Science 9: Unit B – Matter
and Chemical Change
Topic 7: Writing Chemical
Equations
Background
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All compounds are created, changed, or
broken apart through chemical reactions.
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.
In a chemical reaction, the BONDS between
atoms are broken and new bonds are formed
so that atoms are rearranged to form new
compounds.
Eg. hydrogen + oxygen = water
2H2 + O2 → 2H2O
or: H-H + H-H + O-O = H-O-H + H-O-H
Reactants and Products
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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.
The chemicals that come out of a chemical
reaction are called the products. In the above
example, two water molecules are the product.
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
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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.
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
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To describe what occurs during a chemical
reaction, scientists use chemical
equations.
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.
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Eg.
CH4(g) + O2(g) → CO2(g) + H2O(l)
Exothermic and Endothermic
Reactions
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Exothermic reactions RELEASE energy when reacting.
Reactants → Products + Energy
Endothermic reactions ABSORB energy from the surrounding
environment.
Reactants + Energy → Products
An exothermic example is the starting of a barbecue involves
the igniting of propane and oxygen. This releases a large
amount of energy.
C3H8(l) + 5O2(g) → 3CO2(g) + 4H2O(g) + Energy
An endothermic reaction example is the splitting up of water
into hydrogen and oxygen molecules using an electric current.
2H2O(l) + Energy → 2H2(g) + O2(g)
An Important Note about
Endothermic and Exothermic
Reactions
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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.