6.3 Conserving Mass in
chemical reactions
Balancing Chemical Equations
Conserving Mass



Chemical reactions cause a rearrangement of atoms in
the reactants to form different products
Molecules might be broken apart and new ones might be
put together, but the atoms in the product(s) are the
same as the ones that were present in the reactants
Notice that all the atoms present at the beginning of the
chemical reaction are present after the reaction. The
total mass of the atoms in the reactants remains equal
to the total mass of the atoms in the products.
The Law of Conservation of Mass
The total mass of the reactants is equal to the
total mass of the products!
Reactant A + Reactant B
45 g
25 g
calcium + hydrochloric acid
16 g
33 g
Product C + Product D
50 g
?
calcium chloride + hydrogen gas
28 g
?
Review: How to count atoms
1.
The symbol of an element represents one
atom of that element.
e.g. Ca =
2.
A subscript indicates more than one
atom of the element.
e.g. N2 =
Review: How to count atoms
3.
A subscript outside a bracket
multiplies all the elements inside the
brackets.
e.g. Ba3(PO4)2 =
Review: How to count atoms
4.
a) A coefficient is a number written in
front of a chemical symbol and indicates the
number of atoms of that element.
e.g. 3C =
b) A coefficient written in front of a
chemical formula multiplies the number of
atoms of each element in the formula.
e.g. 4Cu(NO3 )2 =
Balancing Chemical Equations
Word Equation
methane + oxygen gas
Skeleton Equation
CH4
+
O2
water + carbon dioxide
H2O
+
CO2

The chemical equation must obey the law of conservation of mass.
Thus the total number of one type of atom must be the same on
both sides of the equation.

To ensure this one balances the equation by putting
COEFFICIENTS in front of the substances.
Balanced Chemical Equation
CH4 (g) +
2 O2 (g)
2 H2O (g)
+
CO2 (g)
Steps to Balancing
Chemical Equations:
1.
Write out the skeleton equation.
2.
Balance atoms that occur in the largest number
first. Leave oxygen and hydrogen until the end.
3.
Balance polyatomic ions next (only if they are the
same on both sides) and balance them as an ion
unit (do not split into individual atoms).
4.
Balance hydrogen and oxygen last.
5.
Make sure you have the same number of each
element on both sides of the equation.
Practice:

Balance the following equation:

Lithium sulphide reacts with magnesium phosphate to
produce lithium phosphate and magnesium sulphide.
Practice:

1.
Balance the following skeleton equations:
Fe +
O2

Fe3O4
More Practice:
2.
Mg +
HNO3

H2 +
Mg(NO3)2
Last one:
3.
Na +
H 2O

NaOH +
H2