Question

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

Short answer

The product formed at the cathode is (c) H_2.

Step-by-step solution

Understanding Electrolysis

In electrolysis, an electric current is used to drive a non-spontaneous chemical reaction. During the electrolysis of aqueous NaCl, water molecules can also participate in the reactions because NaCl is dissolved in water.

Analyzing Cathode Reactions

At the cathode, reduction occurs. In the solution, the possible cations available to be reduced are Na⁺ and H₂O. However, Na⁺ is not reduced because its reduction potential is lower than that of water. Therefore, water is reduced at the cathode.

Writing Reduction Reaction

The reduction of water at the cathode is represented by the equation: 2H₂O + 2e⁻ → H₂ + 2OH⁻. This shows that hydrogen gas is produced at the cathode when aqueous NaCl is electrolysed.

Determining the Product

Based on the reduction reaction equation and standard reduction potentials, the product formed at the cathode is hydrogen gas, H₂, because water is reduced instead of sodium ions.

Key concepts

Aqueous NaCl solution

A solution of sodium chloride (NaCl) dissolved in water is referred to as an aqueous NaCl solution. In this mixture, the sodium chloride dissociates into its respective ions, namely sodium ions (Na⁺) and chloride ions (Cl⁻).
The presence of water is crucial because it also dissociates into hydrogen ions (H⁺) and hydroxide ions (OH⁻). This means there are now several types of ions present to participate in the electrolysis process.
Understanding this setup is important for predicting which reactions will occur at the electrodes during electrolysis.

Platinum electrodes

When performing electrolysis, the choice of electrode is significant. Platinum electrodes are often used because they are inert.
Being inert means they do not actively participate chemically in the reactions taking place. Instead, they act as a medium for the transfer of electrons.
This property makes platinum an excellent choice for processes like the electrolysis of aqueous NaCl, ensuring that the reactions occur smoothly without interference from the electrodes themselves.

Cathode reactions

During electrolysis, the cathode is the electrode where reduction occurs. In the case of an aqueous NaCl solution, the reduction reaction can involve either sodium ions (\( \text{Na}^+ \)) or water molecules.
However, considering the reduction potentials, it is more favorable for water to undergo reduction rather than sodium ions.
The reduction of water at the cathode is given by the reaction: \[2\text{H}_2\text{O} + 2\text{e}^- \rightarrow \text{H}_2 + 2\text{OH}^-.\]This reaction generates hydrogen gas (\( \text{H}_2 \)) at the cathode, confirming the product formed during the electrolysis of aqueous NaCl.

Reduction potentials

Reduction potentials indicate the tendency of a substance to gain electrons and be reduced. They are also a crucial factor in determining which reactions predominate during an electrolysis process.
In aqueous NaCl solutions, water and sodium ions compete for reduction at the cathode.
The standard reduction potential for water (\( E° \approx -0.83 \text{ V} \)) is more positive compared to that of sodium ions (\( E° = -2.71 \text{ V} \)).
This means water is more readily reduced than sodium ions in this setup, making hydrogen gas the primary product at the cathode. Understanding reduction potentials is key to predicting outcomes in electrolysis reactions.