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Chemical Reactions
What is a chemical reaction?
• A chemical reaction is a chemical change
where chemical substances (called
reactants) react to give new chemical
substances (called products).
• Example
– The combustion of hydrogen in oxygen is a
chemical reaction which gives water.
• Hydrogen and Oxygen are the reactants.
• Water is the product.
Chemical reaction and molecules
• The chemical reaction of hydrogen and
oxygen is done between the molecules of
these two gases.
• In this chemical reaction, the bonds of the
hydrogen molecules H2 and oxygen
molecules O2 break down.
• The atoms H and O form new bonds to
give water molecules H2O.
The three molecules
• Hydrogen molecule H2
is made up of 2 atoms
• Oxygen molecule O2
is made up of 2 atoms
• Water molecule H2O
Chemical equation
• A chemical equation is a way which represents
the rearrangement of the atoms during a
chemical reaction.
• In a chemical equation, we write the formulas of
the reactants and the formulas of the products
separated by an arrow.
hydrogen + oxygen
H2
+
+
O2
water
H2O
Lavoisier’s Law
• Antoine Lavoisier, a french chemist, stated
the following law:
– « the number of atoms in the reactants = the
number of atoms in the products. »
• This law makes it possible to balance
chemical equations
Balance a chemical equation
H2
+
O2
H2O
- There are 2 atoms H and 2 atoms O in the reactants.
- There are 2 atoms H and 1 atom O in the product.
To balance this equation, we take 2 water molecules
containing 2 atoms O and 4 atoms H, and we take 2
hydrogen molecules and 1 oxygen molecule.
H2 and H2
+
O2
H2O and
H2O
We should write it:
2 H2
+
O2
2 H2O
To read a chemical equation
2 H2
+
O2
2 H2O
Read this equation as follow:
2 hydrogen molecules react with 1 oxygen
molecule to form 2 water molecules.
Attention:
- The number of atoms is the same in the
reactants and the products.
- The number of molecules is not necessarily
the same.
Problems
1. Methane CH4 reacts with oxygen O2 to
form carbon dioxide CO2 and water H2O.
a. Balance the equation of this reaction
CH4 +
O2
CO2
+
H2O
b. Determine the number of reactants and
products molecules.
Problems
2. Balance and read the following equations:
a.
b.
c.
d.
Na
Fe
N2
Al
+
+
+
+
O2
Cl2
H2
O2
Na2O
FeCl3
NH3
Al2O3
Note : FeCl3 = Iron III chloride
NH3 = Ammonia
Al2O3 = Aluminum oxide
Answers
1. CH4 +
2 O2
CO2
+ 2 H2O
1 methane molecule react with 2 oxygen
molecules to form 1 dioxide carbon molecule
and 2 water molecules.
answers
2.
a. 4Na + O2
2 Na2O
4 sodium molecules react with 1 oxygen
molecule to form 2 sodium oxide molecules.
b. 2Fe + 3Cl2
2FeCl3
2 iron molecules react with 3 chlorine
molecules to form 2 iron III molecules.
Answers
c. N2 +
3H2
2NH3
1 nitrogen molecule react with 3 hydrogen
molecules to form 2 ammonia molecules.
d. 4Al +
3O2
2Al2O3
4 aluminum molecules react with 3 oxygen
molecules to form 2 aluminum oxide
molecules.