In an experiment 0.04 F was passed through 400ml of a 1m solutionof NaCl. what would be the pH of the solution after electrolysis ?
NaCl is a strong electrolyte, while H2O is a weak electrolyte. In moderately concentrated solution (1M), the following ions will prevail:
NaCl breaks down into Na+ and Cl- ions
H2O breaks down into H+ and OH- ions.
When electric current is passed through the solution, both Na+ and H+ ions move towards the cathode. But Na+ being more electropositive, exhibits lesser tendency to gain electrons, hence H+ ions will be preferntially discharged at the cathode
H+ + e gives H; H+H gives H2(stable, gas)
Similarly, both OH- and Cl- ions will move towards the anode. Although OH- has slightly lower discharge potential, owing to higher concentration of Cl- ions, they will be preferntially discharged at the anode
Cl- - e gives Cl; Cl + Cl gives Cl2(stable, gas)
Due to the discharge of H+ ions, the concentration of OH- ions keep on increasing and the pH rises. The exact amount can be calculated from Farady's conditions which states that for a monovalent ion, one Faraday current discharges one gram-ions of it.
Here 0.04 F should discharge 0.04 gram-ions of H+
Hence, [OH]- = 0.04 M
or, pOH = - log 0.04
hence pH = 14-1.40 = 12.60 (as pH + pOH = 14 at 25 degree Celsius, for aqueous systems).