Problem 8 - Chemistry An Atoms First Approach

Question

Consider an iron bar on a balance as shown. $$75.0 \mathrm{g}$$ As the iron bar rusts, which of the following is true? Explain your answer. a. The balance will read less than \(75.0 \mathrm{g}\). b. The balance will read \(75.0 \mathrm{g}\). c. The balance will read greater than \(75.0 \mathrm{g}\). d. The balance will read greater than \(75.0 \mathrm{g},\) but if the bar is removed, the rust is scraped off, and the bar replaced, the balance will read 75.0 g.

Step-by-Step Solution

Answer

The correct answer is option c: The balance will read greater than \(75.0 \mathrm{g}\). As the iron bar rusts, it combines with atmospheric oxygen, increasing its mass due to the formation of hydrated iron(III) oxide (rust).

Step 1: Recall the Chemical Reaction for Rust Formation

Rusting is a chemical reaction in which iron (Fe) reacts with atmospheric oxygen (O₂) and water (H₂O) to form hydrated iron(III) oxide, also known as rust (Fe₂O₃ ⋅ nH₂O). This reaction adds mass to the iron bar since oxygen from the atmosphere is combined with the iron.

Step 2: Evaluate Option a

Option a states that the balance will read less than 75.0 g. This statement is false because the chemical reaction that produces rust adds mass to the iron bar, not reducing it.

Step 3: Evaluate Option b

Option b states that the balance will read 75.0 g. This statement is false because, as we mentioned previously, the rusting process adds mass to the iron bar. Therefore, the balance reading will not remain at 75.0 g.

Step 4: Evaluate Option c

Option c states that the balance will read greater than 75.0 g. This statement is true because the process of rusting adds mass to the iron bar. The bar's mass increases as a result of iron reacting with atmospheric oxygen, resulting in the formation of hydrated iron(III) oxide (rust). Therefore, the balance will read more than the initial mass of 75.0 g.

Step 5: Evaluate Option d

Option d states that the balance will read greater than 75.0 g, but if the bar is removed, the rust is scraped off, and the bar is replaced, the balance will read 75.0 g. This statement is false because when the rust is scraped off, some of the original iron is also removed in the process. As a result, the mass of the remaining iron bar will be less than the initial mass of 75.0 g.

Step 6: Choose the correct answer

Based on the evaluation of all options, the correct statement is option c: The balance will read greater than 75.0 g.

Key Concepts

Rust Formation

When we observe an iron object over time, we may notice a reddish-brown coating forming on its surface, which is commonly known as rust. This is not merely a sign of age but a clear indication of a chemical process taking place.

Rust formation is the result of iron reacting with oxygen in the presence of water or moisture in the environment. This reaction is scientifically termed as the oxidation of iron, and the chemical equation for the formation of rust can be written as:
\[\begin{equation}4Fe + 3O_2 + xH_2O \rightarrow 2Fe_2O_3 \text{⋅} xH_2O\text{(rust)}\text{{Hydrated iron(III) oxide}}\text{{(iron + oxygen + water \(\rightarrow\) rust)}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}Ă̈Ă̈n\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}\text{{}}Ă̈Ă̈\end{equation}\]It's important to understand that rust is essentially hydrated iron(III) oxide, which means it includes not only iron and oxygen but also water molecules incorporated into its structure. This contributes to the increase in mass during the rusting process. Additionally, factors like environmental conditions and the presence of electrolytes can accelerate the rate of rust formation.

Understanding rust formation gives us insight into how certain materials deteriorate over time and why protection methods, such as painting or galvanization, are necessary to prevent the material damage caused by this process.

Conservation of Mass

The concept of conservation of mass is a fundamental principle that dictates that mass cannot be created or destroyed in an isolated system during a chemical reaction. This principle was formulated by Antoine Lavoisier in the 18th century and is foundational to the field of chemistry.

In the context of the rusting iron bar, the conservation of mass implies that the total mass of the system must remain constant, provided no material is lost or added from outside the system. When iron rusts, oxygen from the air combines with the iron, which means the mass of the resulting rust must equal the mass of the original iron plus the mass of the oxygen that was added.

• Initial Mass: Mass of iron bar before rusting.
• Final Mass: Mass of rust (iron bar + oxygen + water).

Thus, if we were to perfectly capture all the rust and reweigh the iron bar with its rust, the mass would be greater than the original weight of the iron bar alone. This is because the additional oxygen and water from the air have become part of the rusted bar, contributing to its overall mass. However, if any rust flakes off and is lost from the system, then the observed mass would decrease from the original mass, but this would not violate the conservation of mass because the system was not isolated (mass was allowed to leave the system).

Chemical Reactions

A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking of chemical bonds between atoms, without any change to the nuclei (no change to the elements present), and can often be described by a chemical equation.

For the iron bar, the rusting process is a chemical reaction involving the iron atoms in the bar, the oxygen molecules from the air, and water (usually from the humidity in the atmosphere). The atoms in the reacting substances (reactants) rearrange themselves to create new substances (products). In the case of the rusting equation provided earlier, the reactants are iron, oxygen, and water, and the single product is rust (hydrated iron(III) oxide).

• Reactants: Iron (Fe), Oxygen (O2), and Water (H2O).
• Products: Rust (Hydrated iron(III) oxide).

The chemical equation provides a shorthand way of expressing the substances involved, the relative quantities, and the direction of the reaction. The arrow in the equation can be read as 'reacts to form' or 'yields.' This particular reaction is also categorized as an oxidation-reduction (redox) reaction, with iron being oxidized (losing electrons) and oxygen being reduced (gaining electrons). Understanding the nature of chemical reactions is essential for predicting the behavior of matter under various conditions and is a critical component in fields ranging from materials science to biochemistry.