Question
9 Explain why hydrogen is formed at the cathode in the electrolysis of some solutions rather than a product from the discharge of ions in the compound dissolved in the solution? __
Answer
4.2
(252 Votes)
Lydia
Veteran · Tutor for 11 years
Answer
Hydrogen is formed at the cathode in the electrolysis of certain solutions because hydrogen ions (from the water in the solution) are easier to reduce than the cations from the compound dissolved in solution. They require less energy to discharge, thus get reduced preferentially forming H2 gas. This happens when the other cations are more reactive or act as spectators in the given electrolytic cell setup.
Explanation
The formation of hydrogen at the cathode in the electrolysis of certain solutions is connected to the electrochemical series/standard electrode potential and involves the processes at the cathode. In electrolysis, the cathode, which is a negatively charged electrode, attracts cations, or positive ions. Predicting the products of electrolysis involves understanding the easier discharge of ions.During the electrolysis of aqueous solutions, where water is present like in the case of dilute acids or in many salts, both H+(aq) ions and the cations from the compound being electrolysed are attracted to the cathode. However, assuming no reactive hazards and considerable inertness of the electrodes, if the positive ions are either more reactive than hydrogen or a spectator then hydrogen is preferentially discharged given it requires less energy to reduce, forming H2 gas. Hydrogen is preferentially formed because it's easier to reduce water, whose reduction half-equation is 2H2O(l) + 2e- —> H2(g) + 2OH- ; Eθ= -0.83V , representing higher likelihood under standard conditions, than many other possible cations (the discharge of ions from the compound having an even more negative reduction potential).In simpler terms, hydrogen is easier to discharge before a certain higher energy threshold needed for the discharge of many other cations.