A Closer Look at the Electrolysis of Water
Most of our textbooks and lab manuals use the electrolysis of water to provide evidence for its molecular formula, but common student questions such as why is “hydrogen gas coming out of the negative end” and ‘why does the water turn yellowish at the positive end” reveal some intriguing chemistry.
Aside from water’s high surface tension, high boiling point, high specific heat and its exceptional ability to dissolve a wide variety of substances, it can also accept or lose electrons. Of course, it will not do either at the drop of a hat.
+ 2e- H2 (g) + 2
Its reduction potential is a negative value, but in the presence of a strong reducing agent such as sodium, calcium or a metal hydride, water obliges, and the incoming electrons serve to bind diatomic hydrogen gas. Coming from two different water molecules, the two hydrogen atoms are drawn to the incoming electrons and leave their original ones behind, forming hydroxides. Alkali or alkaline are derived from the Arabic word for ashes, which are basic like hydroxide, and for this reason these names are given to the first two groups of the periodic table.
In electrolysis, a strong voltage also induces the above half reaction. Since electrons flow out of the negative end of the battery, it answers our first student question as to why hydrogen is found there. If the water is relatively pure, the reaction is still slow. After performing electrolysis with a strong electrolyte, we repeated the experiment with distilled water and with 9.0 volts, we obtained less than a milliliter of hydrogen after 7 hours, and only 2.6 ml after 24 hours.
A battery, of course will never surrender electrons unless a different part of the battery receives them back. Water is not the best at giving up electrons.
2 H2O(l) O2 (g) + 4 H+(aq) + 4e- E o red = -1.23 V
From its oxidation potential, we see that it is even worse a
reducing agent than it is an oxidizing agent. But the energy barrier is not
insurmountable. In fact water accomplishes the feat not only with sufficient
battery voltage but everywhere on the planet from the algae on Antarctican ice
to the trees of
But why did the water turn yellowish at the anode? If there are chlorides in water they will compete with water and also surrender electrons back to the battery’s positive side. Iron ions (Fe+2) in solution will get also get further oxidized. As chlorine or Fe+3are formed, they not only discolour the water, but they lower the amount of oxygen produced, skewing the ratio of hydrogen to oxygen volumes from 2:1 to anywhere from 2.4 to 2.9 to 1, as obtained by 63 students. We verified the presence of chlorine not only through its characteristic smell but by the orthotolidine test. Upon adding the reagent, the colour of the solution surrounding the anode went from a pale yellow to a deep yellow orange.