REACTIONS CONSERVE MASS
• The Law of Conservation of Mass
• States that in ordinary chemical or physical changes,
mass is neither created nor destroyed.
• React vinegar and baking soda
• Produces a gas (which “floats” away).
• The products including this gas, if captured, is the
same mass per mole as the reactants consumed.
REACTIONS REARRANGE ATOMS
• Products and reactants of a reaction are made up of
the same number and types of atoms.
• The molecules may change but the atoms within them
do not.
• If you have H, O, and C are the reactant side of a
reaction, you must have ___ , ___ , and ____ on the
product side.
• The ONLY thing we can change is the quantity of
MOLECULES, or the number of moles of molecules
CONSIDER THE FOLLOWING:
• Sodium metal reacts with water to yield sodium
hydroxide and hydrogen gas.
2Na + 2H2O  2NaOH + 1H2
• What are the reactants in this reaction? What are the
products?
• Reactants:
• Products:
• How many of each atom? Na: ____ , H: ____ , O: ____
BALANCING EQUATIONS
• To show that a reaction satisfies the law of
conservation of mass, its equation must be
BALANCED.
Notice the “Numbers” out front in the last
reaction
• As we stated before, we can change ONLY the number
of moles of molecules present
• Both of the reactants and products
• We change this by adding a coefficient out front of the
reactant or product.
STOICHIOMETRIC COEFFICIENTS
• To balance an equation, you need to make the number if
atoms for each element the same on the reactants’ side
and on the products’ side.
• You CANNOT change the formulas of any of the substances.
• Only change the COEFFICIENT
• A small whole number that appears as a factor in front
of a formula in a chemical equation.
MORE ON COEFFICIENTS
• H2O represents water (2 H, 1 O)
• 2 H20 represents two moles of water (4 H, 2 O)
• H202 represents hydrogen peroxide (2 H, 2 O)
• A small change of the subscripts within the molecule
will change the compound completely AND alter the
entire reaction.
MORE ON COEFFICIENTS
• Determine the number of each atom in the following
examples (just give the number of each atom) :
• _1_Fe2O3 + _3_H2 
• _3_ Ca(NO3)2 +
• _1_ C6H12O6 + _6_ O2  _6_ H20 + _6_ CO2
RULES TO CONSIDER WHEN BALANCING
1. Identify reactants and products
• If no equation is provided, identify the reactants and
products and write an unbalanced equation for the
reaction. (You may find it helpful to write a word
equation first.)
• If not all chemicals are described in the problem, try
to predict the missing chemicals based on the type of
reaction.
RULES TO CONSIDER WHEN BALANCING
2. Count atoms
• Count the number of atoms of each element in the
reactants and in the products, and record the results
in a table.
• Identify elements that appear in only one reactant
and in only one product, and balance the atoms of
those elements first. Delay the balancing of atoms
(often hydrogen and oxygen) that appear in more that
one reactant or product.
• If a polyatomic ion appears on both sides of the
equation, treat it as a single unit in you counts.
RULES TO CONSIDER WHEN BALANCING
3. Insert coefficients
• Balance atoms one element at a time by inserting
coefficients.
• Count the atoms of each element frequently as you
try different coefficients. Watch for elements whose
atoms become unbalanced as a result of your work.
• Try the odd-even technique (double the odd number)
if you see an even number of a particular atom on
one side of an equation and an odd number of that
atom on the other side.
RULES TO CONSIDER WHEN BALANCING
4. Verify your results
• Double-check to be sure that the numbers of atoms
of each element are equal on both sides of the
equation. Make sure your own work did not mess any
“previously balanced” atoms.
PRACTICE:
• Balance the following example:
• Iron (III) oxide and hydrogen react to yield iron and water.
___Fe2O3 + ___H2  ___Fe + ___H2O
Put numerical “stoichiometric coefficients” out front on the lines
to make the elements equal out
(make the left be the same as the right of the arrow)
PRACTICE:
• Balance the following examples:
___P4 + ___O2  ___P2O5
___C3H8 + ___O2  ___CO2 + ___H2O
NEVER CHANGE SUBSCRIPTS TO BALANCE A
REACTION!!!
TRY THE FOLLOWING
___NH3 + ___O2  ___NO + ___ H2O
Iron (II) hydroxide reacts with hydrogen peroxide to yield
iron (III) hydroxide.
POLYATOMICS
• Polyatomics are no different.
• Let’s Practice 
• ___HgCl2 + ___AgNO3  ___Hg(NO3)2 + ___AgCl
• Calcium phosphate and water are produced when
calcium hydroxide reacts with phosphoric acid.
___Ca(OH)2 + ___ H3PO4  ___Ca3(PO4)2 + ___H2O
PRACTICE, PRACTICE, PRACTICE
This will only get better with practice. You will soon spot
clues and patterns that will allow you to balance
reactions almost instantly.
Be patient, it will come!!