Question

If electrolysis is carried out on an aqueous solution of cadmium iodide, what products are expected at the electrodes? Write the equation for the net cell reaction.

Short answer

At the anode, iodide ions are oxidized to iodine (I2). At the cathode, cadmium ions (Cd^2+) are reduced to cadmium metal (Cd). The net cell reaction is Cd^2+ + 2I^- \( \rightarrow \) Cd + I2.

Step-by-step solution

Identify the Electrolyte

Begin by identifying the substance undergoing electrolysis. In this case, it is an aqueous solution of cadmium iodide (CdI2), which means that the solution contains Cd^2+ and I^- ions along with H2O molecules.

Determine the Products at the Anode

At the anode, oxidation occurs. In aqueous solutions, iodide ions (I^-) can be oxidized to iodine (I2), but water (H2O) can also be oxidized to oxygen (O2). Compare the standard electrode potentials: I^- to I2 (E° = +0.54 V) and H2O to O2 (E° = +1.23 V). The lower potential oxidation will occur, leading to the formation of iodine.

Determine the Products at the Cathode

At the cathode, reduction occurs. Between cadmium ions (Cd^2+) and water, cadmium has a more positive potential for reduction, so cadmium will be reduced before hydrogen ions from water. Cd^2+ ions gain electrons to form cadmium metal (Cd).

Write the Half-cell Reactions

Anode half-reaction: 2I^- \( \rightarrow \) I2 + 2e^-Cathode half-reaction: Cd^2+ + 2e^- \( \rightarrow \) Cd

Combine the Half-cell Reactions

Combine the two half-reactions to obtain the overall cell reaction by ensuring that the electrons lost in the oxidation reaction are equal to those gained in the reduction reaction. The combined balanced reaction is Cd^2+ + 2I^- \( \rightarrow \) Cd + I2.

Key concepts

Anode and Cathode Products in Electrolysis

Understanding what happens at the anode and cathode during electrolysis is crucial. Here, we'll delve into the products formed at each electrode when you electrolyze an aqueous solution of cadmium iodide (CdI₂).

At the anode, which is the positive electrode, oxidation takes place. For cadmium iodide, the possible oxidation reactions involve iodide ions (I^-) and water (H₂O). The iodide ions are oxidized to form iodine (I₂), rather than oxygen from water due to the former having a lower standard electrode potential. This makes iodine production at the anode more favorable.

At the cathode, the negative electrode, reduction occurs. Cadmium ions (Cd²⁺) compete with hydrogen ions from water to gain electrons. Cadmium's higher reduction potential means it is reduced to metallic cadmium (Cd) before hydrogen, which would otherwise form hydrogen gas (H₂). So, in our specific example, the cathode product is solid cadmium metal.

Half-Cell Reactions in Electrolysis

Half-cell reactions are at the heart of understanding electrolysis. These reactions showcase the changes occurring at each electrode separately. For cadmium iodide (CdI₂), the anode half-cell reaction can be represented as:

Anode Half-Cell Reaction

2I^- → I₂ + 2e^-
This shows the oxidation of iodide ions to iodine, releasing electrons in the process. On the other hand, the cathode half-cell reaction is all about reduction, where cadmium ions pick up electrons:

Cathode Half-Cell Reaction

Cd²⁺ + 2e^- → Cd
Breaking down the overall reaction into these half-reactions enables us to understand the electron flow in the process and prepares us for combining them into the full cell reaction.

Standard Electrode Potentials

The standard electrode potential is key to predicting which direction a half-reaction will favor. It's measured under standard conditions (1 M concentration, 1 atm pressure, and 25°C) and indicates a substance's tendency to gain or lose electrons.

For our example with cadmium iodide, the standard electrode potentials tell us that iodine has a potential of +0.54 V for the oxidation to occur (I^- to I₂), while water's potential is +1.23 V (H₂O to O₂). Since reactions with lower (more negative) electrode potentials occur first, iodine is produced at the anode. Understanding these potentials is crucial for predicting the products of electrolysis and can be used to compare the reactivity of different ions in solution.

Balancing Redox Reactions

Balancing redox reactions is a fundamental aspect of electrochemistry, ensuring that the number of electrons lost is equal to the number of electrons gained. In the electrolysis of cadmium iodide, we balance the anode and cathode half-cell reactions by equalizing the number of electrons involved in each:

Anode: 2I^- → I₂ + 2e^-
Cathode: Cd²⁺ + 2e^- → Cd
As there are two electrons involved in both the oxidation and reduction half-reactions, they are already balanced and can be combined to give the net cell reaction:

Cd²⁺ + 2I^- → Cd + I₂.
When balancing redox reactions, it's crucial to also consider the conservation of mass and charge, ensuring that the products and reactants have the same number of atoms for each element and that overall charge is neutral.