The reaction of hydrogen gas and oxygen gas produces water. This
reaction requires an input of energy and can be written as:
a word equation (shown below in blue) or
a chemical equation (shown below in black using chemical formulas).
As we learned in unit 2, section
2.5.2, this is an exothermic
reaction because energy is released.Contrast
to the above exothermic reaction is the reverse reaction, the decomposition
reaction of water.
The decomposition reaction of water,
as given below, is an endothermic
reaction. To decompose water into its elements, energy is required.
A process to decompose water is by electrolysis.
We will cover the different types of chemical reactions,
including decomposition reactions in section
8.4.
For now, let's focus on the symbols
used to write a chemical reaction. The following table summarizes the symbols
and terms used in chemical equations:
are numbers written in front of each substance.
If nothing is written in front, the default value is 1.
eg.
the 2 written in front of H2 (g)
is a coefficient.
the 2 written in front of H2O (l)
is a coefficient.
In the decomposition reaction of water, it means that:
there are two molecules H2 (g) and 1 molecule of O2
(g) formed from the decomposition of two molecules of H2O
(l). OR, scaled up,
there are two moles H2 (g) and 1 mole of O2
(g) formed from the decomposition of two moles of H2O (l).
Let's count the number of each type of atoms on both
sides of the decomposition reaction of water:
The number of hydrogen atoms on the left-hand-side
of the chemical equation:
Each water molecule has 2 hydrogen atoms.The coefficient of '2'
tells us that there are two water molecules at the start of the reaction.
Therefore, there are 2 H atoms x 2 =
4 hydrogen atoms on the left-hand-side
.
The number of oxygen atoms on the left-hand-side
of the chemical equation:
each water molecule has 1 oxygen atom. The coefficient of '2'
tells us that there are two water molecules at the start of the reaction.
Therefore, there are 1 O atom x 2 =
2 oxygen atoms on the left-hand-side
.
The number of hydrogen atoms on the right-hand-side
of the chemical equation:
two H2 (g) molecules are formed. Each H2 (g) molecules
has 2 hydrogen atoms. The coefficient of '2'
tells us that there are two H2 (g) produced. Therefore, there
are 2 H atoms x 2 =
4 hydrogen atoms on the right-hand-side
.
The number of oxygen atoms on the right-hand-side
of the chemical equation:
one O2 (g) molecules is formed. Each O2 (g) molecules
has 2 oxygen atoms. There is no coefficient written in from of theO2
(g). By default, the coefficient is '1'.
Therefore, there are 2 O atoms x 1 =
2 oxygen atoms on the right-hand-side
Note: The number of each type of atoms
on both sides of the chemical equation is the SAME.
No of atoms:
LHS
RHS
Hydrogen atoms
4
4
Oxygen atoms
2
2
This is not a coincident. The above reaction is called a 'balanced'
chemical reaction. All chemical reactions must be 'balanced' because they must
obey the Law
of Conservation of Mass. Read on ....
Section
9.1 Definition
of a Chemical Equation ..p216 Section
9.2 Terms,
Symbols, and
TheirMeanings
..p217
Section
9.1 
