Problem 33 - Chemistry An Atoms First Approach

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.

Step-by-Step Solution

Answer

The decomposition of water can be represented by the chemical equation: \(2H_2O \rightarrow 2H_2 + O_2\). According to Dalton's atomic theory, atoms cannot be created, divided, or destroyed. In this reaction, the total number of hydrogen (4) and oxygen (2) atoms remains the same before and after the reaction, which illustrates the law of conservation of mass. The mass of 2 molecules of water (2 H₂O) equals the combined mass of 2 molecules of hydrogen gas (2 H₂) and 1 molecule of oxygen gas (O₂).

Step 1: Understanding Dalton's atomic theory

Dalton's atomic theory is a fundamental concept in chemistry that states that all matter is composed of small, indivisible particles called atoms. The main ideas of Dalton's atomic theory are: 1. Elements are made up of tiny particles called atoms. 2. Atoms of the same element are identical and have the same properties. 3. Atoms of different elements have different properties. 4. Atoms cannot be created, divided or destroyed. 5. Chemical reactions involve the rearrangement of atoms.

Step 2: Understanding the law of conservation of mass

The law of conservation of mass states that the total mass of the reactants in a chemical reaction is equal to the total mass of the products formed. In other words, mass cannot be created or destroyed during a chemical reaction.

Step 3: Examining the decomposition of water

The decomposition of water can be represented by the following chemical equation: \[2H_2O \rightarrow 2H_2 + O_2\] In this reaction, two molecules of water (H₂O) decompose into two molecules of hydrogen gas (H₂) and one molecule of oxygen gas (O₂).

Step 4: Applying Dalton's atomic theory to the decomposition of water

According to Dalton's atomic theory, atoms cannot be created, divided, or destroyed. In the given chemical reaction, we can see that the total number of hydrogen atoms and oxygen atoms remains the same before and after the reaction. There are: 1. Four hydrogen atoms in 2 molecules of water (2 H₂O) before the reaction, and four hydrogen atoms in 2 molecules of hydrogen gas (2 H₂) after the reaction. 2. Two oxygen atoms in 2 molecules of water (2 H₂O) before the reaction, and two oxygen atoms in 1 molecule of oxygen gas (O₂) after the reaction. As the total number of atoms of each element remains the same, no atoms are created or destroyed in this reaction, which is consistent with Dalton's atomic theory.

Step 5: Connecting the decomposition of water to the law of conservation of mass

The law of conservation of mass states that mass cannot be created or destroyed during a chemical reaction. Since we have established that the total number of hydrogen and oxygen atoms remains the same in the given decomposition of water, the mass of the reactants should equal the mass of the products. The mass of 2 molecules of water (2 H₂O) is equal to the combined mass of 2 molecules of hydrogen gas (2 H₂) and 1 molecule of oxygen gas (O₂). Therefore, the given representation of the decomposition of water illustrates the law of conservation of mass.

Key Concepts

Law of Conservation of Mass

The Law of Conservation of Mass is a foundational principle in chemistry that has stood the test of time since it was first articulated by Antoine Lavoisier in the late 18th century. Put simply, this law states that mass is neither created nor destroyed in a chemical reaction.

During any reaction, the mass of the substances before the reaction (reactants) is always equal to the mass of the substances after the reaction (products). Even if a substance changes form or combines with another, the total mass remains constant. This concept is visualized in the equation \[2H_2O \rightarrow 2H_2 + O_2\], which represents the decomposition of water.

If we weigh the reactants (2 H₂O) and the products (2 H₂ and O₂) of the reaction, we'll find their masses to be identical. This tangible measure reaffirms Dalton's atomic theory, which provides a microscopic explanation for this macroscopic law - atoms are simply rearranged, not created or destroyed, during chemical reactions.

Chemical Reaction

A chemical reaction is a process where substances, known as reactants, transform into new substances called products. The transformation occurs as atoms are rearranged into new configurations.

The decomposition of water (\[2H_2O \rightarrow 2H_2 + O_2\]) is an excellent example. Initially, water molecules are made up of two hydrogen atoms bonded to one oxygen atom. During the reaction, energy is applied (often as electricity or heat), breaking these bonds and allowing the atoms to form new substances: hydrogen gas and oxygen gas.

Visual Representation of Reactions

Diagrams, symbols, and chemical equations are used to represent chemical reactions. These representations are not just arbitrary illustrations; they are grounded in the scientific understanding of atoms and molecules, making them essential for students to comprehend the underlying mechanisms of reactions.

Decomposition of Water

When water (\[H_2O\]) decomposes, it breaks down into hydrogen gas (\[H_2\]) and oxygen gas (\[O_2\]). Decomposition is a type of chemical reaction where a single substance breaks down into two or more simpler substances.

The decomposition of water can be catalyzed by electricity, a process known as electrolysis.

Counting Atoms

Following Dalton's atomic theory, we can count the atoms to ensure the reaction obeys the law of conservation of mass. Two water molecules have a total of four hydrogen atoms and two oxygen atoms. After decomposition, there are exactly four hydrogen atoms in the hydrogen gas and two oxygen atoms in the oxygen gas - no more, no less. This conservation mirrors the unchangeable nature of atoms during chemical reactions as stated in Dalton’s atomic theory.

Understanding the decomposition of water not only explains a specific chemical reaction but also illustrates larger principles, such as the law of conservation of mass and the enduring relevance of Dalton's atomic theory.