reactants. If the reaction is
    endothermic the products are LESS stable
    than the reactants. Since all elements are
    trying to become more stable exothermic
    reactions are more natural while
    endothermic reactions need an outside
    energy source.

								
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