Question

Explain each of the following in terms of Dalton’s atomic theory: a. the law of conservation of mass b. the law of definite proportions c. the law of multiple proportions

Short answer

Dalton’s theory states atoms are indivisible and conserved in reactions (Law of Conservation of Mass), combine in fixed ratios (Law of Definite Proportions), and form compounds in whole number ratios (Law of Multiple Proportions).

Step-by-step solution

Understanding Dalton’s Atomic Theory

Dalton’s atomic theory states that elements are composed of tiny, indivisible particles called atoms. Atoms of the same element are identical in mass and properties, while atoms of different elements are different. Chemical reactions involve the rearrangement of these atoms, and compounds are formed from the combination of atoms.

Explaining the Law of Conservation of Mass

According to Dalton’s atomic theory, atoms are neither created nor destroyed in a chemical reaction. This means that the total mass of reactants will equal the total mass of the products because the same atoms are simply rearranged. Thus, the total mass remains conserved.

Explaining the Law of Definite Proportions

Dalton’s theory asserts that a given compound is always composed of the same kind and number of atoms. Therefore, any sample of a specific compound will have the same proportion by mass of the constituent elements. This explains why a compound has a fixed ratio of elements by mass.

Explaining the Law of Multiple Proportions

According to Dalton, when two elements form more than one compound, they do so in ratios of small whole numbers. This means that if element A combines with element B to form more than one compound, the different masses of B that combine with a fixed mass of A will be in the ratio of small whole numbers. This is because atoms combine in simple whole-number ratios.

Key concepts

Law of Conservation of Mass

The law of conservation of mass states that mass is neither created nor destroyed during a chemical reaction. This principle is foundational in chemistry.
According to Dalton's atomic theory, each element consists of atoms that are indivisible and indestructible.
This means that during any chemical reaction, the same atoms are present before and after the reaction, only rearranged.
Thus, the total mass of all reactants must equal the total mass of all products because no atoms are lost or gained.
For example, when you burn wood, the mass of the ash, gases, and any other products equal the mass of the original wood and the oxygen from the air that reacted with it.
Simply put, thanks to Dalton’s theory, we know the law of conservation of mass holds true because atoms are conserved in quantity but merely change partners.

Law of Definite Proportions

The law of definite proportions states that a given chemical compound always contains its component elements in a fixed ratio by mass, regardless of the source or method of preparation.
Dalton’s atomic theory clarifies this law by proposing that a particular compound is always composed of the same type and number of atoms.
This means that water (H₂O) always has 2 hydrogen atoms and 1 oxygen atom, resulting in a consistent mass ratio of hydrogen to oxygen.
For instance, water from a river and water produced in a laboratory both consist of the same proportions of hydrogen and oxygen by mass.
This consistent composition is due to the fixed arrangement and combination of specific atoms defined in Dalton’s theory.
This is why a compound like CO₂ (carbon dioxide) always has the same ratio of carbon to oxygen, no matter how it is obtained.

Law of Multiple Proportions

The law of multiple proportions states that if two elements form more than one compound, the masses of one element which combine with a fixed mass of the other are in ratios of small whole numbers.
Dalton's atomic theory supports this by suggesting that atoms combine in simple, whole-number ratios to form compounds.
For example, carbon combines with oxygen to form both CO (carbon monoxide) and CO₂ (carbon dioxide).
If you have 12 grams of carbon, it can combine with 16 grams of oxygen to form CO, or with 32 grams of oxygen to form CO₂.
The ratio of the masses of oxygen that combine with a fixed mass of carbon (12 grams) in these two compounds is 16:32 or 1:2.
This whole-number ratio is explained by Dalton’s idea that atoms combine in straightforward numerical relationships, solidifying our understanding of the law of multiple proportions.
Such ratios validate the predictable and structured nature of elements combining to form different compounds.