Understanding Chemical Reactions: Mass Conservation & Constant Proportions
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Understanding Chemical Reactions
Law of Conservation of Mass
In a chemical reaction, mass is conserved. This means that the total mass of the products obtained is equal to the total mass of the reactants that have reacted.
Law of Constant Proportions
The reagents involved in a chemical reaction always react in fixed proportions.
Example: If hydrogen and oxygen react to form water, 1 g of hydrogen always reacts per 8 g of oxygen, and that is always going to be the proportions necessary to form water. The proportions are:
| Hydrogen | Oxygen | Water |
| 1 g | 8 g | 9 g |
| 2 g | 16 g | 18 g |
| 3 g | 24 g | 27 g |
| 4 g | 32 g | 36 g |
And so on. If there is more of one of them, it will stay without reacting. If we put 4 g of hydrogen and 24 g of oxygen, only 3 g of hydrogen will react with the 24 g of oxygen, and about 1 g of hydrogen will not react.
A Model to Explain the Laws: Atoms
In 1808, John Dalton gave an explanation of why chemical reactions follow the law of conservation of mass and of constant proportions. It is based on the following ideas:
- Matter is made of atoms.
- All atoms of an element are the same, but different from other elements.
- In a chemical reaction, initially the atoms are rearranged to form different substances. However, the atoms are the same (neither created nor destroyed), hence the mass does not change in the reaction.
From the atomic point of view, in a chemical reaction, atoms are rearranged differently in reactive substances, giving rise to new substances called products. By not changing the number and kind of atoms, the mass does not change in the reaction.
Balancing Chemical Reactions
To adjust chemical equations, it should be borne in mind that in the reaction, atoms are neither "won" nor "lost". You should have the same number of atoms of each class in the reactants and products. This is the condition of conservation of mass. However, we cannot change at will the formulas of the substances to equal the number of atoms.
Example: When iron oxidizes, it produces a different compound called iron oxide (II). If we write the reaction symbolically, we have:
Fe + O2 → FeO
As written, this reaction shows 1 atom of iron reacting with 2 oxygen atoms, forming a molecule with 1 iron and 1 atom of oxygen. This cannot be; there must be the same number of atoms on both sides.
Therefore, we must use some numbers called coefficients that will be in front of each formula or symbol, indicating the number of atoms of each species that must react in order to keep the mass and number of atoms:
2Fe + O2 → 2FeO
That is, you need 2 iron atoms to react with an oxygen molecule to form 2 molecules of FeO. This process is called balancing a chemical reaction, and it is necessary to put the integers in front of the symbols and formulas of the substances involved in the reaction, so as to meet at the atomic scale the law of conservation of mass.