Does changing the electrolyte concentration in the electrolysis of water affect the speed that hydrogen gas is produced at the cathode?

Question

I am currently completing a Year 12 Extended Practical Investigation. Due to the lock-down restrictions we are unable to complete our own practical experiment and have been given sample answers with made up lab conditions. The experiment I chose was to investigate the difference of time taken to produce 25mL of H2 gas in an electrolytic cell when the concentration of the potassium hydroxide electrolyte was changed. The lab conditions given were a pressure of 101kPa, a temperature of 26.0 celcius and in addition the current in the system was kept at a constant 0.600A throughout all trials of different concentrations.

When analysing the results there was a trend that showed the higher the concentration of the potassium hydroxide the faster the 25mL of H2 gas was produced. However, Through using the formulas Q=It, Q=n(e-)F and the electrochemical half equation for H2 production it seemed that due to the current and the amount of Hydrogen being formed both being constant the time taken to form the hydrogen should also remain constant.

I have found a write up for the exact experiment on https://www.qcaa.qld.edu.au/downloads/senior-qce/sciences/snr_chemistry_19_ia2_asr_high.pdf however when it comes to talk about the effect of increasing the KOH concentration it seems that there is no mention that the current was kept constant.

I would like to know if the trend shown should have occured and why.

Answer 1

With decreasing electrolyte concentration, either the needed voltage raises at constant current, either the passing current decreases at the constant voltage.

Both happens because at lower concentration, a higher potential gradient across the electrolyte is needed to maintain the same current, as the smaller ion count requires ions to move faster for the same current..

This potential gradient and the resulting potential drop acts against the external voltage. It pushes forced electrode potentials closer to their equilibrium potential. The net electrode reactions are then slower.

Answer 2

Interesting question. Remember for future calculations that in electrolysis either voltage can be kept constant or the current but not both. Now if you were actually doing this experiment you would have noticed that in order to maintain the same current in different KOH solutions you would have needed different voltages.

So the hint is: What is changing when you are changing the KOH concentration?

Answer: The resistance of the solution. Think whether low KOH solution will have high resistance or low resistance.

The ideal situation is that you have high current and low resistance at a given voltage. The voltage should be above the decomposition voltage of water.