CONSERVATION OF MASS IN CHEMICAL CHANGES
o
A chemical change always involves the conversion of substances, called reactants, into other
substance, called products.
o
A CHEMICAL REACTION is a process in which new substances with new properties are formed.
o
In a chemical reaction, atoms are conserved (they are neither created nor destroyed).
The atoms are merely rearranged to form new substances.
o
A French chemist, named Antoine Lavoisier, proved that mass remains unchanged in a
chemical reaction. He continuously repeated the same experiment in which he heated
mercury (II) oxide in a closed container to produce mercury and oxygen. Time after time,
his results indicated that the mass of the reactants was the same as the mass of the
products.
o
The LAW OF CONSERVATION OF MASS states that in a chemical reaction, the total
mass of the products is always the same as the total mass of the reactants.
o
A REACTANT is a pure substance that undergoes a chemical change (it is a beginning material in
a chemical change).
o
A PRODUCT is a pure substance that is formed in a chemical change (it is the final material in a
chemical change). The properties of the product(s) are different from the properties of the
reactants.
WRITING CHEMICAL EQUATIONS
o
A CHEMICAL EQUATION is a representation of what happens to the reactants and products
during a chemical change. It is used to represent a chemical reaction.
o
Chemical equations can be represented in three ways:
o
WORD EQUATION: The name of each reactant is written to the left of an arrow and the
name of each product is written to the right of the arrow.
o
A plus sign on the reactants side means “reacts with”. A plus sign on the product
sign means “and”. The arrow stands for “yields” or “reacts to produce”.
o
o
Example) hydrogen + oxygen →water
SKELETON EQUATION is when you replace the words (from a word equation) with
chemical formulas. This is done because a word equation shows the reactants and products
of a reaction, but it does not provide information about the chemical composition of the
substances.
o
Example)
hydrogen + oxygen →water (word equation)
H2 + O2 → H2O
(skeleton equation)
o
BALANCED CHEMICAL EQUATION demonstrates the law of conservation of mass,
which requires the same number of atoms of each element to appear on both sides of a
chemical equation.
o
A COEFFICIENT is a number that is placed in front of a chemical formula in a
balanced chemical equation.

The Coefficient is multiplied by every atom’s subscript (remember that if
there is not a subscript present, we assume there is a subscript of “1”)

The ONLY way to balance a chemical equation is to add coefficients, as you
cannot alter the subscripts of the atoms (since this would alter the actual
substance involved in the reaction).
o
Example)
hydrogen + oxygen →water (word equation)
H2 + O2 → H2O
(skeleton equation)
There are 2 hydrogen and 2 oxygen in the reactants, and
2 hydrogen, but only 1 oxygen in the products.
2H2 + O2 → 2H2O
(balanced chemical equation)
Now there are 4 hydrogen and 2 oxygen in the reactants, and 4
hydrogen and 2 oxygen in the products. Both sides of the
equation are balanced.
SHOWING THE STATE OF A SUBSTANCE
o
In order to properly complete a balanced chemical equation, the states of the reactants and
products at the temperature of the chemical reaction, may be included.
o
So, we add an abbreviation at the end of the chemical formulas indicating the state of each
chemical formula.
State
Abbreviation
Example (at room temperature)
Solid
Liquid
(s)
Sodium chloride: NaCl(s)
(l)
Water (l)
Gas
Aqueous solution
(g)
(aq)
Hydrogen: H2 (g)
Aqueous sodium chloride solution:
NaCl (aq)
BALANCING CHEMICAL EQUATIONS
Step 1) H2 (g) + O2 (g) → H2O (l)
o
In the skeleton equation, there is the same number of hydrogen atoms on both
sides of the equation. However, there are more oxygen atoms in the reactants than
there are in the products.
o Checking the Atom Balance
Element
# of atoms in the
# of atoms in the
Equal?
H
O
Reactants
2
2
Products
2
1
Yes
No
Step 2) H2 (g) + O2 (g) → 2H2O (l)
o
Placing the coefficient “2” in front of H2O causes the number of oxygen atoms on
both sides of the equation to be the same.
o
Because the coefficient “2” in front of H2O applies to all the elements in the
compound, it will cause the number of hydrogen atoms in the products to equal 4.
o Checking the Atom Balance
Element
# of atoms in the
# of atoms in the
Equal?
H
O
Reactants
2
2
Products
4
2
No
Yes
Step 3) 2H2 (g) + O2 (g) → 2H2O (
o
l)
Placing another coefficient of “2” in front of H2 will now make the number of
hydrogen atoms in both the reactants and products the same.
o Checking the Atom Balance
Element
# of atoms in the
# of atoms in the
Equal?
H
O
Reactants
4
2
Products
4
2
Yes
Yes
TIPS FOR BALANCING CHEMICAL EQUATIONS
o
Remember that these elements exist as diatomic molecules: hydrogen (H 2), nitrogen (N2), fluorine (F2),
chlorine (Cl2), bromine (Br2), iodine (I2), and oxygen (O2).
o
First balance a compound (example: H2O) before individual elements (example: N2).
o
Balance hydrogen and oxygen last. They often appear in more than one reactant or more than one product,
so they are easier to balance after other elements are balanced.
o
If a polyatomic ion appears in both a reactant and a product, think of it as a single unit to balance the
chemical equation faster (keep it in the brackets and just add a coefficient in front of the bracket).
WRITING BALANCED CHEMICAL EQUATIONS
Practice Questions:
A) Ammonia, NH3 (g) is produced from the reaction of nitrogen gas and hydrogen gas. Write a balanced
chemical equation for this reaction.
o
Begin by writing a word equation
nitrogen + hydrogen → ammonia
o
Next, write a skeleton equation
N2 + H2 → NH3
o
Finally, balance the equation using coefficients. If the states of the products and
reactants are provided, make sure to include them.
N2 (g) + 3H2 (g) → 2NH3 (g)
B) A solid piece of magnesium reacts with oxygen gas to produce solid magnesium oxide.
C) Iron reacts with oxygen to produce rust, Fe2O3.
D) Nitrogen gas reacts with bromine gas to form gaseous nitrogen tribromide.
E) The combustion of methane gas, CH4 (g), involves its reaction with oxygen to produce carbon dioxide
gas and water vapour.