Question

Write the balanced half-reaction involving \(\mathrm{O}_{2}\) that occurs in acidic waters when it oxidizes organic matter.

Short answer

The balanced half-reaction is \( ext{O}_2 + 4 ext{H}^+ + 4e^- ightarrow 2 ext{H}_2 ext{O}\).

Step-by-step solution

Understanding Half-Reaction

In acidic conditions, the half-reaction involving oxygen as an oxidizing agent is written as it gains electrons. The reaction in acidic water usually involves the conversion of \( ext{O}_2\) to \( ext{H}_2 ext{O}\).

Writing the Unbalanced Half-Reaction

Start by writing the basic half-reaction for oxygen gaining electrons. The unbalanced equation is \( ext{O}_2 ightarrow ext{H}_2 ext{O} \).

Balancing Oxygen Atoms

Balance the oxygen atoms by adding water molecules. Each molecule of \( ext{O}_2\) forms two molecules of \( ext{H}_2 ext{O}\). The equation becomes \( ext{O}_2 + 2 ext{H}_2 ext{O} ightarrow 2 ext{H}_2 ext{O}\).

Balancing Hydrogen Atoms

To balance hydrogen atoms, add hydrogen ions \( ext{H}^+\) to the reactant side. Since we have 4 hydrogen atoms in \(2 ext{H}_2 ext{O}\), we add 4 \( ext{H}^+\). The equation now looks like \( ext{O}_2 + 4 ext{H}^+ ightarrow 2 ext{H}_2 ext{O}\).

Balancing Electrons

Balance the charge by adding electrons on the reactant side. The right side has no net charge, while the left side has a +4 charge from \( ext{H}^+\). To balance, add 4 electrons to the left: \( ext{O}_2 + 4 ext{H}^+ + 4e^- ightarrow 2 ext{H}_2 ext{O}\).

Key concepts

Acidic Conditions

In the context of oxidation-reduction reactions, acidic conditions refer to environments where the concentration of hydrogen ions \(\mathrm{H^+}\) is significant. This is common in various natural and man-made settings, such as acidic waters and industrial processes. When performing reactions under these conditions, it is crucial to consider the role of hydrogen ions. They help balance the equations, especially in redox reactions where electron transfer occurs.

• Hydrogen ions are added to either side of the chemical equation to maintain mass and charge neutrality.
• They aid in converting molecular oxygen \(\mathrm{O}_2\) into water \(\mathrm{H}_2\mathrm{O}\).
• Acidic conditions are a common setting for redox reactions in water bodies like rivers and lakes.

Maintaining balance in these systems is important for predicting the behavior of pollutants and understanding biochemical cycles.

Half-Reaction Balancing

Balancing half-reactions is a methodical process in redox chemistry used to ensure that both matter and charge are conserved. When balancing half-reactions, particularly in acidified conditions, several steps must be taken. First, determine the basic form of the reaction equation.
Next, balance the oxygen atoms typically using water molecules. This is followed by hydrogen, adjusted using hydrogen ions \(\mathrm{H^+}\). Finally, balance the charge with electrons, which is vital for the integrity of the equation.

• Identify the initial and final forms of the reactant and product.
• Add water to balance oxygen or hydrogen ions to balance hydrogen.
• Add electrons to ensure both sides of the equation are electronically neutral.

This procedure is essential for accurately describing how substances are transformed in a reaction.

Oxygen as an Oxidizing Agent

Oxygen has a strong tendency to accept electrons, making it a powerful oxidizing agent. In chemical reactions like those in acidic waters, oxygen plays a critical role by driving the oxidation process forward. It achieves this by undergoing reduction itself, transforming from \(\mathrm{O}_2\) to \(\mathrm{H}_2\mathrm{O}\) after accepting electrons.
This transformation illustrates the versatility and strength of oxygen in facilitating redox reactions.

• The reduction of \(\mathrm{O}_2\) to \(\mathrm{H}_2\mathrm{O}\) is a common reaction in both biological and industrial processes.
• Oxygen acts as the electron acceptor, leading to the oxidation of organic matter.
• This process is crucial for converting energy in cells and impacts environmental and engineering systems.

Understanding oxygen's role helps in designing systems that leverage these reactions, such as wastewater treatment and energy generation.

Electron Transfer in Chemical Reactions

The transfer of electrons is a fundamental aspect of redox reactions, governing the transformation of reactants into products. During these reactions, electrons move from a reductant (which loses electrons) to an oxidant (which gains electrons like oxygen in acidic conditions). This electron flow is what drives the chemical change and energy release or consumption in a system.
Key aspects of electron transfer include:

• Understanding the donor and acceptor of electrons is critical in predicting reaction paths.
• Electron transfer is often associated with energy exchange, forming the basis of metabolic and industrial processes.
• Controlled electron transfer is essential for battery technology, corrosion prevention, and biochemical pathways.

Recognizing how electrons facilitate these transformations enables scientists and engineers to innovate and optimize various chemical and biological systems.