Problem 53 - Chemistry: The Central Science

Question

Ferrous sulfate \(\left(\mathrm{FeSO}_{4}\right)\) is often tued as a coagulant in water purification. The iron(II) salt is dissolved in the water to be purified, then oxidized to the iron(III) state by dissolved exygen, at which time gelatinous \(\mathrm{Fe}(\mathrm{OH})_{3}\) forms, assuming the \(\mathrm{pH}\) is abeve approximately 6 . Write balanced chemical equations for the oxidation of \(\mathrm{Fe}^{2+}\) to \(\mathrm{Fe}^{3+}\) by dissolved oxygen and for the formation of \(\mathrm{Fe}(\mathrm{OH})_{3}(s)\) by reaction of \(\mathrm{Fe}^{3+}(a q)\) with \(\mathrm{HCO}_{3}^{-}(a q)\) -

Step-by-Step Solution

Answer

The balanced chemical equations for the given processes are: 1. Oxidation of Fe²⁺ to Fe³⁺ by dissolved oxygen: \( 4Fe^{2+} + O_2 \rightarrow 4Fe^{3+} + 2O^{2-} \) 2. Formation of Fe(OH)₃ by the reaction of Fe³⁺ with HCO₃⁻: \( 6HCO_3^- + 2Fe^{3+} \rightarrow 2Fe(OH)_3 + 6CO_2 \)

Step 1: Oxidation of Fe²⁺ to Fe³⁺ by dissolved oxygen

First, we will write the unbalanced chemical equation for the oxidation of Fe²⁺ to Fe³⁺ by dissolved oxygen: \( Fe^{2+} + O_2 \rightarrow Fe^{3+} \) To balance this equation, we need to make sure there is an equal number of atoms of each element on both sides: \( 4Fe^{2+} + O_2 \rightarrow 4Fe^{3+} + 2O^{2-} \)

Step 2: Formation of Fe(OH)₃ by the reaction of Fe³⁺ with HCO₃⁻

Now, we will write the unbalanced chemical equation for the formation of Fe(OH)₃ by the reaction of Fe³⁺ with HCO₃⁻: \( Fe^{3+} + HCO_3^- \rightarrow Fe(OH)_3 + CO_2 \) To balance this equation, we need to make sure there is an equal number of atoms of each element on both sides: \( 6HCO_3^- + 2Fe^{3+} \rightarrow 2Fe(OH)_3 + 6CO_2 \) So, the balanced chemical equations for the given processes are: 1. Oxidation of Fe²⁺ to Fe³⁺ by dissolved oxygen: \( 4Fe^{2+} + O_2 \rightarrow 4Fe^{3+} + 2O^{2-} \) 2. Formation of Fe(OH)₃ by the reaction of Fe³⁺ with HCO₃⁻: \( 6HCO_3^- + 2Fe^{3+} \rightarrow 2Fe(OH)_3 + 6CO_2 \)

Key Concepts

Ferrous Sulfate in Water Treatment

Ferrous sulfate \(\mathrm{FeSO}_{4}\) serves a critical role in the purification of water. It acts as a coagulant, a substance that causes particles to clump together, so they can be more easily removed from water. When ferrous sulfate dissolves in water, the iron(II) ions \(\mathrm{Fe}^{2+}\) are released. These iron ions interact with contaminants, helping to remove them from the solution. Moreover, the iron(II) is oxidized to iron(III), which forms compounds that precipitate out of the water, further aiding in the purification process. This chemical adjustment is dependent on certain pH conditions; a pH above approximately 6.0 is generally required for optimal coagulation and subsequent removal of contaminants.

Understanding the role of ferrous sulfate in water treatment helps students grasp the larger concept of how chemistry is applied in practical and environmentally beneficial ways. The specific chemical reactions involved highlight the importance of reaction conditions, such as pH levels, in achieving desired outcomes in real-world applications.

Oxidation Reactions

Oxidation reactions are processes where a substance loses electrons, which is commonly associated with a gain of oxygen or loss of hydrogen. In water treatment, the oxidation of iron plays a pivotal role. Iron in the ferrous state \(\mathrm{Fe}^{2+}\) is oxidized to the ferric state \(\mathrm{Fe}^{3+}\) by dissolved oxygen in the water. This oxidation reaction helps to form compounds that precipitate and can be filtered out of the water.

During this process, the iron goes from a +2 to a +3 oxidation state, indicating it has lost an electron. Dissolved oxygen in water facilitates this electron transfer, making it instrumental in converting iron to a form that promotes coagulation. Oxidation reactions like this are not just limited to water treatment; they are central to many biological and environmental processes, emphasizing the importance of understanding this type of chemical reaction for students.

Chemical Equations Balancing

Chemical equations need to be balanced to adhere to the law of conservation of mass, which states that matter cannot be created or destroyed in a chemical reaction. Students are often tasked with balancing chemical equations as a fundamental exercise in chemistry that requires understanding the stoichiometry of reactions.

In the context of water treatment using ferrous sulfate, we balance the chemical equations to ensure that the number of atoms of each element is the same on both sides of the equation. This process involves adding coefficients to the reactants and products such that the mass and charge are conserved. The balancing of equations can be thought-provocative, but it's a critical skill for students to master as it underpins all stoichiometric calculations in chemistry.

Iron Coagulation Process

The iron coagulation process is a series of reactions that involve the transformation of soluble iron compounds into insoluble ones, ultimately aiding in the removal of impurities from water. When ferrous iron \(\mathrm{Fe}^{2+}\) is oxidized to ferric iron \(\mathrm{Fe}^{3+}\), it reacts with hydroxide ions to form ferric hydroxide \(\mathrm{Fe(OH)}_{3}\), which is a gelatinous precipitate.

This precipitate can adsorb and trap particulate impurities, leading to their removal from the water upon filtration. The coagulation process is also sensitive to the pH of the water; at lower pH levels, the reaction may not proceed efficiently. Thus, this process is not just a set of chemical reactions but a practical application that requires careful control of reaction conditions to achieve the desired outcome of clean, safe water.