Question

During the electrolysis of an aqueous salt solution, the \(\mathrm{pH}\) in the space near one of the electrode was increased and the other one was decreased. The salt solution was (a) \(\mathrm{NaCl}\) (very dilute) (b) \(\mathrm{ZnCl}_{2}\) (c) \(\mathrm{NaCl}\) (Conc.)

Short answer

The pH increase is due to the formation of hydroxide ions (OH-) at the cathode and the decrease is due to the formation of hydrogen ions (H+) at the anode. This happens during the electrolysis of dilute NaCl and ZnCl2, and for concentrated NaCl, the decrease is mostly due to chlorine evolution at the anode.

Step-by-step solution

Identify the Processes at Each Electrode

During the electrolysis of an aqueous salt solution, at the cathode, water is reduced to hydrogen gas and hydroxide ions, increasing the pH. At the anode, water is oxidized to oxygen gas and hydrogen ions, decreasing the pH.

Examine the Effect of the Salt Solution

For very dilute NaCl, water molecules are preferentially discharged both at the anode and cathode resulting in the formation of OH- ions at the cathode and H+ ions at the anode. Similarly for ZnCl2, zinc will be deposited at the cathode, while chlorine evolution occurs at the anode. For concentrated NaCl, chloride ions are preferentially discharged at the anode to form chlorine gas.

Conclude the Observation for Each Salt

(a) For very dilute NaCl, increase in pH at the cathode is due to OH- ions and decrease at the anode is due to H+ ions formation.(b) For ZnCl2, increase in pH at the cathode is due to Zn deposition, and decrease at the anode is due to chlorine gas evolution with some oxygen and H+ ions formation.(c) For concentrated NaCl, increase in pH at the cathode is also due to OH- ions, and decrease in pH at the anode is predominantly due to chlorine gas evolution.

Key concepts

pH Changes During Electrolysis

Understanding pH changes during the electrolysis of aqueous salt solutions is essential in grasping the underlying chemical processes that occur. During electrolysis, the electric current passing through the solution drives non-spontaneous chemical reactions at the electrodes. This leads to the generation of gases and the movement of ions, which can significantly alter the pH near the electrodes.

For instance, with very dilute sodium chloride (NaCl), water molecules are reduced at the cathode to form hydrogen gas (H2) and hydroxide ions (OH-), which causes the pH to increase due to the basic nature of OH-. At the anode, the oxidation of water releases oxygen gas (O2) and hydrogen ions (H+), making the solution more acidic and thus decreasing the pH. For salts like zinc chloride (ZnCl2), the cathode reaction involves the deposition of metal, and does not greatly affect pH; however, the anode reaction involves the evolution of chlorine gas, which along with some oxygen production can contribute to a pH decrease due to corresponding reactions that generate H+ ions.

It is important to note that the concentration of the salt can influence which ions are preferentially discharged at the electrodes, thereby affecting the pH. For example, in concentrated NaCl, chloride ions (Cl-) are discharged at the anode over water, resulting in chlorine gas (Cl2), and overall, there's a more significant decrease in pH at the anode compared to the dilute solution.

Electrode Reactions in Electrolysis

Electrode reactions are at the heart of the electrolysis process. At the cathode, the negatively charged electrode, cations are typically reduced owing to the gain of electrons. Conversely, at the anode, the positively charged electrode, anions are oxidized as they lose electrons. The specific reactions at each electrode, however, depend on the ions present in the solution and their respective discharge potentials.

In the case of dilute NaCl, water, having a lower discharge potential compared to sodium ions, gets reduced at the cathode. The reaction can be represented as: \(2H_2O(l) + 2e^- \rightarrow H_2(g) + 2OH^-\). At the anode, water is again involved in the reaction because hydroxide ions (formed at the cathode) are more readily discharged than the chloride ions. Hence, the anodic reaction is: \(2H_2O(l) \rightarrow O_2(g) + 4H^+ + 4e^-\). These reactions collectively contribute to the changes in pH observed during electrolysis.

For concentrated NaCl, chloride ions are preferentially discharged at the anode to produce chlorine gas according to the reaction: \(2Cl^- \rightarrow Cl_2(g) + 2e^-\). This changes the dynamic of the solution compared to the more passive discharge of water molecules in a dilute solution.

Discharge of Ions in Electrolysis

In electrolysis, the discharge of ions involves the removal of ions from an electrolyte solution by their conversion into a different form—usually a gas or a solid—by the passage of an electric current. The order in which ions are discharged is determined by their discharge potentials. Typically, ions with lower discharge potentials are discharged first.

For example, in a dilute NaCl solution, hydrogen ions from water are reduced over sodium ions at the cathode because they have a lower discharge potential. This selective discharge governs the overall product of the reaction and the resulting chemical landscape in the solution. Similarly, at the anode, hydroxide ions formed at the cathode are oxidized in preference to chloride ions.

However, in concentrated solutions such as concentrated NaCl, the higher concentration of chloride ions tips the balance and leads to Cl- ions being preferentially discharged over water. This results in the aforementioned chlorine gas evolution. The discharge of ions is not only critical in terms of the products formed but also influences conductivity and the efficiency of the electrolysis process. Careful consideration of ionic discharge order is thus crucial for predicting and controlling the outcome of the electrolytic process.