Question

Explain why chlorine gas can be prepared by electrolyzing an aqueous solution of \(\mathrm{NaCl}\) but fluorine gas cannot be prepared by electrolyzing an aqueous solution of NaF.

Short answer

Chlorine is produced because Cl⁻ has a lower oxidation potential than water, while F⁻ needs a higher one than water, preventing fluorine gas formation.

Step-by-step solution

Understanding Electrolysis of NaCl

When aqueous NaCl is electrolyzed, the NaCl dissociates into Na⁺ and Cl⁻ ions in the solution. During electrolysis, Cl⁻ ions are oxidized at the anode to produce chlorine gas (Cl₂). The equation for this is \( 2Cl^- \rightarrow Cl_2 + 2e^- \). Water is also available as a source of oxygen and hydrogen.

Comparing Oxidation Potentials

The oxidation potential for the chlorine ion (Cl⁻) is higher than that of water (H₂O), allowing Cl⁻ ions preferentially to be oxidized into chlorine gas (Cl₂) when sodium chloride is used. Whereas in the case of fluoride ions (F⁻), fluorine gas (F₂) has a very high oxidation potential.

Explaining Electrolysis of NaF

For NaF, the fluoride ions (F⁻) do not preferentially discharge because the oxidation potential of water (to form O₂) is lower than that of fluoride ions. Therefore, during the electrolysis of aqueous NaF, water is oxidized at the anode instead, producing oxygen gas rather than fluorine gas.

Conclusion

Since fluorine has a higher oxidation potential compared to water, water is oxidized before fluoride ions, preventing the formation of fluorine gas through electrolysis in an aqueous solution. Hence, only chlorine gas can be reliably prepared by this method, not fluorine.

Key concepts

Oxidation Potentials

In electrochemistry, oxidation potentials are critical in determining which substances will undergo oxidation during electrolysis. Oxidation potential is a measure of the tendency of a chemical species to lose electrons and become oxidized. The higher the oxidation potential, the more likely that species will be oxidized at the anode during the electrolysis process.
For chlorine ions (Cl⁻), the oxidation potential is sufficiently high, allowing them to oxidize before other ions like those from water molecules. This makes it feasible to produce chlorine gas easily from an aqueous solution of sodium chloride (NaCl). However, fluorine ions (F⁻) have an even higher oxidation potential than water molecules, which complicates their reduction to fluorine gas in an aqueous environment.

Chlorine Gas Production

The production of chlorine gas through electrolysis involves the oxidation of chloride ions at the anode. In an aqueous solution such as one derived from NaCl, the dissolved chloride ions split into chlorine gas and electrons:

• Equation: \( 2Cl^- \rightarrow Cl_2 + 2e^- \)

The electrons released during this reaction are then carried to the cathode where reduction reactions occur. Because the oxidation of water requires a higher voltage, chloride ions are preferentially oxidized when an electric current is passed through the solution.
This preferential oxidation is what makes the electrolysis of an aqueous NaCl solution efficient for producing chlorine gas. This process is beneficial and widely used in industries for the production of chlorine, especially through cost-effective means relying on simple aqueous solutions.

Fluorine Gas Challenges

The primary challenge in preparing fluorine gas through electrolysis of an aqueous solution lies in the high oxidation potential of fluoride ions. When attempting to electrolyze an aqueous solution of NaF, water oxidizes at the anode instead of fluoride ions due to its lower oxidation potential.

• Water oxidation equation: \( 2H_2O \rightarrow O_2 + 4H^+ + 4e^- \)

This results in the production of oxygen gas rather than fluorine gas. The challenge is exacerbated by the fact that fluorine’s very high reactivity and high oxidation potential mean it competes poorly against water in an aqueous environment.
Consequently, to produce fluorine gas, it is necessary to use non-aqueous methods, such as molten salts, which provide a suitable environment where water is absent and thus cannot interfere with fluorine production.

Aqueous Solution Electrolysis

Electrolysis in aqueous solutions is a common method to drive non-spontaneous chemical reactions using electricity. When a soluble ionic compound like NaCl or NaF is dissolved, it dissociates into constituent ions. These ions move towards the respective electrodes when electrical current is applied, facilitating chemical reactions to occur.
The choice of reaction that occurs at the anode (oxidation) and cathode (reduction) depends heavily on oxidation potentials. A key observation is that only species with lower oxidation potential than water can typically be oxidized in aqueous solutions.
Electrolysis of aqueous NaCl leads to chlorine gas production due to favorable oxidation potentials whereas, in aqueous NaF, water’s lower oxidation potential results in oxygen gas production instead of the intended fluorine gas, highlighting the importance of understanding oxidation reactions in aqueous systems.