– Show that during a chemical reaction the material is preserved.
– Write and balance a balance equation.
1. CONCEPT OF CHEMICAL REACTION
A chemical reaction is a chemical transformation in which pure bodies disappear and simultaneously new pure bodies are formed.
The pure bodies that disappear during a chemical reaction are called the reagents and the pure bodies that form are called the products.
- During sulfur combustion, sulfur and oxygen are lost (reagents) and sulfur dioxide (product) is formed which is a gas that discolors a solution of potassium permanganate in a violet color.
- When a mixture of iron (gray) and sulfur (yellow) in powder is heated, a new chemical species is formed in which it is no longer possible to separate iron from sulfur: It is iron sulphide ( dark grey).
1.2. BALANCE EQUATION OF THE CHEMICAL REACTION
The two reactions can be schematized as follows:
Sulfur + dioxygen ——-> Sulfur dioxide
Iron + sulfur ——-> iron sulphide
Using the formulas of the chemical elements, we obtain the following equilibrium equations:
S + O 2 ——-> SO 2 ;
Fe + S ——-> FeS.
A balance equation is the representation of a reaction with the formulas of pure bodies involved. It expresses the quantities of materials involved during the reaction.
Indeed, the reactions above are read:
- One mole of sulfur (S) is reacted with one mole of oxygen (O 2 ) to give one mole of sulfur dioxide molecule (SO 2 ).
- One mole of iron (Fe) reacts with one mole of sulfur (S) to give one mole of iron sulfide (FeS).
2. CONSERVATION OF THE MATERIAL DURING A CHEMICAL REACTION
- During a chemical reaction, there is a conservation of atoms.
S + O 2 ——-> SO 2 ;
Fe + S ——-> FeS
H 2 + 1/2 O 2 —-> H 2 O is 2H 2 + O 2 —–> 2H 2 O.
The coefficients 2, 1 and 1/2 are called stoichiometric coefficients . They are placed before the raw formula of the bodies to translate the conservation of atoms into a balance equation. The balance equation thus obtained is said to be balanced.
When the coefficient is the number 1, it is not written.
- During a chemical reaction, the mass of the reagents disappeared is equal to that of the products formed: It is the law of Lavoisier which is stated as follows:
” The mass of products formed during a chemical reaction is equal to the mass of reagents consumed “.
Balance the following reactions:
Al + O 2 ——-> Al 2 O 3
Cu 2 S + Cu 2 O ——-> Cu + SO 2
C + CO 2 ——-> CO
H 2 + N 2 ——-> NH3
3. ELECTROLYSIS AND WATER SYNTHESIS
3.1. ELECTROLYSIS OF WATER
The electrolysis of a substance is the decomposition of this substance thanks to the electric current. Electrolysis is also an analysis. To analyze a substance is to break it down into its constituents.
Assembly for the electrolysis of pure water
- 1st case: Let’s close the switch: Nothing happens.
Conclusion: pure water does not conduct electrical current.
- 2nd case: Pour into the electrolyser a few drops of soda solution or sulfuric acid, we observe the following phenomena:
- The lamp is shining.
- Gas bubbles form on the surface of the electrodes.
- The level of the solution in the tubes drops.
The lamp shines because the liquid mixture (+ solution of soda or water + sulfuric acid) conducts the electric current. This electric mixture is called electrolyte (solution that drives the electric current).
The level of the solution drops in the tubes because a chemical reaction occurs at the electrodes; reaction releasing gas bubbles.
The analysis of the gases collected at the electrodes shows that the gas collected at the anode reignites a matchbox with a red dot : It is oxygen O 2 .
The gas collected at the cathode causes a detonation in the presence of a flame: It is H 2 dihydrogen .
Experience shows that the volume of dihydrogen collected is twice that of dioxygen.
VH 2 = 2VO 2 ↔ VO 2 = 1VH 2 /2.
During the electrolysis of the water, the water disappears and at the same time dihydrogen and dioxygen are formed. The balance equation that is written is:
H 2 O ——-> H 2 +1/2 O 2 is 2H 2 O ——-> 2H 2 + O 2 .
3.2. Synthesis of water
The synthesis of a substance is a chemical transformation during which the substance is formed from its constituent. The balance equation of the synthesis of water is written:
2H 2 + O2 ——-> 2H 2 O. This is the inverse equation of water synthesis.
Assembly for the synthesis of water: