4.1 INTRODUCTION TO CHEMICAL REACTIONS
CHEMICAL REACTIONS
Chemical Reactions are the result of one or more substances undergoing a chemical change
to produce new substances. How can you tell that new substances have been formed?
Indicators of a Chemical Change:
The problem with these indicators is that they don’t 100% verify that you have undergone a
chemical change. Think about two examples where this may be true:
Elements and compounds often interact together in a chemical reaction.
are a way of communicating what is occurring in a chemical reaction. The
are the substances that undergo the chemical reaction (on the left) and
the
are the substances formed in a chemical reaction (on the right).
Figure 1: According to the following chemical reaction, two molecules of hydrogen peroxide decompose to form
two molecules of water and one molecule of oxygen gas. The bonds in hydrogen peroxide break and rearrange to
form new bonds. A catalyst (MnO2) can be used in this reaction to speed up a reaction, without being used up
itself.
DESCRIBING CHEMICAL REACTIONS
A. WORD EQUATIONS:
hydrogen peroxide
MnO2
catalyst
water + oxygen + energy*
The disadvantage of a word equation is that you do not have information about the chemicals
themselves, including the number of atoms being reacted and produced.
B. SKELETON EQUATIONS:
H2O2(aq)
solid
MnO2
catalyst
H2O(l) + O2(g) + energy*
liquid
gas
aqueous
The disadvantage of a skeleton equation is that it does not show how the numbers of atoms
have balanced each other on the reactant side and the product side.
*Even though energy is not considered a chemical, it is still absorbed or released in a reaction, and therefore we
include this in a reaction. You will see more of this in Grade 12.
C. THE LAW OF CONSERVATION OF MASS AND BALANCING CHEMICAL EQUATIONS
This is also true for the number of atoms. We know from atomic
theory that matter cannot be created or destroyed, only changed.
The number of atoms is always the same. This law has been
proven using experimental evidence.
2 H2O2(aq)
MnO2
catalyst
2 H2O(l) + O2(g) + energy
Balancing equations is possible, because of the Law of Conservation of Mass. Occasionally,
you will have a skeleton equation that is already balanced.
C(s) + O2(g) → CO2(g)
Not all chemical equations work out this way. In the equation below, there is one magnesium
on both sides of the equation. There are 2 O on the reactant side, but only 1 O on the
product side.
Mg(s) + O2(g) → MgO(s)
To balance the equation, you can add numbers called
in front of the
chemical formulas. This will tell us how many molecules or formula units are present. You
cannot change the formulas, only the coefficient in front of it.
2 Mg(s) + O2(g) → 2 MgO(s)
RULES TO BALANCING CHEMICAL EQUATIONS (Using Trial and Error)
Here is a diagram showing the balancing of an equation with molecular structures:
Example Problems:
1. ___ As4S6(s) + ___ O2 → ___ As4O6(s) + ___ SO2(g)
2. ___ Sc2O3(s) + ___ H2O(l) → ___ Sc(OH)3(s)
3. ___ C2H5OH(l) + ___ O2(g) → ___ CO2(g) + ___ H2O(l)
4. ___ C4H10(g) + ___ O2(g) → ___ CO2(g) + ___ H2O(g)
5. ___ Fe2(SO4)3(aq) + ___ KSCN(aq) → ___ K3Fe(SCN)6(aq) + ___ K2SO4(aq)
* You can use an alternate way of balancing equations by making a table for each element on the reactant side
and the product side. You may have learned this format last year.