Sodium hydroxide remains in solution after electrolysis primarily because hydrogen ions are preferentially discharged over sodium ions at the negative electrode (cathode), leaving the sodium ions to combine with hydroxide ions that are continuously generated.
Understanding the Electrolysis Process
Electrolysis involves using electrical energy to drive non-spontaneous chemical reactions. When an aqueous solution of a salt like sodium chloride (often called brine) undergoes electrolysis, several types of ions are present in the solution:
- Positive ions (cations): Sodium ions (Na⁺) and hydrogen ions (H⁺) from the dissociation of water.
- Negative ions (anions): Chloride ions (Cl⁻) and hydroxide ions (OH⁻) from the dissociation of water.
These ions migrate to oppositely charged electrodes where they can gain or lose electrons, leading to the formation of new substances.
The Role of the Cathode: Preferential Discharge
The key to understanding why sodium hydroxide is left behind lies at the negative electrode, also known as the cathode. At this electrode, positively charged ions are attracted. In an aqueous solution containing sodium ions (Na⁺) and hydrogen ions (H⁺):
- Hydrogen ions are more easily reduced (discharged) than sodium ions. This is due to their relative positions in the electrochemical series; hydrogen is less reactive than sodium.
- Therefore, the hydrogen ions gain electrons in preference to sodium ions, forming hydrogen gas:
2H⁺(aq) + 2e⁻ → H₂(g)
Crucially, the sodium ions (Na⁺) are not discharged and thus remain in the solution.
Formation of Sodium Hydroxide
As hydrogen ions are continuously removed from the solution at the cathode, the delicate balance of water dissociation (H₂O ⇌ H⁺ + OH⁻) is disturbed. To re-establish equilibrium, more water molecules dissociate, producing more hydroxide ions (OH⁻).
Meanwhile, at the positive electrode (anode), negative ions migrate. In the case of sodium chloride solution, chloride ions (Cl⁻) are typically discharged in preference to hydroxide ions (OH⁻) (unless the solution is very dilute), producing chlorine gas:
2Cl⁻(aq) → Cl₂(g) + 2e⁻
The continuous removal of hydrogen ions at the cathode, coupled with the non-discharge of sodium ions, leads to an accumulation of both sodium ions (Na⁺) and hydroxide ions (OH⁻) in the solution. These two ions combine to form sodium hydroxide (NaOH), which remains dissolved in the water.
Key Takeaways
- Preferential Discharge: Hydrogen ions are reduced at the cathode instead of sodium ions.
- Ion Accumulation: Sodium ions remain in the solution.
- Hydroxide Generation: The removal of H⁺ ions shifts the water dissociation equilibrium, increasing OH⁻ concentration.
- Formation: Undischarged Na⁺ ions combine with the increasing concentration of OH⁻ ions to form NaOH.
This process is fundamental in the industrial production of chlorine, hydrogen, and sodium hydroxide, such as in the chlor-alkali process.
