Download Les Équations Chimiques

Chemical
Equations
Chemical Equation:
A method of representing reactants and
products of a reaction by showing the formulas.
Example:
2H2 + O2 → 2H2O
In the reaction above, we can see that hydrogen
reacts with oxygen to form water.
Writing Chemical Equations
The simplest form of a chemical equation
is called the nominative equation
(in this type of equation we use words, not symbols)
hydrogen + oxygen
water
The + sign means “ reacts with”
The arrow
means “ to produce”
However, the nominative formula tells you
nothing about the composition (formulas) of the
reactants and products…
So, a better formula is :
H 2 + O2
H2O (skeleton eqtn)
Can you notice anything wrong with the
equation above?
(look at the number of atoms involved)
There are some elements that are always
found in groups of two if they are not
bonded with other elements. These
include:
H N O F Cl Br I
Groups of these atoms are called
DIATOMIC MOLECULES
To balance an equation we need to consider
the Law of Conservation of Mass
The Law of Conservation of Mass states that the
mass of the reactants in a chemical equations
must equal the mass of the products.
So, lets consider again
H2 + O 2
H 2O
How many hydrogen and oxygen atoms in the
a) reactants?
b) products?
Balancing Equations…
You need to add coefficients to balance
the equation…to make sure that there are
the same number of atoms of each kind on
both sides.
Sometimes, you must indicate the state of the
reactants and products…
For solids add (s) after the element symbol
ex. Fe (s)
For liquids add (l), ex. H2O (l)
For gases, add (g), ex. O2 (g)
For a substance dissolved in water (aq)
ex. NaCl (aq)
(aq represents “aqueous” which is latin for “in water”)
Here is what magnesium + oxygen
would look like:
The following are examples of
balanced equations:
1. H2 + Cl2
2. Zn + 2HCl
2HCl
ZnCl2 + H2
TIP:
Create a Reactants / Products Chart to see
whether the number of atoms of each element
are equal on both sides of the equation