Question

If mercury is used as cathode in the electrolysis of aqueous \(\mathrm{NaCl}\) solution, the ion discharged at cathode is : (a) \(\mathrm{H}^{+}\) (b) \(\mathrm{Na}^{+}\) (c) \(\mathrm{OH}^{-}\) (d) \(\mathrm{Cl}^{-}\)

Short answer

The ion discharged at the cathode is (a) \(\mathrm{H}^{+}\)

Step-by-step solution

Understand the electrolysis of aqueous NaCl

When an aqueous NaCl solution undergoes electrolysis, both Na+ and water are present at the cathode. Since water is a better conductor of electricity than Na+, it's more likely that H+ ions from water will be discharged rather than Na+.

Apply the Electrochemical Series

According to the electrochemical series, the ion that is discharged at the cathode is the one which requires less energy to be reduced. H+ ions require less energy to gain electrons compared to Na+, which means that H+ will be reduced over Na+.

Consider the Role of Mercury Cathode

Mercury tends to form amalgams with many metals, but it doesn't have any specific effect on the selectivity of the H+ ion discharge process. Therefore, even with a mercury cathode, H+ ions from water will be reduced in preference to Na+.

Key concepts

Electrochemical Series

Understanding the electrochemical series is crucial when studying electrolysis. The electrochemical series is a list of elements sorted according to their standard electrode potentials. Elements at the top of the series have a greater tendency to lose electrons, making them good reducers, while those at the bottom have a greater tendency to gain electrons, acting as good oxidizers. During the electrolysis of aqueous solutions, this series can predict which ions are more likely to be reduced at the cathode or oxidized at the anode. For instance, when aqueous NaCl is electrolyzed, both H+ and Na+ are candidates for reduction at the cathode, but since H+ ions are higher in the electrochemical series than Na+ ions, they get reduced preferentially, leading to the release of hydrogen gas.

Cathode Ion Discharge

Cathode ion discharge refers to the process where ions gain electrons and become reduced at the cathode during electrolysis. Ions that are positively charged are attracted to the cathode where they acquire electrons. The ion's position in the electrochemical series, concentration, and the nature of the electrode material all influence which ion will be discharged. In the given example of aqueous NaCl electrolysis using a mercury cathode, the ions available for discharge at the cathode are H+ from water and Na+. However, due to the lower reduction potential of H+ as indicated by the electrochemical series, H+ ions are preferentially discharged, resulting in the formation of hydrogen gas.

Influence of Mercury Cathode

While the mercury cathode poses no significant influence on the discharge preference of H+ ions, its presence is crucial for other reasons. A mercury cathode can create a different environment which could potentially affect the overall electrochemical cell performance, including aspects like overpotential and cell efficiency.

Mercury Cathode Amalgams

Mercury cathode amalgams are relevant when discussing the electrolysis of solutions containing metal ions. An amalgam is an alloy of mercury with another metal. In some electrolytic processes, especially those involving the manufacture of chlorine and caustic soda, mercury cathodes are used to prevent the formation of metal deposits on the cathode which can lead to diminished efficiency. Instead, the metal forms an amalgam with mercury that can be easily processed to retrieve the pure metal. While sodium amalgam could potentially form in the electrolysis of aqueous NaCl solution, it is the H+ ions that are reduced due to their higher position in the electrochemical series. The use of a mercury cathode in such cases is largely to facilitate the collection and later recovery of the sodium from the amalgam rather than to impact the discharge order of ions.