Why is twice as much hydrogen produced than chlorine in the electrolysis of brine?

Question

I have seen a great many number of diagrams which show the electrolysis of brine in a diaphragm cell, but show twice the volume of hydrogen produced. For example:

On the other hand, other diagrams show that the same volume of hydrogen and chlorine is produced. For example:

So who's right and why?

These are just qualitative schemes; they don't specify volumes at all. At least, I don't see any indication of it whatsoever. — Ivan Neretin, Aug 01 '16 at 13:31
No they did not pay attention to the volumes (otherwise they would have shown them equal, as the equation requires). You are inferring meaning which is simply not there. — Ivan Neretin, Aug 01 '16 at 14:16
@IvanNeretin I appreciate your response. Are you suggesting that there is evidence of systematic error in the creation of these diagrams relating to the electrolysis of brine? — Turbo, Aug 01 '16 at 14:17
I would not even call that an error. They never said their volumes are 2:1 or whatever; you inferred that from some random details which (I think) simply aren't supposed to mean anything. — Ivan Neretin, Aug 01 '16 at 14:24
@IvanNeretin I think I've worked it out - see my answer below — Turbo, Aug 01 '16 at 14:32
@SnailMail Note that your first picture shows the diaphragm process and your second picture shows the membrane process; however, the difference is not necessarily related to the volume of generated gases. Also note that both images are simplified; the set-up of actual chlor-alkali cells is more complex and complicated. — , Aug 01 '16 at 15:01
@Loong I agree with you :) Please see my response below for clarification. — Turbo, Aug 01 '16 at 15:03
I would rather have considered just the chemistry of brine electrolysis instead, so no question. By the way, concomitant hydrolysis will indeed cause different volumes of gases to evolve at the cathode and anode compartments. This is not related to the drawing in which no set up to measure a gas volume is depicted and the drawers were just suggesting what you say in your answer. Surprisingly all this got 14 plus 4 + actions. — Alchimista, Feb 18 '19 at 10:41
In a simple experiment of electrolysis of brine the gases can be collected in test tubes originally filled with the solution. I normally find that the volume of hydrogen gas produced is more than that of chlorine. It appears to me that since chlorine is formed initially as free reactive atoms, they immediately react with the water molecules nearby and have no time to form chlorine molecules. As a result very little of the gas is formed — Chris, Dec 14 '20 at 14:27

score 15 · Answer 1 · answered Aug 01 '16 at 14:31

It appears, as the others have said that the same number of moles of gas of chlorine and hydrogen is produced.

Then why are there different levels of solution?

Since the solution is taken out in the cathode chamber, the level of solution present is lower than the anode chamber where sodium chloride solution is being added in.

score 11 · Answer 2 · edited Dec 15 '20 at 15:14

I'm not sure how the first diagram shows a $2:1$ volume ratio. But I am sure about the balanced chemical equations:

$$ \begin{align} \ce{2 Cl^- &→ Cl2 + 2 e-} \\ \ce{2 H2O + 2 e- &→ H2 + 2 OH-} \\ \hline \ce{2 H2O + 2 Cl- &→ H2 + Cl2 + 2 OH-} \end{align} $$

Thus equal amounts of both gases are produced, and at the same temperature and pressure that implies equal volumes by the ideal gas law.

Why is twice as much hydrogen produced than chlorine in the electrolysis of brine?

2 Answers2