Chemistry
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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.
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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