Question

Ferrous sulfate \(\left(\mathrm{FeSO}_{4}\right)\) is often used 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 oxygen, at which time gelatinous \(\mathrm{Fe}(\mathrm{OH})_{3}\) forms, assuming the \(\mathrm{pH}\) is above 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 \(\mathrm{Fe}(\mathrm{OH})_{3}(\mathrm{~s})\) by reaction of \(\mathrm{Fe}^{3+}(a q)\) with \(\mathrm{HCO}_{3}^{-}(a q)\).

Short answer

The balanced chemical equations for the water purification process using Ferrous sulfate are: 1. Oxidation of iron(II) to iron(III) by dissolved oxygen: \(4 \mathrm{Fe}^{2+}(a q)+\mathrm{O}_{2}(g) \rightarrow 4 \mathrm{Fe}^{3+}(a q)\) 2. Formation of iron(III) hydroxide by reaction of iron(III) with bicarbonate ions: \(\mathrm{Fe}^{3+}(a q)+3 \mathrm{HCO}_{3}^{-}(a q) \rightarrow \mathrm{Fe}(\mathrm{OH})_{3}(s)+3 \mathrm{CO}_{2}(g)\)

Step-by-step solution

Write the unbalanced equation for oxidation of iron(II) to iron(III) by dissolved oxygen

We can start by writing the unbalanced equation for the oxidation of iron(II) (Fe2+) to iron(III) (Fe3+) by dissolved oxygen (O2). Fe2+(aq) + O2(g) -> Fe3+(aq)

Balance the equation for the oxidation of iron(II) to iron(III) by dissolved oxygen

To balance the equation from step 1, we need to make sure that the number of atoms of each element is the same on both sides of the equation. We achieve this by adjusting the coefficients of the reactants and products. The balanced equation for the oxidation of iron(II) to iron(III) by dissolved oxygen is: 4 Fe2+(aq) + O2(g) -> 4 Fe3+(aq)

Write the unbalanced equation for iron (III) hydroxide formation

Now, we need to write the unbalanced equation for the formation of iron(III) hydroxide (Fe(OH)3) by the reaction of iron(III) (Fe3+) with bicarbonate ions (HCO3-): Fe3+(aq) + HCO3-(aq) -> Fe(OH)3(s) + CO2(g)

Balance the equation for iron (III) hydroxide formation

To balance the equation from step 3, we need to make sure that the number of atoms of each element is the same on both sides of the equation. We achieve this by adjusting the coefficients of the reactants and products. The balanced equation for the formation of iron(III) hydroxide (Fe(OH)3) by the reaction of iron(III) (Fe3+) with bicarbonate ions (HCO3-) is: Fe3+(aq) + 3 HCO3-(aq) -> Fe(OH)3(s) + 3 CO2(g)

Key concepts

Redox Reactions

Redox reactions, short for reduction-oxidation reactions, are chemical processes that involve the transfer of electrons between two substances. They are essential to many biological processes, industrial applications, and even in environmental chemistry, as we can see in the example of water treatment.

In a redox reaction, one substance gets oxidized by losing electrons, while the other gets reduced by gaining electrons. The exercise mentioned focuses on the oxidation of iron(II) ions (Fe²⁺) to iron(III) ions (Fe³⁺). Oxygen from the air plays the role of the oxidizing agent, accepting electrons from the iron(II) ions which are being oxidized.

Understanding redox reactions is crucial because they are not only about electron transfer, but they also involve a change in the oxidation state of the elements involved. For this transformation from Fe²⁺ to Fe³⁺, we see iron's oxidation state increase, indicating it has lost electrons to oxygen, a sign of oxidation.

Iron Compounds in Water Treatment

Iron compounds are frequently used in the treatment of water due to their ability to remove contaminants and improve water quality. Specifically, ferrous sulfate (FeSO₄) is used as a coagulant. In the water, this compound reacts to form gelatinous iron(III) hydroxide (Fe(OH)₃), which helps to coagulate and remove suspended particles.

The efficacy of iron compounds in water treatment relies on controlled chemical reactions and the pH of the water. Iron(II) sulfate is dissolved in water and is then oxidized to iron(III), usually by dissolved oxygen in the water. This is a redox reaction where iron's oxidation state changes. If the pH is above approximately 6, iron(III) hydroxide precipitates out and can absorb or trap impurities. This process is central to removing pathogens and particulates, making the water safer for consumption.

Furthermore, this chemical reaction demonstrates the importance of understanding how different substances interact in varying environmental conditions and why mastering these reactions is crucial for professionals working in environmental sciences and public health.

Chemical Stoichiometry

Chemical stoichiometry involves the quantitative relationship between reactants and products in a chemical reaction. It allows us to predict the amount of each substance required and produced in a reaction. The steps highlighted in the solution exemplify stoichiometry by balancing chemical equations to ensure that the law of conservation of mass is upheld.

In the provided solution, the process includes writing the unbalanced equations for the oxidization of iron and the formation of iron(III) hydroxide, followed by balancing these equations. When balancing, coefficients are adjusted to have an equal number of atoms for each element on both sides of the equation. This balancing act is the essence of chemical stoichiometry and is crucial in predicting the outcomes of reactions in practical applications, like water treatment.

For students, learning to balance chemical equations is not just about solving homework problems; it is about understanding the underlying principles that govern all chemical reactions. This foundational knowledge is vital across many scientific fields, from engineering to environmental science.