Question

When iron rusts in moist air, the product is typically a mixture of two iron- oxygen compounds. In one compound, there is an equal number of iron and oxygen atoms. In the other compound, there are three oxygen atoms for every two iron atoms. Write the formulas for the two iron oxides.

Short answer

The chemical formulas for the two iron oxide compounds are \(FeO\) and \(Fe_2O_3\).

Step-by-step solution

Understand the elements and their valencies

We are working with two elements: iron (Fe) and oxygen (O). They have valencies of 3 or 2 (Fe) and 2 (O). Valency represents how capable an element is of combining with others to form chemical compounds.

Write the chemical formulas based on valencies

For the first compound where there is an equal number of iron and oxygen atoms, we choose the lowest common multiple of the valencies of Fe and O. Since iron can have valencies of either 2 or 3, and oxygen has a valency of 2, the lowest common multiple is 2. Therefore, the formula for the first iron oxide compound is FeO. For the second compound where there are 3 oxygen atoms for every 2 iron atoms, we multiply the valencies (2 or 3 for Fe, and 2 for O) by the ratio (2:3). If we use the valency of 2 for Fe, we get Fe₂O₃, but if we use the valency of 3, we get Fe₃O₆, which is not in the desired 2:3 ratio. So, the formula for the second iron oxide compound is Fe₂O₃.

Final answer

The chemical formulas for the two iron oxide compounds are FeO and Fe₂O₃.

Key concepts

Iron Oxides

Iron oxides are fascinating compounds formed when iron (Fe) reacts with oxygen (O) in the presence of moisture. These compounds are commonly found as rust on metals exposed to air and moisture, showcasing their importance in real-world applications. Iron oxides are named based on the number of iron and oxygen atoms present in their chemical structure. The two most known types are ferrous oxide (FeO) and ferric oxide (Fe₂O₃).

• Ferrous oxide (FeO): This compound is formed with equal numbers of iron and oxygen atoms, which gives it the simplest form among iron oxides. It appears as a black powder and differs from rust in its color and properties.
• Ferric oxide (Fe₂O₃): This red-brown compound is composed of three oxygen atoms for every two iron atoms. It's the most common form of iron oxide found in rust, providing that characteristic reddish-brown hue we associate with corrosion.

Understanding these compound formulas offers insight into how iron interacts with its environment, resulting in different forms based on oxygen intake.

Valency

Valency is a critical concept in chemistry that explains how atoms of a given element can combine with others to form compounds. It represents the ability of an atom to bond with other atoms, signifying the number of chemical bonds it can form.

The element iron, for instance, possesses valencies of 2 and 3. Oxygen typically has a valency of 2. This means that when forming compounds,

• Iron with valency of 2 can pair with one oxygen atom each (FeO).
• Iron with valency of 3 is combined with three oxygen atoms to achieve stability, as in Fe₂O₃.

The variation in valency allows an element like iron to form multiple compounds, each with distinct chemical and physical properties. Understanding how valency dictates compound formation helps explain why we encounter different iron oxides, each differently interacting with the environment.

Compound Formation

The formation of chemical compounds occurs when elements with different valencies bond together. This process involves the sharing or transferring of electrons between atoms.

For example, iron and oxygen combine to form iron oxides through valency interactions:

• In FeO, both iron and oxygen satisfy their valency needs by sharing electrons equally. This forms a stable bond with one oxygen atom per iron atom.
• In Fe₂O₃, the ratio is 2:3, where two iron atoms share electrons with three oxygen atoms. This satisfies the valency of iron (3 in this case) and the valency of oxygen (2).

Understanding how elements form compounds is crucial for predicting chemical behavior, synthesizing new materials, and interpreting natural phenomena like rusting. In practical terms, recognizing the requirement of a stable ratio eases the prediction of potential compounds iron may form, based on its environment and chemical interactions.