Question

Elevation of boiling point of 1m kcl solution is nearly double than that of 1 m sugar solution

Answer 1

We take cathode and H3O+ ions as an example. Initially near the cathode surface water molecules can be dissociated into H3O+ and OH- ions. H3O+ ions obtain electrons from cathode leading to hydrogen evolution; while newly-generated OH- ions can only transport very slowly through the bulk solution by slow diffusion or hopping process. These lead to local OH- ions accumulation (so that the solution near cathode turns alkaline) especially at the cathode surface, reducing the reaction rate of hydrogen evolution and thus water splitting. In other words, the reaction becomes very slow or even self-limited, showing a large equivalent resistance between the cathode and the anode. That is why pure water in macrosystem cannot be split efficiently.

2. Pure water electrolysis in Nanogap cells

In pure water, when the counter-electrode is placed within the Debye-length, double layer regions of the cathode and the anode are overlapping with each other so that high electric field exists in the entire gap. Still at cathode, newly-generated OH- ions can be migrated rapidly from cathode towards anode due to large electric field in the entire gap. When the gap distance is small enough, initially the transport rate can be even higher than the electron-transfer rate. Once OH- ions are generated, they are immediately drawn from cathode to anode, leading to such OH- ions waiting for electron-transfer at the anode, rather than accumulated at the cathode. In this way, the whole reactions would continue even in pure water, but now are limited by electron-transfer.