Chemical Equations
What is a reaction?
• A chemical reaction occurs when the bonds
between the outermost parts of atoms are
formed or broken
• Chemical reactions involve changes in matter,
the making of new materials with new
properties, and energy changes
• The equation for a reaction shows the
reactants and products as well as their relative
amounts within the reaction
Reactants
• The reactants in a chemical equation are the
atoms that react to form a different substance
• They will typically be on the left hand side of
the equation
• In word equations the reactants will also
typically be mentioned first
Example
• Iodine crystals react with chlorine gas to
produce iodine trichloride.
• The reactants are iodine crystals and chlorine
gas
I2 and Cl2
Products
• The products in a chemical reaction are what
is formed from the reaction
• In a chemical equation they will typically be
on the right
• In word equations they are usually listed after
the reactants to show what the reactants
“created” (created is in quotes for a reason)
Example
• We will use the same previous example:
Iodine crystals react with chlorine gas to
produce iodine trichloride.
• The product in this reaction would be iodine
trichloride
ICl3
Word Equations
• When writing a word equation, and eventually
a chemical equation, you will place reactants
on one side and products on the other
• Iodine crystals react with chlorine gas to
produce iodine trichloride.
In our example we would have end up having:
Iodine + Chlorine  Iodine trichloride
• The plus sign indicates that two items on that
particular side of the equation are separate
but within the same reaction
• The arrow indicates the direction of the
reaction
• There are also situations where there are two
arrows pointing in the opposite directions, but
we are not going into those yet
Practice
1. Sodium metal and chlorine gas react to form
sodium chloride
2. Iron metal and chlorine gas yield iron III
chloride
3. Water is formed when hydrogen and oxygen
gases react
4. Sodium chloride and lead II nitrate react to
produce lead II chloride and sodium nitrate
Answers
1. Sodium + Chlorine  Sodium Chloride
2. Iron + Chlorine  Iron III Chloride
3. Hydrogen + Oxygen  Water (dihydrogen
monoxide)
4. Sodium Chloride + Lead II Nitrate  Lead II
Chloride + Sodium Nitrate
Real chemical equations
• Chemists do not write out the words each
time for two reasons:
• First, its time consuming. Using shorthand
symbols like the chemical symbols we have
been using for the last few months is quicker
• Second, the chemical symbols tell us more
about the molecule and make spotting
reaction types often easier than when using
words
• So in order to write a word equation as a
chemical equation, all you simply have to do is
replace the words with their respective
chemical symbols
• Remember that there will be ionic
compounds, covalent compounds, and there
will be polyatomic ions present in many
equations as well. Do not forget how to
identify them and when to use parenthesis!
Example
• Iodine crystals react with chlorine gas to
produce iodine trichloride.
• The word equation was:
Iodine + Chlorine  Iodine trichloride
• This means our chemical equation will be:
I2 + Cl2  ICl3
Our practice examples
1. Sodium + Chlorine  Sodium Chloride
2. Iron + Chlorine  Iron III Chloride
3. Hydrogen + Oxygen  Water (dihydrogen
monoxide)
4. Sodium Chloride + Lead II Nitrate  Lead II
Chloride + Sodium Nitrate
Answers
1.
2.
3.
4.
Na + Cl2  NaCl
Fe + Cl2  FeCl3
H2 + O2  H2O
NaCl + Pb(NO3)2  PbCl2 + NaNO3
What looked Wrong?
• There was something off about those
equations
• Did you see what it was?
• Look again.
Answers
1.
2.
3.
4.
Na + Cl2  NaCl
Fe + Cl2  FeCl3
H2 + O2  H2O
NaCl + Pb(NO3)2  PbCl2 + NaNO3
• Did you see it that time?
• What was it?
Balancing an equation
• The law of conservation of matter states that
matter can neither be created nor destroyed
• This means that the previous chemical
equations were not correct due to their lack
conservation
• Some of them had one or two different
amounts of the same elements on the
opposite sides of the SAME EQUATION!
• This cannot happen
• When balancing an equation you can only
manipulate the coefficients
• Fe2O3 + 3 CO  3 CO2 + 2 Fe
• The underlined numbers are the coefficients
• You CANNOT CHANGE THE SUBSCRIPTS!
What is on one side must be on the
other
• This means you cannot have sodium on one
side and not on the other
• Back to our group example:
Iodine + Chlorine  Iodine trichloride
Which became:
I2 + Cl2  ICl3
Steps to Balancing Equations
1) Assemble the correct formulas for all the
reactants and products, using “+” and “→”
2) Count the number of atoms of each type
appearing on both sides
3) Balance the elements one at a time by adding
coefficients (the numbers in front) where you
need more - save balancing the H and O until
LAST!
(hint: Some prefer to save O until the very
last)
4) Double-Check to make sure it is balanced.
Balance it
• I2 + Cl2  ICl3
• First, we’ll balance out the chlorines by adding a
coefficient
• I2 + 3 Cl2  ICl3
• Still not balanced yet. We have six chlorines in
the reactant, but only 3 in the products. Add
another coefficient
• I2 + 3 Cl2  2 ICl3
Now count the atoms
• Reactants: 2 I and 6 Cl
• Products: 2 I and 6 Cl
Balanced!
Your Turn
1.
2.
3.
4.
Na + Cl2  NaCl
Fe + Cl2  FeCl3
H2 + O2  H2O
NaCl + Pb(NO3)2  PbCl2 + NaNO3
Answers
1.
2.
3.
4.
2 Na + Cl2  2 NaCl
2 Fe + 3 Cl2  2 FeCl3
2 H2 + O2  2 H2O
2 NaCl + Pb(NO3)2  PbCl2 + 2 NaNO3