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Chemistry

Electricity

Optics

Mechanics
Mechanics lessons
Water
Water on Earth
Changes of state in the nature: the water cycle
Water in human body
Test for water
Properties of water in different states
States of matter
States of matter and its changes
Boiling water
Water: freezing and melting
Changes of state: mass and volume
Molecules in different states of matter

Mixtures and solutions
Heterogeneous mixtures
Homogeneous mixtures
Decantation
Centrifugation
Filtration
Vaporization of water
Distillation
Chromatography
Dissolving a solid in water
Conservation of mass on dissolving
Miscibility in water

Mass and volume
Volume and its units
Measuring volume with a graduated cylinder
Mass and its units
Measuring the mass of a liquid
Mass of a liter of water

Air and atmosphere
Atmosphere
Earth's atmosphere protect us
Threats to the Earth's atmosphere
Composition of air
Air and life
Pressure
Atoms and molecules
Molecules
Molecules in mixtures and pure substances
Molecules and states of matter
Atoms
Composition of molecules
Constituents of the atoms
The electrical neutrality of the atom

Combustions
Basics of combustion
The combustion of carbon
The combustion of butane
Atoms and chemical reactions
Chemical reactions
Chemical equations
Law of conservation of mass
Chemical synthesis
Metals

Most common metals

How to distinguish metals ?

Corrosion of metals

Electrical conductivity of solid materials

Electrons and free electrons

Electric current in metals
Ions
The conductivity of aqueous solutions
Aqueous solutions and ions
The direction of movement of ions
Formation of ions
Tests for ions
Acidic and basic solutions
pH of aqueous solutions
Ions in acidic and basic solutions
Dilution of acids and bases
Composition of hydrochloric acid
Chemical reaction between iron and hydrochloric acid

Electrochemical cells and chemical energy
Chemical reaction beteween a copper sulphate solution and zinc
Copper sulfate and zinc battery
Basics of electrochemical cell



















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Atoms and chemical reactions

Chemical equation

 
1) What is a chemical reaction ?

The reaction equation is another way (more accurate than a simple word equation) to describe a chemical transformation.

The reaction equation is written starting from the simple word equation:
- The names of products and reactants are replaced by their chemical formula.
- The law of conservation of matter must be respected.

The law of conservation of matter:
The number of atoms of each type is the same before and after the reaction.

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2) Chemical equation for combustion of carbon
Simple word equation: Carbon + dioxygen      carbone dioxide

Chemical equation:

Step 1: names are replaced by their chemical formula
C + O2      CO2

Step 2: verification of law of conservation of matter


Total number of atoms before          Total number of atoms after
chemical transformation:                    chemical transformation:
* Carbon atoms: 1                            *  Carbon atoms: 1      
* Oxygen atoms:   2                          *  Oxygen atoms: 2
 

Law of conservation of matter is then respected: it is said that the equation is balanced

This equation is read:
A carbon atom and an oxygen molecule react together to give a molecule of oxygen.


3) Chemical equation for combustion of méthan
Simple word equation: methane + dioxygen      water + carbon dioxid

Chemical equation (temporary)
 CH4 + O2       H2O + CO2

Total number of atoms before                  Total number of atoms after
chemical transformation:                          chemical transformation:
Carbon atoms: 1                                     *  Carbon atoms: 1            
Oxygen atoms:  2                                    *   Oxygen atoms:  1 + 2 = 3
* Hydrogen atoms:  4                                *   Hydrogen atoms:  2

This equation is not balanced since the number of oxygen atoms and hydrogen atoms are not the same in the reactants and products.

It should be added a coefficient before some formulas to modify the number of molecules:

 CH4 + 2 O2    2 H2O + CO2

Total number of atoms before                  Total number of atoms after
chemical transformation:                          chemical transformation:
Carbon atoms: 1                                     *  Carbon atoms: 1            
Oxygen atoms:  2 x 2 = 4                       *   Oxygen atoms:  2 x 1 + 2 = 4
* Hydrogen atoms:  4                                *   Hydrogen atoms:  2 x 2 = 4


The equation is then balanced

The chemical reaction can be read as follow:
A methane molecule and two dioxygen molecules give two molecules of water and one molecule of carbon dioxide
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