Balancing Chemical Equations

Contributed by:

Jonathan James Mon, Jan 17, 2022 08:20 PM UTC

The highlights are:
1. Law of conservation of mass
2. Steps to balance a chemical reaction

1. Balancing Chemical Equations
What goes in must
come out!

2. Balancing Chemical Equations
 Balancinga chemical equation is much like
the work of an accountant who has to show
every penny that comes in and where it has
gone to.

3.  Learn the steps to balancing chemical
equations.
 Take notes to help you understand.
 Test yourself with a set of equations to
balance.
 Enter your own equations to see if they
balance.

4. Law of Conservation of Mass
You need to remember this law!
 The Law of Conservation of Mass states:
that mass is neither created nor
destroyed in any chemical reaction.
Therefore balancing of equations requires
the same number of atoms on both sides of a
chemical reaction.
 The number of atoms in the Reactants must
equal the Number of atoms in the Products

5. Chemical Equations
Because of the principle of the
Conservation of Matter,
an equation must be
balanced.
It must have the same
number of atoms of the
same kind on both sides.
Lavoisier, 1788

6. Law of Conservation of Mass
 Themass of all the reactants (the
substances going into a reaction) must equal
the mass of the products (the substances
produced by the reaction).
 Reactant + Reactant = Product

7. A simple equation, such as the
synthesis of Iron (II) sulfide,
 iron + sulfur Iron (II) sulfide
 Fe + S FeS
 Note that in a chemical equation, by
convention, we use the arrow “ " instead
of the equals “ = ".

8.  The last stage is to put in state of matter
symbols, (s, l, g, aq), as appropriate (solid,
liquid, gas, aqueous or dissolved in water)
 Fe(s) + S(s) FeS(s)

9. Equations
2 3
___ Al(s) + ___ Br2(l) ---> ___ Al2Br6(s)

10. Steps to Balancing a Chemical Equation
1. Write all reactants 2. Use coefficients in
on the left and all front of each formula
products on the to balance the
number of atoms on
right side of the each side.
equation arrow.
Make sure you
write the correct
formula for each
element

11. Steps to Balancing a Chemical Equation
3. Multiply the 4. It is often easiest to start
coefficient of each balancing with an element
that appears only once on
element by the each side of the arrow.
subscript of the These elements must have
element to count the the same coefficient. Next
atoms. Then list the balance elements that
number of atoms of appear only once on each
side but have different
each element on numbers of atoms. Finally
each side. balance elements that are
in two formulas in the
same side.

12. Re-cap of steps from rule 4:
 Balance elements that appear only
once on each side of the arrow.
 Next balance elements that appear only
once on each side but have different
numbers of atoms.
 Finally balance elements that are in two
formulas in the same side.

13. Balancing Chemical
Equations
An easier way

14. First you need an equation with the correct “formulae”
………. You’ll probably be given this in the question
Just like this one
Mg + O2  MgO
Then all you do is list the atoms that are involved
on each side of the arrow
Mg + O2  MgO
Mg Mg
O O

15. Then start balancing:
[1] Just count up the atoms on each side
Mg + O2  MgO
1 Mg 1
2 O 1
[2] The numbers aren’t balanced so then add “BIG”
numbers to make up for any shortages
Mg + O2  2 MgO
1 Mg 1 2
2 O 1 2
And adjust totals

16. But the numbers still aren’t equal, so add
another “BIG” number
2 Mg + O2  2 MgO
2 1 Mg 2
2 O 2
And adjust totals again
NOW BOTH SIDES HAVE EQUAL
NUMBERS OF ATOMS
WE SAY THAT THE
EQUATION IS BALANCED!!

17. Try to balance these equations using the same
[1] Na + Cl2  NaCl
[2] CH4 + O2  CO2 + H2O
[3] Li + HNO3  LiNO3 + H2
[4] Al + O2  Al2O3

18. How did you get on??
Here are the answers:
[1] 2 Na + Cl2  2 NaCl
[2] CH4 + 2 O2  CO2 + 2 H2O
[3] 2 Li + 2 HNO3  2 LiNO3 + H2
[4] 4 Al + 3 O2  2 Al2O3
HOPE YOU’VE GOT THE IDEA…
REMEMBER TO CHECK THAT
YOU CAN DO ELECTROLYSIS
EQUATIONS TOO

19.  NH3+ O2 NO + H2O
Reactants Products
N appears once on both sides in equal
numbers, so the coefficient for NH3 is the
same as for NO.

20. Example: NH3 + O2 NO + H2O
 Next look at H which appears only once on
each side but has different numbers of
atoms, 3 on the left and 2 on the right. The
least common multiple of 3 and 2 is 6, so
rewrite the equation to get 6 atoms of H on
both sides:
 2NH3 + O2 NO + 3H2O

21. Example: 2NH3 + O2 NO + 3H2O
 There are 2 oxygen atoms on the left and 5 on
the right — the least common multiple of 2 and 5
is 10, so rewrite the equation as:
 2NH3 + 5O2 4NO + 6H2O

22. Now count the atoms on each side:
 2NH3 + 5O2 4NO + 6H2O
 Write them out keeping them on the
appropriate side of the chemical equation
 2 N (nitrogen atoms) 4 N (nitrogen atoms)
 6 H (hydrogen atoms) 12 H (hydrogen atoms)
 10 O (oxygen atoms) 10 O (oxygen atoms)
 This shows the equation not to be balanced
“YET”

23. Check the number again:
 Ifyou double the N and H on the left the
equation will be balanced:
 4NH3 + 5O2 4NO + 6H2O

24.  4NH3 + 5O2 4NO + 6H2O
4 N (nitrogen atoms) 4 N (nitrogen atoms)
 12 H (hydrogen atoms) 12 H (hydrogen atoms)
 10 O (oxygen atoms) 10 O (oxygen atoms)
 The equation is Balanced

25. Balancing Practice
 For more help go to:
http://richardbowles.tripod.com/chemistry/bal
ance.htm#part0
 For some fun balancing equations go to:
 http://www.mpcfaculty.net/mark_bishop/bala
ncing_equations_tutorial.htm

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