Problem 64 - Chemistry The Molecular Science

Question

For each of these solutions, tell what reactions take place at the anode and at the cathode during electrolysis. (a) \(\mathrm{NiBr}_{2}(\mathrm{aq})\) (b) \(\mathrm{NaI}(\mathrm{aq})\) (c) \(\mathrm{CdCl}_{2}(\mathrm{aq})\) (d) \(\mathrm{CuI}_{2}(\mathrm{aq})\) (e) \(\mathrm{MgF}_{2}(\mathrm{aq})\) (f) \(\mathrm{HNO}_{3}(\mathrm{aq})\)

Step-by-Step Solution

Answer

Cathode: reduction of cations or water; Anode: oxidation of anions or water.

Step 1: Determine the components of each electrolyte

For each solution, identify the cation and anion that results from the dissolution of the salt in water:(a) \( \text{NiBr}_2 \) dissociates into Ni²⁺ and 2Br⁻(b) \( \text{NaI} \) dissociates into Na⁺ and I⁻(c) \( \text{CdCl}_2 \) dissociates into Cd²⁺ and 2Cl⁻(d) \( \text{CuI}_2 \) dissociates into Cu²⁺ and 2I⁻(e) \( \text{MgF}_2 \) dissociates into Mg²⁺ and 2F⁻(f) \( \text{HNO}_3 \) dissociates into H⁺ and NO₃⁻.

Step 2: Identify the reaction at the cathode

At the cathode, reduction takes place, meaning that cations gain electrons:(a) \( \text{Ni}^{2+} + 2e^- \rightarrow \text{Ni} \)(b) \( \text{2H}_2\text{O} + 2e^- \rightarrow \text{H}_2 + 2OH^- \)(c) \( \text{Cd}^{2+} + 2e^- \rightarrow \text{Cd} \)(d) \( \text{Cu}^{2+} + 2e^- \rightarrow \text{Cu} \)(e) \( \text{2H}_2\text{O} + 2e^- \rightarrow \text{H}_2 + 2OH^- \)(f) \( \text{2H}_2\text{O} + 2e^- \rightarrow \text{H}_2 + 2OH^- \)Note that in some cases, water can also be reduced instead of the cation, depending on the electrode potential.

Step 3: Identify the reaction at the anode

At the anode, oxidation takes place, meaning that anions lose electrons:(a) \( \text{2Br}^- \rightarrow \text{Br}_2 + 2e^- \)(b) \( \text{2I}^- \rightarrow \text{I}_2 + 2e^- \)(c) \( \text{2Cl}^- \rightarrow \text{Cl}_2 + 2e^- \)(d) \( \text{2I}^- \rightarrow \text{I}_2 + 2e^- \)(e) \( \text{2F}^- \rightarrow \text{F}_2 + 2e^- \) (However, this reaction is less favorable compared to water oxidation)(f) Nitrate ions are stable, so water is oxidized: \( \text{2H}_2\text{O} \rightarrow \text{O}_2 + 4H^+ + 4e^- \).

Key Concepts

Cathode reactions

In an electrolysis process, the cathode is the electrode where reduction occurs. At this point, cations in the solution gain electrons, converting them into neutral atoms. Let's explore what happens at the cathode during electrolysis with different aqueous solutions.

In electrolysis, it’s crucial to determine what component undergoes reduction at the cathode. This primarily involves cations, which are positively charged. When a cation approaches the cathode, it gains electrons to become a neutral atom. This is known as a reduction reaction. For example:

\[\text{Ni}^{2+} + 2e^- \rightarrow \text{Ni} \]
Here, the nickel ion (\text{Ni}^{2+}) receives two electrons to become neutral nickel at the cathode.

Sometimes, water itself can be reduced at the cathode instead of the cation, depending on their respective electrode potentials. An example of water being reduced is:

\[\text{2H}_2\text{O} + 2e^- \rightarrow \text{H}_2 + 2OH^-\]
This happens because water is a more favorable reactant for reduction than some cations, like Na⁺ in NaI (\text{aq}). The choice depends on the comparative ease (or electrode potentials) of reducing a particular ion versus water.

Understanding cathode reactions is crucial as it defines the products formed in electrolysis, deeply impacting industrial processes such as metal plating and hydrogen production.

Anode reactions

At the anode, oxidation occurs. This means anions, which are negatively charged, lose electrons, converting into neutral molecules or atoms. Let's examine the anode reactions for some aqueous solutions during electrolysis.

For every cation reduced at the cathode, an anion must be oxidized at the anode. This involves the loss of electrons, and often results in the forming of gaseous elements. Consider the reaction:

\[\text{2Br}^- \rightarrow \text{Br}_2 + 2e^- \]
In this case, bromide ions (\text{Br}^-) lose electrons to form bromine gas (\text{Br}_2) at the anode.

Similarly, iodide ions (\text{I}^-) and chloride ions (\text{Cl}^-) are oxidized to form iodine (\text{I}_2) and chlorine (\text{Cl}_2) gases, respectively. However, not all anions undergo oxidation easily. An example is fluoride where even though theoretically it can oxidize to form fluorine gas, it’s energetically unfavored compared to the oxidation of water:

\[\text{2H}_2\text{O} \rightarrow \text{O}_2 + 4H^+ + 4e^-\]
This reaction reflects how water oxidation is often more feasible when working with solutions like (\text{NaI}\text{(aq)}) where the oxidation of iodide is more favorable.

Anode reactions determine the nature of the gas produced during electrolysis. Understanding them is critical for applications ranging from chemical synthesis to wastewater treatment.

Aqueous solutions

An important context for discussing electrolysis is whether it is conducted in aqueous solutions or molten ones. Here, we focus on aqueous solutions, where water is the solvent, affecting the electrolysis process.

In an aqueous solution, the dissolved substance dissociates into ions. For example:\(\text{NiBr}_2(\text{aq}) \rightarrow \text{Ni}^{2+} + 2\text{Br}^-\)
This dissociation into ions allows for conduction of electricity, as the ions migrate towards the electrodes. Aqueous solutions introduce water into the system, which can also participate in electrode reactions.

An interesting aspect of aqueous solutions is the competition between water molecules and dissolved ions during electrolysis. For cations, water can provide an alternative reaction path by itself being reduced. Similarly, at the anode, if cost-effectively feasible, water oxidation might prevail over the oxidation of less reactive anions.

Mechanisms in aqueous solutions enrich the study and application of electrolysis in products such as hydrogen gas. Aqueous electrolysis can also be seen in processes like electroplating, where metal ions are deposited onto a surface. Knowing how substances behave in these solutions helps in designing efficient and economical electrochemical processes.