Question

What reaction will take place at the cathode and the anode when each of the following is electrolyzed? (Assume standard conditions.) a. $1.0\mathrm{M}$ KF solution b. $1.0\mathrm{M}{\mathrm{CuCl}}_2$ solution c. $1.0\mathrm{M}{\mathrm{MgI}}_2$ solution

Short answer

For the electrolysis of different solutions, the following reactions occur at the cathode and anode: a) 1.0 M KF solution: Cathode: $K^{+}+e^{-}\to K$ Anode: $2F^{-}\to F_2+2e^{-}$ b) 1.0 M CuCl₂ solution: Cathode: $2H^{+}+2e^{-}\to H_2$ Anode: $2C{l}^{-}\to C{l}_2+2e^{-}$ c) 1.0 M MgI₂ solution: Cathode: $M{g}^{2+}+2e^{-}\to Mg$ Anode: $2I^{-}\to I_2+2e^{-}$

Step-by-step solution

Solution a: 1.0 M KF

Step 1: Identify ions in solution In the 1.0 M KF solution, we have the following ions: K+ and F-. Step 2: Determine half-reactions at the cathode (reduction) and anode (oxidation) Since potassium (K) is higher on the activity series of metals than hydrogen (H), reduction of K+ ions to K will take place at the cathode: Cathode reaction: $K^{+}+e^{-}\to K$ At the anode, oxidation of F- ions to form F2 gas will occur: Anode reaction: $2F^{-}\to F_2+2e^{-}$

Solution b: 1.0 M CuCl2

Step 1: Identify ions in solution In the 1.0 M CuCl2 solution, we have the following ions: Cu2+ and 2Cl-. Step 2: Determine half-reactions at the cathode (reduction) and anode (oxidation) Copper (Cu) is lower on the activity series of metals than hydrogen (H), so reduction of H+ ions to H2 gas will take place at the cathode instead of Cu2+ ions to Cu: Cathode reaction: $2H^{+}+2e^{-}\to H_2$ At the anode, oxidation of Cl- ions to form Cl2 gas will occur: Anode reaction: $2C{l}^{-}\to C{l}_2+2e^{-}$

Solution c: 1.0 M MgI2

Step 1: Identify ions in solution In the 1.0 M MgI2 solution, we have the following ions: Mg2+ and 2I-. Step 2: Determine half-reactions at the cathode (reduction) and anode (oxidation) Magnesium (Mg) is higher on the activity series of metals than hydrogen (H), so reduction of Mg2+ ions to Mg will take place at the cathode: Cathode reaction: $M{g}^{2+}+2e^{-}\to Mg$ At the anode, oxidation of I- ions to form I2 molecules will happen (instead of O2 gas since I- is more easily oxidized than water): Anode reaction: $2I^{-}\to I_2+2e^{-}$

Key concepts

Cathode and Anode Reactions

Electrolysis involves two key half-reactions at the electrodes: reduction at the cathode and oxidation at the anode. During electrolysis of a substance like potassium fluoride (KF), positive potassium ions (K⁺) are attracted to the cathode, where they gain electrons in a reduction process:
Cathode reaction: $K^{+}+e^{-}\to K$.
At the anode, fluoride ions (F^-) give up electrons through oxidation, producing fluorine gas:
Anode reaction: $2F^{-}\to F_2+2e^{-}$.
In solutions where metal ions are less reactive than hydrogen ions, like copper(II) chloride (CuCl₂), hydrogen will be reduced at the cathode instead of the metal ion.

Activity Series of Metals

The activity series of metals is a chart that ranks metals by their reactivity in displacement reactions and, by extension, in electrolysis. More active metals, such as potassium (K) and magnesium (Mg), are more likely to be reduced at the cathode compared to hydrogen (H), which is intermediate in the series. Conversely, less active metals, like copper (Cu), will not reduce in the presence of H⁺ ions, leading to the reduction of H⁺ rather than the metal ion itself.

Half-Reactions in Electrolysis

Understanding half-reactions is crucial for predicting the outcomes of electrolysis. A half-reaction is either the reduction or oxidation process that occurs at an electrode. Reduction half-reactions always occur at the cathode, as electrons are gained by the chemical species. Oxidation half-reactions happen at the anode, where electrons are lost. For example, the reduction of magnesium ions to magnesium metal (Mg²⁺ to Mg) and the oxidation of iodide ions to iodine (I^- to I₂) are half-reactions in the electrolysis of magnesium iodide (MgI₂).

Standard Electrode Potentials

Standard electrode potentials are essential for predicting the direction of half-reactions in electrolysis. They are a measure of a half-cell’s potential under standard conditions, with higher potentials indicating greater tendency to be reduced. In a cell, the difference in standard electrode potential between two half-cells will drive the flow of electrons from the anode to the cathode. For instance, because Cu has a higher standard electrode potential than H₂, copper ions in CuCl₂ solution will be less likely to reduce than hydrogen ions from water present in the solution.