Problem 105 - Chemistry The Science in Context

Question

Earth's Crust The following chemical reactions have helped to shape Earth's crust. Determine the oxidation numbers of all the elements in the reactants and products, and identify which elements are oxidized and which are reduced. a. \(3 \mathrm{SiO}_{2}(s)+2 \mathrm{Fe}_{3} \mathrm{O}_{4}(s) \rightarrow 3 \mathrm{Fe}_{2} \mathrm{SiO}_{4}(s)+\mathrm{O}_{2}(g)\) b. \(\operatorname{SiO}_{2}(s)+2 \operatorname{Fe}(s)+\mathrm{O}_{2}(g) \rightarrow \mathrm{Fe}_{2} \mathrm{SiO}_{4}(s)\) c. \(4 \mathrm{FeO}(s)+\mathrm{O}_{2}(g)+6 \mathrm{H}_{2} \mathrm{O}(\ell) \rightarrow 4 \mathrm{Fe}(\mathrm{OH})_{3}(s)\)

Step-by-Step Solution

Answer

Question: In each of the following reactions, identify which elements are oxidized and which elements are reduced: (a) $3 \mathrm{SiO}_2(s)+2 \mathrm{Fe}_3 \mathrm{O}_4(s) \rightarrow 3 \mathrm{Fe}_2 \mathrm{SiO}_4(s)+\mathrm{O}_2(g)$ (b) $\mathrm{SiO}_2(s)+2 \mathrm{Fe}(s)+\mathrm{O}_2(g) \rightarrow \mathrm{Fe}_2 \mathrm{SiO}_4(s)$ (c) $4 \mathrm{FeO}(s)+\mathrm{O}_2(g)+6 \mathrm{H}_2 \mathrm{O}(l) \rightarrow 4 \mathrm{Fe}(\mathrm{OH})_3(s)$ Answer: (a) Fe is reduced and O is oxidized. (b) Fe is oxidized and O in SiO2 is reduced. (c) Fe is oxidized and O in FeO is reduced.

Step 1: Assign oxidation numbers to all the elements in the reactants

The oxidation numbers for each element are as follows: Si in SiO2: +4 O in SiO2: -2 Fe in Fe3O4: +8/3 (average) O in Fe3O4: -2

Step 2: Assign oxidation numbers to all the elements in the products

The oxidation numbers for each element are as follows: Fe in Fe2SiO4: +2 Si in Fe2SiO4: +4 O in Fe2SiO4: -2 O in O2: 0

Step 3: Identify which elements are oxidized and which are reduced

Comparing the oxidation numbers of the elements in the reactants and products, we can see that: Fe is reduced (from +8/3 to +2) O is oxidized (from -2 in Fe3O4 to 0 in O2) For reaction (b): $\operatorname{SiO}_{2}(s)+2 \operatorname{Fe}(s)+\mathrm{O}_{2}(g) \rightarrow \mathrm{Fe}_{2} \mathrm{SiO}_{4}(s)$

Step 1: Assign oxidation numbers to all the elements in the reactants

The oxidation numbers for each element are as follows: Si in SiO2: +4 O in SiO2: -2 Fe: 0 O in O2: 0

Step 2: Assign oxidation numbers to all the elements in the products

The oxidation numbers for each element are as follows: Fe in Fe2SiO4: +2 Si in Fe2SiO4: +4 O in Fe2SiO4: -2

Step 3: Identify which elements are oxidized and which are reduced

Comparing the oxidation numbers of the elements in the reactants and products, we can see that: Fe is oxidized (from 0 to +2) O in SiO2 is reduced (from -2 to -2 in Fe2SiO4) For reaction (c): $4 \mathrm{FeO}(s)+\mathrm{O}_{2}(g)+6 \mathrm{H}_{2} \mathrm{O}(\ell) \rightarrow 4 \mathrm{Fe}(\mathrm{OH})_{3}(s)$

Step 1: Assign oxidation numbers to all the elements in the reactants

The oxidation numbers for each element are as follows: Fe in FeO: +2 O in FeO: -2 O in O2: 0 H in H2O: +1 O in H2O: -2

Step 2: Assign oxidation numbers to all the elements in the products

The oxidation numbers for each element are as follows: Fe in Fe(OH)3: +3 O in Fe(OH)3: -2 H in Fe(OH)3: +1

Step 3: Identify which elements are oxidized and which are reduced

Comparing the oxidation numbers of the elements in the reactants and products, we can see that: Fe is oxidized (from +2 to +3) O in FeO is reduced (from -2 to -2 in Fe(OH)3)

Key Concepts

Chemical Reactions

Chemical reactions are processes where substances are transformed into new substances with different properties. They involve the rearrangement of atoms to form new compounds.

In the context of the Earth's crust, these reactions can have a significant impact. The Earth's crust is constantly changing due to chemical reactions between minerals. For example, when silicon dioxide \((SiO_2)\) reacts with iron oxide \((Fe_3O_4)\), it forms a new compound, ferrous silicate \((Fe_2SiO_4)\), and oxygen \((O_2)\).

The reactants, \(SiO_2\) and \(Fe_3O_4\), lose and gain atoms to form the products. This rearrangement of atoms is the essence of a chemical reaction.

• Reactants are the starting materials in a chemical reaction.
• Products are the substances formed as a result of the reaction.

Understanding chemical reactions helps us grasp how matter changes and is fundamental to the study of chemistry.

Earth's Crust

The Earth's crust is the outermost layer of our planet. It is a thin shell compared to the Earth's other layers but plays a crucial role in the natural environment.

Composed mostly of rocks and minerals, the crust is constantly being shaped by both natural processes and human activity.

• The crust contains elements like oxygen, silicon, aluminum, iron, calcium, and more.
• Many chemical reactions, such as oxidation, occur naturally within the crust.

These reactions help form the diverse range of minerals that we find in the Earth's crust. For example, the oxidation of iron can turn it from its metallic state into rust, which is an iron oxide mineral. Understanding the chemical reactions that shape the Earth's crust gives us insight into geological processes, mineral formation, and even the availability of natural resources.

Oxidation and Reduction

Oxidation and reduction are chemical processes that often occur together and are known as redox reactions. In these reactions, there is a transfer of electrons between elements, which alters their oxidation numbers.

Oxidation involves losing electrons, resulting in an increase in oxidation number. For example, when iron \((Fe)\) changes from an oxidation state of 0 to +3, it is oxidized.

Reduction, on the other hand, involves gaining electrons, resulting in a decrease in oxidation number. In redox reactions, one substance is oxidized while another is reduced.

• The substance that gives away electrons and becomes oxidized is known as the reducing agent.
• The substance that accepts electrons and becomes reduced is known as the oxidizing agent.

These reactions are vital in many natural processes, including those that shape the Earth's crust. They can also be observed in everyday phenomena, like the rusting of iron or the browning of an apple.