Question

${\mathrm{O}}_3(\mathrm{g})$ is a powerful oxidizing agent. Write equations to represent oxidation of $(a)I^{-}$ to $I_2$ in acidic solution; (b) sulfur in the presence of moisture to sulfuric acid; (c) ${[\mathrm{Fe}(\mathrm{CN}{)}_6]}^{4-}$ to ${[\mathrm{Fe}(\mathrm{CN}{)}_6]}^{3-}$ in basic solution. In each case ${\mathrm{O}}_3(\mathrm{g})$ is reduced to ${\mathrm{O}}_2(\mathrm{g})$.

Short answer

The balanced equations are: (a) 2I^- + O3 + H2O = I2 + 2OH- + O2. (b) S + O3 + H2O2 = H2SO4 + 2H2O + O2. (c) \([\mathrm{Fe}(\mathrm{CN})_{6}]^{4-} + O3 + H2O = [\mathrm{Fe}(\mathrm{CN})_{6}]^{3-} + 2OH- + O2.

Step-by-step solution

Identify the Oxidation State Changes

Identify the changes in oxidation state of the species involved in the reaction. (a) $I^{-}$ is oxidized to $I_2$. (b) Sulfur is oxidized to sulfuric acid $H_{2}S{O}_4$. (c) ${[\mathrm{Fe}(\mathrm{CN}{)}_6]}^{4-}$ is oxidized to ${[\mathrm{Fe}(\mathrm{CN}{)}_6]}^{3-}$. In each case, ${\mathrm{O}}_3(\mathrm{g})$ is reduced to ${\mathrm{O}}_2(\mathrm{g})$.

Write Unbalanced Equations

(a) $I^{-}+O_3\to I_2+O_2$ (b) $S+O_3\to H_{2}S{O}_4+O_2$ (c) ${[\mathrm{Fe}(\mathrm{CN}{)}_6]}^{4-}+O_3\to {[\mathrm{Fe}(\mathrm{CN}{)}_6]}^{3-}+O_2$

Balance The Equations using half-reaction method

(a) 2I^- + O3 + H2O = I2 + 2OH- + O2. (b) S + O3 + H2O = H2SO4 + O2. Balance the sulfur and then the oxygen atoms by adding hydrogen peroxide (H2O2) to the left side of the equation and water (H2O) to the right side. (c) \([\mathrm{Fe}(\mathrm{CN})_{6}]^{4-} + O3 = [\mathrm{Fe}(\mathrm{CN})_{6}]^{3-} + O2. Balance the iron and then the oxygen atoms by adding one water (H2O) molecule to the left side of the equation and two hydroxide (OH-) ions to the right side.

Key concepts

Half-reaction method

The half-reaction method is a systematic way to balance oxidation-reduction reactions. It involves dividing the overall reaction into two separate half-reactions: one for oxidation and one for reduction. Each half-reaction is balanced individually before being combined to give the balanced overall equation.

This method helps clearly identify which species are losing or gaining electrons. Here’s how you can approach each part of the half-reactions:

• Write separate half-equations for the oxidation and reduction processes.
• Balance the atoms in each half-reaction, starting with elements other than oxygen and hydrogen.
• Balance oxygen atoms by adding water ( H2O ) molecules, and hydrogen atoms by adding hydrogen ions ( H^+ ).
• Adjust the electron count to ensure charge balance by adding electrons ( e^- ).

Finally, the two balanced half-reactions are combined so that the electrons cancel out, resulting in a balanced overall reaction.

Oxidation state changes

Understanding oxidation state changes is crucial in identifying which species are oxidized or reduced. Oxidation refers to the loss of electrons, while reduction refers to the gain of electrons. Oxidation states can help track these electron movements.

For instance, when you analyze the transformation:$$I^{-}\to I_2$$
Each iodine ion starts with an oxidation state of -1 and changes to 0 in the final iodine molecule.

Identifying oxidation state changes involves:

• Determining the oxidation state of each element in the reactants and products.
• Recognizing which species undergo an increase in oxidation state (oxidation) or a decrease (reduction).
• Pairing the oxidized and reduced species to form full redox reactions.

This knowledge lays the foundation for successfully balancing and understanding redox reactions.

Balancing chemical equations

Balancing chemical equations ensures that the number of atoms for each element is the same on both sides of the equation. This reflects the conservation of mass principle, a fundamental concept in chemistry. In redox reactions, this includes balancing both mass and charge.

Typically, in the context of redox reactions, balancing includes:

• First, addressing individual atoms, typically starting with elements that appear in only one reactant and one product.
• Next, balancing oxygen atoms by adding water molecules.
• Then, balancing hydrogen atoms with hydrogen ions in acidic solutions or hydroxide ions in basic solutions.
• Finally, ensuring that the charges are balanced by adding electrons.

Using these steps ensures that both matter and charge are conserved, providing an accurate representation of the chemical process.