5 3 lecture

5.3 Balancing Chemical Equations

Pages 161 - 165

Goal 1

Describe and balance chemical equations

• Word equation

Methane + Oxygen Carbon dioxide + Water

Reactants Products

• Chemical Equations– Balanced to show conservation of mass– #Atoms per element go in = # Atoms per

element that come out

• Chemical equationsUse chemical formulas and coefficients

CH4 + 2O2 CO2 + 2H2O

• Coefficients used to show– # of molecules/compounds – # of moles– Multiply subscripts by coefficients to

determine the # of atoms

CH4 + 2O2 CO2 + 2H2O

• C =

• H =

• O =

• C =

• H =

• O =

CH4 + 2O2 CO2 + 2H2O

# of atoms on left MUST = # of atoms on right

Is this equation balanced?

CH4 + 2O2 CO2 + 2H2O

Yes! It’s balanced

• H =

• O =

• H =

• O =

H2 + O2 H2O

# of atoms on left MUST = # of atoms on right

Is this equation balanced?

Uh-Oh!

• H =

• O =

• H =

• O =

H2 + O2 H2O

# of atoms on left MUST = # of atoms on right

How can we balance it?

• Rules for balancing chemical equations

– 1. Only change coefficients

– 2. Never never never ever change a subscript

– 3. Multiply subscripts by coefficients

– 4. Compounds FIRST

– 5. Elements LAST

• H =

• O =

• H =

• O =

__ H2 + __ O2 __ H2O

# of atoms on left MUST = # of atoms on right

Try it…

• H = 4 atoms

• O = 2 atoms

• H = 4 atoms

• O = 2 atoms

_2_ H2 + __ O2 _2_ H2O

# of atoms on left MUST = # of atoms on right

Try it…

• Mg =

• O =

• Mg =

• O =

__ Mg + __ O2 __ MgO

# of atoms on left MUST = # of atoms on right

Let’s try it again…

• Mg = 2 atoms

• O = 2 atoms

• Mg = 2 atoms

• O = 2 atoms

_2_ Mg + __ O2 _2_ MgO

# of atoms on left MUST = # of atoms on right

How did you do?

• C =

• H =

• C =

• H =

__ CH4 __ C3H8 + __ H2

# of atoms on left MUST = # of atoms on right

Let’s try it one more time…

• C =

• H =

• C =

• H =

_3_ CH4 __ C3H8 + _2_ H2

# of atoms on left MUST = # of atoms on right

How did you do?