Problem 44 - General Chemistry: Principles and Modern Applications

Question

Write a balanced equation for the redox reactions. (a) The reaction of aluminum metal with hydroiodic acid. (b) The reduction of vanadyl ion ( \(\mathrm{VO}^{2+}\) ) to vanadic ion \(\left(\mathrm{V}^{3+}\right)\) in acidic solution with zinc metal as the reducing agent. (c) The oxidation of methanol by chlorate ion in acidic solution, producing carbon dioxide gas, water, and chlorine dioxide gas as products.

Step-by-Step Solution

Answer

(a) Balanced equation: 2Al + 6HI = 2AlI3 + 3H2. (b) Balanced equation: VO2+ + Zn + 2H+ = V3+ + Zn2+ + H2O. (c) Balanced equation: 3CH3OH + 5ClO3- + 12H+ = 3CO2 + 14H2O + 5ClO2.

Step 1: Part (a) Reaction of Aluminum Metal with Hydroiodic Acid

The unbalanced chemical reaction can be written as follows: \[Al + HI \rightarrow AlI_3 +H_2.\] Now, balance the equation: \[2Al + 6HI \rightarrow 2AlI_3 + 3H_2.\]

Step 2: Part (b) Reduction of Vanadyl Ion to Vanadic Ion

The unbalanced half-reactions are: \[VO^{2+} \rightarrow V^{3+}\] and \[Zn \rightarrow Zn^{2+}.\] After balancing, we find: \[VO^{2+} + Zn + 2H^{+} \rightarrow V^{3+} + Zn^{2+} + H_2O.\]

Step 3: Part (c) The Oxidation of Methanol by Chlorate Ion

Unbalanced reactions are: \[ CH_3OH \rightarrow CO_2 + H_2O\], and \[ClO_3^- \rightarrow ClO_2 + O_2.\] After balancing we have: \[ 3CH_3OH + 5ClO_3^- + 12H^{+} \rightarrow 3CO_2 + 14H_2O + 5ClO_2.\]

Key Concepts

Balancing Chemical Equations

Balancing chemical equations is a fundamental skill in chemistry that ensures the law of conservation of mass is followed in reactions. When chemical reactions occur, atoms are neither created nor destroyed. Instead, they are rearranged.
Hence, it's crucial to have the same number of each type of atom on both sides of the chemical equation.
In the provided examples, balancing the equations involved several steps:

• The reaction of aluminum with hydroiodic acid requires ensuring that aluminum and hydrogen atoms balance correctly on both sides, achieved by adjusting the coefficients, resulting in the balanced equation: \[2Al + 6HI \rightarrow 2AlI_3 + 3H_2.\]
• For the oxidation of methanol by chlorate ion, a similar balancing act is performed by adjusting coefficients to ensure the mass and charge balance in the acidic solution, giving \[3CH_3OH + 5ClO_3^- + 12H^{+} \rightarrow 3CO_2 + 14H_2O + 5ClO_2.\]

Careful adjustments of coefficients are necessary to match atom numbers on both sides of the equation.

Oxidation-Reduction

Oxidation-reduction reactions, or redox reactions, are essential in chemistry as they involve the transfer of electrons between chemical species.
In a redox process, one species will undergo oxidation (lose electrons), and another will undergo reduction (gain electrons).
Identifying which components gain or lose electrons helps in determining the overall reaction.

• In the case of vanadyl ion reducing to vanadic ion with zinc, the vanadyl ion \( \mathrm{VO}^{2+} \) is reduced (gains electrons), while the zinc metal is oxidized (loses electrons).
• For the methanol oxidation by chlorate ion, methanol undergoes oxidation (loses electrons), while the chlorate ion undergoes reduction (gains electrons).

Recognizing these electron transitions is key in solving and understanding redox reactions.

Half-Reaction Method

The half-reaction method is a systematic approach used to balance redox reactions, focusing on the separate oxidation and reduction processes. By isolating these processes, the balancing of both mass and charge becomes more manageable.
Each redox reaction is split into two half-equations: one for oxidation and one for reduction.
These are then balanced individually before being recombined.

• For example, in the reduction of vanadyl ion to vanadic ion, the half-reactions for \( \mathrm{VO}^{2+} \rightarrow \mathrm{V}^{3+} \) and \( \mathrm{Zn} \rightarrow \mathrm{Zn}^{2+} \) are balanced separately before adding them together to get the final balanced equation: \[VO^{2+} + Zn + 2H^{+} \rightarrow V^{3+} + Zn^{2+} + H_2O.\]
• This method is particularly useful in more complex redox reactions, such as the oxidation of methanol by chlorate ion, where different elements and charges are involved.

The half-reaction method simplifies balancing by treating oxidation and reduction as distinct processes before uniting them into a complete balanced equation.