Question

In Section 1.1 of the text, the concept of a chemical reaction was introduced with the example of the decomposition of water, represented as follows: Use ideas from Dalton’s atomic theory to explain how the above representation illustrates the law of conservation of mass.

Short answer

The decomposition of water (\(2H_2O \rightarrow 2H_2 + O_2\)) illustrates the law of conservation of mass through Dalton's atomic theory, which states that atoms are neither created nor destroyed during chemical reactions. In this case, 4 hydrogen atoms and 2 oxygen atoms are present in both reactants and products, with their masses remaining constant. The atoms are only rearranged to form hydrogen and oxygen molecules, adhering to the law of conservation of mass.

Step-by-step solution

Understand Dalton's Atomic Theory

Dalton's atomic theory has several postulates, but the ones most relevant to this exercise are: 1. All matter is composed of atoms, which are indivisible and indestructible particles. 2. All atoms of a given element are identical in mass and properties. 3. Chemical reactions involve the rearrangement of atoms, which are combined or separated but never created or destroyed.

Understand the Chemical Reaction

The decomposition of water is a chemical reaction represented by the equation: \[2H_2O \rightarrow 2H_2 + O_2\] This means that two water molecules (2H₂O) decompose into two hydrogen molecules (2H₂) and one oxygen molecule (O₂).

Apply Dalton's Atomic Theory to the Chemical Reaction

According to Dalton's atomic theory, atoms are neither created nor destroyed during a chemical reaction. In this case, the decomposition of water involves the rearrangement of hydrogen and oxygen atoms, which separate and combine to form hydrogen and oxygen molecules.

Analyze the Reaction in Terms of the Law of Conservation of Mass

The law of conservation of mass states that the mass of the reactants (the initial substances) must equal the mass of the products (the final substances) in a chemical reaction. In the decomposition of water, the mass of the hydrogen and oxygen atoms in the water molecules must equal the mass of the hydrogen and oxygen molecules that are formed.

Show That the Law of Conservation of Mass is Upheld in the Reaction

We can show that the law of conservation of mass is upheld in the decomposition of water by looking at the number and mass of the atoms on both sides of the reaction. On the reactants side (2H₂O): - There are 4 hydrogen atoms and 2 oxygen atoms. On the products side (2H₂ + O₂): - There are 4 hydrogen atoms and 2 oxygen atoms. The number and mass of the hydrogen and oxygen atoms remain unchanged during the decomposition of water, and the atoms are merely rearranged to form hydrogen and oxygen molecules. Thus, the law of conservation of mass is upheld in this reaction, in accordance with Dalton's atomic theory.

Key concepts

Dalton's Atomic Theory

Dalton's Atomic Theory is foundational to understanding chemical reactions. It proposes that all matter is composed of tiny indivisible particles called atoms. According to this theory, atoms cannot be created or destroyed; they only rearrange during chemical reactions. This concept is critical when examining reactions like the decomposition of water.
In Dalton's view, chemical reactions are merely the rearrangement of atoms. For instance, when water decomposes, hydrogen and oxygen atoms simply reorganize into different molecules. Dalton suggested that all atoms of a particular element are identical in mass and properties, which is why they form predictable combinations in chemical reactions, such as forming water from hydrogen and oxygen.
This theory laid the groundwork for the modern understanding of chemistry and helped scientists predict the outcome of chemical reactions with accuracy.

Law of Conservation of Mass

The Law of Conservation of Mass is an essential principle in chemistry. It states that in any chemical reaction, the total mass of the reactants—the starting substances—must equal the total mass of the products—the substances formed. This is apparent in the chemical equation for the decomposition of water: \[2H_2O \rightarrow 2H_2 + O_2\].
In this reaction, we start with water molecules and end with hydrogen and oxygen molecules. Throughout the process, the number and mass of the atoms are conserved. This means no atoms are lost or gained from start to finish; they only rearrange to create new molecules.

• The reactants have 4 hydrogen atoms and 2 oxygen atoms.
• The products have 4 hydrogen atoms and 2 oxygen atoms as well.

This constant balance is why the mass remains unchanged, proving the law holds true in every chemical reaction, just as Dalton's theory suggests.

Decomposition of Water

Decomposition of water is a classic example of a chemical reaction where water molecules break down into hydrogen and oxygen gases. This reaction can be expressed by the equation: \[2H_2O \rightarrow 2H_2 + O_2\].
Here, water starts as the reactant, and the products are hydrogen gas and oxygen gas. By observing this reaction, we see how atoms separate and reassemble into new compounds.

• Initially, two water molecules contain 4 hydrogen atoms and 2 oxygen atoms.
• After decomposition, they form 2 hydrogen molecules (2H₂) and 1 oxygen molecule (O₂).

This decomposition is a straightforward demonstration of Dalton’s notion of atomic rearrangement during chemical reactions. As any chemical reaction, it reflects the law of conservation of mass, maintaining equal mass before and after the reaction, as the atoms are simply redistributed in different molecular forms.