Basic chemistry

Chemical reactions with example?

Chemical reaction with example?

CHEMICAL REACTIONS

GOALS

– Show that during a chemical reaction the material is preserved.

– Write and balance a balance equation.

1. CONCEPT OF CHEMICAL REACTION

1.1. DEFINITION

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.

EXAMPLES

  • 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.

EXAMPLES

S + O 2       ——-> SO  ;

Fe + S ——-> FeS

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.

NOTE

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  “.

APPLICATION EXERCISE

Balance the following reactions:

Al + O 2  ——-> Al 2 O 3

Cu 2 S + Cu 2 O ——-> Cu + SO 2

C + CO 2    ——-> CO

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

Without title1
  • 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.

Interpretation:

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.

Conclusion.

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:

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:

Without title2

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