Question

What reaction will take place at the cathode and the anode when each of the following is electrolyzed? (Assume standard conditions.) a. \(1.0 M\space \mathrm {KF}\) solution b. \(1.0 M\space \mathrm {CuCl}_{2}\) solution c. \(1.0 M \space \mathrm{MgI}_{2}\) solution

Short answer

a. For a \(1.0 M\space KF\) solution, the reactions at the cathode and anode are: Cathode: \(K^{+} + e^{-} \rightarrow K\) Anode: \(2F^{-} \rightarrow F_{2} + 2e^{-}\) b. For a \(1.0 M\space CuCl_{2}\) solution, the reactions at the cathode and anode are: Cathode: \(Cu^{2+} + 2e^{-} \rightarrow Cu\) Anode: \(2Cl^{-} \rightarrow Cl_{2} + 2e^{-}\) c. For a \(1.0 M\space MgI_{2}\) solution, the reactions at the cathode and anode are: Cathode: \(Mg^{2+} + 2e^{-} \rightarrow Mg\) Anode: \(2I^{-} \rightarrow I_{2} + 2e^{-}\)

Step-by-step solution

Identify the ions present

For a 1.0 M KF solution, the dissociation in water will produce K+ and F- ions.

Determine the possible reduction and oxidation reactions

At the cathode (reduction): K+ + e- -> K At the anode (oxidation): 2F- -> F2 + 2e- b. Electrolysis of a 1.0 M CuCl2 solution

Identify the ions present

For a 1.0 M CuCl2 solution, the dissociation in water will produce Cu(2+) and 2Cl- ions.

Determine the possible reduction and oxidation reactions

At the cathode (reduction): Cu(2+) + 2e- -> Cu At the anode (oxidation): 2Cl- -> Cl2 + 2e- c. Electrolysis of a 1.0 M MgI2 solution

Identify the ions present

For a 1.0 M MgI2 solution, the dissociation in water will produce Mg(2+) and 2I- ions.

Determine the possible reduction and oxidation reactions

At the cathode (reduction): Mg(2+) + 2e- -> Mg At the anode (oxidation): 2I- -> I2 + 2e-

Key concepts

Cathode reactions

In electrolysis, the cathode is the electrode where reduction reactions occur. This is the site where positive ions (cations) gain electrons. You might remember that REDUCTION happens at the cathode, which is a way to help remember what occurs here.
- During the electrolysis of a KF solution, potassium ions (K^+ ) migrate towards the cathode. Here, they gain electrons and are reduced to form potassium metal: \[ \mathrm{K^+ + e^- \rightarrow K}\]
- In a CuCl_2 solution, copper ions (Cu^{2+} ) travel to the cathode, where they undergo reduction to form copper metal:\[ \mathrm{Cu^{2+} + 2e^- \rightarrow Cu}\]
- With MgI_2 , magnesium ions (Mg^{2+} ) also move towards the cathode and are reduced: \[ \mathrm{Mg^{2+} + 2e^- \rightarrow Mg} \]
This reaction pathway is a fascinating feature of chemistry as it converts ions in a solution to solid metals through the simple act of electron transfer.

Anode reactions

The anode is the electrode where oxidation reactions occur in electrolysis. In these reactions, negative ions (anions) lose electrons, and you can remember OXIDATION happens at the anode.
- For a KF solution, fluoride ions (F^- ) move toward the anode, where they lose electrons to form fluorine gas:\[ \mathrm{2F^- \rightarrow F_2 + 2e^-} \]
- In the case of CuCl_2 electrolysis, chloride ions (Cl^- ) undergo oxidation to produce chlorine gas: \[ \mathrm{2Cl^- \rightarrow Cl_2 + 2e^-} \]
- Similarly, MgI_2’s , iodide ions (I^- ) undergo oxidation at the anode, releasing iodine:\[ \mathrm{2I^- \rightarrow I_2 + 2e^-} \]
These anode reactions are crucial for the production of gases from ionic solutions, a key process used in various industrial applications.

Reduction and oxidation

The concepts of reduction and oxidation, together known as redox reactions, are fundamental to understanding electrolysis. In simple terms, reduction is the gain of electrons, while oxidation is the loss of electrons. These processes occur simultaneously in any redox reaction.
- In a typical electrolytic cell setup, cations move to the cathode, where they undergo reduction.
- Conversely, anions migrate to the anode and experience oxidation.
Think of a see-saw mechanism, where the loss of an electron by one atom (oxidation) is complemented by the gain of an electron by another (reduction). Redox reactions are in balance, governed by the principle of conservation of charge. In an electrolytic cell, the applied electrical current drives these reactions, resulting in material deposition or gas release at each electrode.

Ion dissociation

Ion dissociation is the process where an ionic compound separates into its individual ions when dissolved in water. This step is necessary for electrolysis as it provides free ions that can move towards different electrodes under the influence of electric current.
When a salt such as KF, CuCl_2, or MgI_2 is dissolved:

• For KF, it separates into \( K^+ \) and \( F^- \) ions.


• In CuCl_2 , it splits into \( Cu^{2+} \) and \( Cl^- \) ions.


• While MgI_2 dissociates into \( Mg^{2+} \) and \( I^- \) ions.

This process of ion dissociation is vital for the ions to be available for the cathode and anode reactions. Without it, the entire electrolysis process would be impossible, as there would be no mobile charges to facilitate electron transfer.

Standard conditions

Standard conditions in the context of electrolysis help ensure that experiments and reactions are carried out in a consistent environment. This predictability helps in calculating and anticipating the outcomes of chemical reactions accurately.
Standard conditions generally refer to a pressure of 1 atmosphere (atm), a temperature of 25°C (298 K), and a concentration of ions in solution being 1.0 Molar (M).
These conditions are vital because they provide a reference point that chemists can use to replicate chemical reactions with expected outcomes. In discussions of electrolysis, it’s important because:

• Variables such as temperature and pressure can significantly influence reaction rates and outcomes.


• The molarity of solutions affects the availability of ions for reaction.

Adhering to standard conditions ensures consistency and reliability in electrolytic experiments, allowing for valid comparisons and understanding across different chemical setups.