Transcript How to Balance an Equation

How to Balance an Equation
2Mg + O2 → 2MgO
Step 1: Starting Out
• Balancing equations is
very simple with some
basic tools.
• When starting to balance
an equation you should
make a list of all of the
elements in the equation
and identify the number
of each element in that
equation on both the
reactant and product
side.
Mg + O2 → MgO
Reactants Products
Mg
1
1
O
2
1
Step 2
• Once you have done step one
you must decide if the equation
is balanced or not. If the
number of elements on each
side of the equation are equal
then the equation is balanced.
• If they are not then you will use
coefficients to balance the
equation. Coefficients are
large numbers that indicate
that we have multiples of that
compound.
• The coefficients will be
multiplied through each
compound and change the
number of elements in that
compound.
Mg + O2 → MgO
Mg
O
Reactants
1
2
Not balanced.
Products
1
1
Step 2: continued
• This will affect the number you
have written in your chart.
• To start, pick an element that
is not equal in your chart.
Once you’ve chosen the
element decide whether the
reactants or products should
be changed. Remember, we
can only multiply to get bigger,
we cannot divide to get
smaller.
• Once you add the coefficient it
changes not just the element
you were looking at but also all
other element attached to it.
Mg + O2 → 2MgO
Mg
O
Reactants
1
2
Products
1 2
1 2
Step 3
•
•
•
•
Once you have changed one set
of elements, you must change
your chart to show the current
number of elements now in the
equation.
This will often affect if your
equation is balanced or not, if it is
balanced then you are done and
do not need to continue.
If it is not balanced then you will
need to add another coefficient to
continue to balance the equation.
In this example we added a 2 to
the product which balanced the O
but did not balance the Mg so we
must go back to the reactants and
place a coefficient in front of the
Mg to balance that element.
2 Mg + O2 → 2MgO
Mg
O
Reactants
2
1
2
Products
1 2
1
2
Step 4: Final Step
• Once all of your
elements in your
balancing chart are
equal across the
reactant and product
sides then your
equation is balanced
and you can stop.
2Mg + O2 → 2 MgO
Reactants
Products
Mg
1 2
1
2
O
2
1
2
Special Circumstances
• Number Neighbors
What if my number of
elements on the reactant
and product side are
number neighbors?
• When this occurs you
must use the least
common multiple (LCM)
to balance your equation.
Your number neighbors
must then be multiplied to
yield the LCM for that pair
of numbers.
4 Al +3 O2 → 2 Al2O3
Reactants
Al
1 4
O
2
6
Products
2 4
3
6
Number Neighbors
Least common multiple: 6
Special Circumstances
• On some combustion reactions 2 C5H10 +15O2 →
10
5 CO2 10
+5 H2O
you will find that you cannot
balance them because you
Reactants Products
end up with an odd number of
element on one side of the
C
5 10
1
5 10
equation and an even number
of the same element on the
10 20
20
H
10
2
other side.
• When this happens if you
double the carbon compound
you will find that the balancing
will work out with no trouble.
O
2 30
3
11
1530
Practice Problems
• Try balancing these equations on your
own and then check your answers.
Ag2O +2 HI → 2 AgI + H2O
2 H2 + O2 → 2 H2O
4 B +3 O2 → 2 B2O3