Ask A Scientist Chemistry Archive

name         James
status       student
age          16

Question -     We are learning about electrolysis in school. We have a
Hoffman apparatus in our classroom, and I was wondering how come the
hydrogen all goes to the left and the oxygen all
goes to the right. Shouldn't some oxygen and some hydrogen be in both
tubes? Since there is water in each tube and because oxygen and hydrogen
are lighter and less dense then water they should both float to the top
instead floating.
I know something I typed has to be wrong because obviously the two
elements are separated.
----------------------------------------
James,

Analyze the situation. In the apparatus, one electrode is (+) and the other
is (-). When a water molecule splits into its elements, which of the
elements needs an electron (from the electrode) to become an atom? Which
element needs to surrender electrons (to the other electrode) to become an
atom? Once the atoms form in the region of their respective electrode, they
will combine with each other to form gaseous H2 and gaseous O2.

One of water's elements is being oxidized -- losing electrons. The other
element is being reduced -- gaining electrons. In these two processes, which
element is being oxidized and which is being reduced?

Consider these equations:

This is the cathode reaction at the (-) electrode
2 H2O + 2 electrons ==> H2 (gas)  +  2 OH (-) ions

This is the anode reaction at the (+) electrode
2 H2O ==>  O2 (gas)  +  4 H (+ ions)  +  4 electrons


As you can see, it is not significant that "hydrogen all goes to the left
and the oxygen all goes to the right." The side on which the gases collect
depends on which side is (+) and which is (-). If the poles were to be
reversed, the gases would "switch sides."

Regards,
ProfHoff 496
==============================================================
The Hoffman apparatus is an electrolytic cell consisting of a battery, an
electrolyte (Na2SO4) in water, an acid / base indicator (Bromothymol Blue).
The electrodes from the battery are connected to two vertical tubes which
form a U the electrolyte is fed through another tube that is connected to
the two electrode tubes at the base of the U. At the cathode,  4 electrons
react with 4 molecules of water:
4H2O + 4 e ----> 2H2(gas) + 4 OH(-1). At the anode 4 electrons are released
by the reaction of two molecules of water to form oxygen: 2H2O ----> O2(gas)
+ 4 e + 4H(+1)
So the electrolyte at the cathode is basic and the bromothymol blue
indicator turns blue. At the anode the release of H(+1) ions makes the
electrolyte acidic and the bromothymol blue indicator turns yellow. In the
middle where the electrolyte Na2SO4 in water enters the pH is about neutral
and the indicator is colored green.

Although water is present at both electrodes the battery is forcing the
water to undergo DIFFERENT chemical reactions one producing 2 volumes of
H2(gas) for every 1 volume of O2(gas).
The website: www.monroecc.edu/wusers/flanzafame/CLHoffman.pdf
gives a nice color diagram of the apparatus for those interested.

Vince Calder
==============================================================