Question

Which product, \(\mathrm{O}_{2}\) or \(\mathrm{F}_{2,}\) is more likely to form at the anode in the electrolysis of an aqueous solution of KF? Explain your reasoning.

Short answer

Oxygen \(\mathrm{O}_2\) is more likely to form at the anode due to its lower oxidation potential compared to fluorine.

Step-by-step solution

Identify the Anion Being Oxidized

In the aqueous solution of potassium fluoride (KF), the fluoride ions (F-) and hydroxide ions (OH-) are potential candidates for oxidation at the anode since hydroxide ions come from water.

Recall Oxidation Reactions and Their Potentials

The typical oxidation half-reaction for fluoride to fluorine is: \(2\mathrm{F}^{-} \rightarrow \mathrm{F}_{2} + 2e^-\) with a standard potential of +2.87 V. For water, the oxidation half-reaction to form oxygen is: \(2\mathrm{H}_2\mathrm{O} \rightarrow \mathrm{O}_{2} + 4\mathrm{H}^{+} + 4e^-\) with a standard potential of +1.23 V.

Determine Which Reaction Occurs at Lower Potential

Compare the standard potential values for the oxidation reactions: forming \(\mathrm{O}_{2}\) from water has a much lower potential (+1.23 V) compared to forming \(\mathrm{F}_{2}\) from fluoride ions (+2.87 V).

Conclude the Likely Product at the Anode

Since the oxidation of water has a lower potential, it means it's more easily achieved under standard electrolysis conditions. Thus, oxygen \(\mathrm{O}_{2}\) is more likely to form at the anode than fluorine \(\mathrm{F}_{2}\).

Key concepts

Anode Reactions

In the process of electrolysis, the anode is where oxidation reactions occur. This involves the loss of electrons by certain ions in the solution. It is the spot where negatively charged ions (anions) migrate to and release electrons to the external circuit.

Commonly, in a solution containing different anions, several potential oxidation reactions could occur. The determining factor is the oxidation potential of these different ions.

During electrolysis, choosing which ion oxidizes at the anode is based on their oxidation potentials. The ion with the lower oxidation potential will oxidize first as it requires less energy to release its electrons, making it more favorable under typical conditions. Understanding this principle can clarify what substances may form at the anode during the process.

Oxidation Potential

Oxidation potential describes the tendency of a substance to lose electrons, an essential part of oxidation reactions at the anode during electrolysis.

These potentials are measured in volts (V) under standard conditions and serve as a guide to predict which reactions may occur in an electrochemical cell.

• Fluoride ions have a high oxidation potential of +2.87 V.
• Water, which forms oxygen, has a lower oxidation potential of +1.23 V.

Since it's easier, energetically, for water to oxidize into oxygen due to its lower potential, it is the more likely reaction at the anode when both water and fluoride ions are present.
This is why in many aqueous electrolysis scenarios, water's conversion to oxygen occurs rather than the formation of fluorine gas.

Fluoride Ions

Fluoride ions ( F^- ) are negatively charged and can be oxidized to form fluorine gas ( F_2 ) during electrolysis.
However, this oxidation demands a significant amount of energy, reflected in the high standard oxidation potential of +2.87 V.

• This high potential indicates that forming fluorine gas is less favorable compared to other possible oxidation reactions.
• It requires the electrolytic cell to exert additional energy to achieve this reaction, making it rare in typical aqueous electrolysis setups.

Understanding these energy requirements aids in predicting that fluoride ions are less likely to undergo oxidation at the anode than substances like water, when both are present.

Hydroxide Ions

Hydroxide ions ( OH^- ) originate from the dissociation of water in aqueous solutions and can also participate in the oxidation reactions at the anode during electrolysis.
These ions transform into oxygen ( O_2 ) because they come from water molecules breaking apart.

Due to the significantly lower oxidation potential at +1.23 V compared to fluoride ions, hydroxide ions often participate in anode reactions.

• This lower potential indicates that the conversion of hydroxide ions to oxygen is favorable and more readily achieved in electrolysis.
• Consequently, the conditions needed to produce oxygen from hydroxide ions are more easily met than those required for the oxidation of fluoride ions.

This makes ( O_2 ) formation more common than ( F_2 ) during such processes.