Question

Aqueous solution of \(\mathrm{NaCl}\) is electrolyzed using platimum electrodes. What is the product formed at the cathode? (a) \(\mathrm{O}_{2}\) (b) \(\mathrm{Cl}_{2}\) (c) \(\mathrm{H}_{2}\) (d) \(\mathrm{Na}\)

Short answer

(c) \(\mathrm{H}_2\)

Step-by-step solution

Understanding Electrolysis

In the electrolysis of aqueous \( ext{NaCl}\), apart from \( ext{Na}^{+}\) and \( ext{Cl}^{-}\) ions, water molecules (\( ext{H}_2 ext{O}\)) are also present. During electrolysis, ions in the solution are attracted to the electrodes where redox reactions occur, giving products at the anode and the cathode.

Identifying Possible Reactions at the Cathode

The cathode is the electrode where reduction occurs (gain of electrons). In aqueous solutions, the potential reactions involving ions are: \(\text{Na}^{+} + e^- \rightarrow \text{Na}\) and \(2\text{H}_2\text{O} + 2e^- \rightarrow \text{H}_2 + 2\text{OH}^-\).

Analyzing the Reduction Potentials

The reduction potential for sodium ion \(\text{Na}^{+}\) is -2.71 V, whereas for the reduction of water to hydrogen gas \(\text{H}_2\), it is -0.83 V. Since reduction occurs more readily at less negative potentials, water is reduced in preference to sodium ions.

Determining the Reaction at the Cathode

Based on the reduction potentials, the reaction at the cathode is \(2\text{H}_2\text{O} + 2e^- \rightarrow \text{H}_2 + 2\text{OH}^-\). This reaction leads to the formation of \(\text{H}_2\), hydrogen gas, as the product at the cathode.

Conclusion

The product formed at the cathode during the electrolysis of aqueous \(\text{NaCl}\) is hydrogen gas (\text{H}_2).

Key concepts

Aqueous Solution

An aqueous solution is a solution in which the solvent is water. In the case of sodium chloride (NaCl), when it is dissolved in water, it forms an aqueous solution, denoted as \( ext{NaCl (aq)}\). This means that the solid NaCl splits into its constituent ions, sodium ions (\( ext{Na}^{+}\)) and chloride ions (\( ext{Cl}^{-}\)), dispersed throughout the water.

Aqueous solutions are incredibly important in chemistry because water is a neutral solvent, making it perfect for facilitating various chemical reactions. Specifically, in electrolysis, water also plays a reactive role with other potential ions in solution, potentially leading to multiple reaction paths in the electrolytic process.

• This means that during electrolysis, not just the ions from the dissolved salt but also water itself may participate in the chemical reactions at the electrodes.

Sodium Chloride

Sodium chloride, commonly known as table salt, is an ionic compound with the formula \( ext{NaCl}\). It consists of sodium ions (\( ext{Na}^{+}\)) and chloride ions (\( ext{Cl}^{-}\)) held together by a strong ionic bond.

When \( ext{NaCl}\) is dissolved in water to form an aqueous solution, these ions disassociate and become free to move. In this electrolytic context, the ions are subjected to electric fields, causing them to migrate toward the electrodes.

• In terms of chemical reactions, understanding the behavior of these ions in aqueous solution under electrolysis is key to predicting the products formed at the cathode and anode.

Reduction Potential

Reduction potential is a measure of the tendency of a chemical species to gain electrons and thereby be reduced. It is measured in volts (V) relative to the standard hydrogen electrode.

In the electrolysis of an aqueous solution of sodium chloride, the reduction potentials inform us which species in the solution is more likely to gain electrons at the cathode.

• Sodium ions (\( ext{Na}^{+}\)) have a reduction potential of -2.71 V, which is highly negative compared to other common species.

• Water, in contrast, has a standard reduction potential of -0.83 V to form hydrogen gas (\( ext{H}_2\)).

In these cases, the species with the higher (less negative) reduction potential is preferred in the reduction process.

• Therefore, water is reduced to form hydrogen gas instead of sodium ions being reduced to sodium metal.

Cathode Reaction

The cathode is the electrode in an electrolytic cell where reduction (gain of electrons) occurs. During the electrolysis of an aqueous \( ext{NaCl}\) solution, two potential reduction reactions can occur at the cathode.

1. Reduction of sodium ions:

• \[ ext{Na}^{+} + e^- \rightarrow ext{Na}\]

2. Reduction of water:

• \[2 ext{H}_2 ext{O} + 2e^- \rightarrow ext{H}_2 + 2 ext{OH}^-\]

Due to the different reduction potentials, the reduction of water is more thermodynamically favorable.

• This results in hydrogen gas (\( ext{H}_2\)) being released at the cathode as the main product of this process.
• This is a common outcome when electrolyzing aqueous solutions containing alkali metals.

Understanding the cathode reactions in electrolytic cells helps predict the products formed and explains why certain elements, such as sodium, do not appear as products even when they are present in the electrolyte.