Chapter 4.3
Chemical Reactions
Chemical Reactions
Chemical Change – the transformation of one or
more substances into different substances
with different properties
Chemical Reactions
-include a change in energy energy is
absorbed or releases (heat, light , sound)
-occur at different rates: some are fast others
are slow
Chemical Equations
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Represent chemical reactions by using
words or symbols and formulas
Starting materials are called “reactants”
New substances are called “products”
An arrrow () is read as “produces”
States of matter appear in brackets
(s)=solid (l)=liquid (g)=gas (aq) dissolved in water
Word Equations
Use words to represent reactions
 Use all of your naming rules to do this!
Ex.
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Magnesium + Hydrochloric acid  Magnesium chloride + Hydrogen
Formula Equations
Use formulas to represent reactions
 Use your formula rules to do this!
 Notice states of matter are always included
 Coefficients are used to show the ratios of the
different substances present in the reaction
Ex.
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Mg (s) + 2HCl (aq)  MgCl2 (aq) + H2 (g)
Law of Conservation of Mass
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States that in a chemical reaction, the
mass of the products ALWAYS EQUALS
the mass of the reactants since
elements cannot be created or
destroyed in a chemical reaction, they
can only be rearranged!
Counting Atoms
Rules:
1.
Just a symbol = 1 atom
Ex. N is one atom of nitrogen
2.
3.
Subscripts tell you the number of atoms
Ex. H2 is two atoms of hydrogen
Coefficients mean to multiply everything after it by
that number
Ex. 2 MgCl2 means 2x1 = 2 atoms of Mg
2x2 = 4 atoms of Cl
Counting Atoms
4. Brackets mean to multiply everything
inside by the subscript number outside
the brackets
Ex. Al2(SO4)3
2= atoms Al
3x1 =3 atoms S
4X3 = 12 atoms O
Counting Atoms Examples
Count the atoms in the following
examples
a.
BaCl2
b.
3 BeF2
c.
Al (NO3)3
d.
4 Mg3PO3
Homework
Counting Atoms Worksheet
Skeleton Equation
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An unbalanced equation that shows all
reactants and products in a chemical
reaction written as formulas
Rules:
-write the symbol of a metal element that
is NOT in a compound ex. “Iron Nail” is
Fe
Skeleton Equations
-If the non-metal is part of Hockey stick and
puck, write it as a diatomic molecule
H2, N2, O2, F2, Cl2, Br2, I2
Ex. Oxygen gas reacts with fluorine gas
O2 + F 2
- Write the formula of any other compound
using the rules you have learned
Example of Skeleton Equations
Ex. Write the skeleton equation for the
reaction of sodium chloride with
Magnesium phosphate to produce
magnesium chloride and sodium
phosphate
Balanced Equations
Include coefficients infront of chemical
formulas to ensure that the number of
each element is the same on both sides
of the equation
WHY??????
You must follow the Law of Conservation
of mass!
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Balancing Rules
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Balance polyatomic ions first if the same
ion appears on both sides of the
equation
Balance all other elements except H an
O
Balance H
Balance O
Examples