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Q & A: hydrolysis

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Most recent answer: 10/22/2007
Q:
When water is electrolyzed, is there a way to make sure that the gases you gather are pure hydrogen or oxygen, and not hydroxide or another polyatomic ion? P.S. When an atom has a positive ionization, does that mean it has extra electrons to give away, or that it could use some more to neutralize itself?
- Peter (age 42)
USA
A:
First, the semantic question: electrons are called negative, so a positive ion is missing one or more electrons.

Second, the hydrolysis question. Whatever gas forms will consist of neutral molecules. I'm no sure what you mean by 'hydroxide' gas. I suppose there could be a tiny amount of hydrogen peroxide, H2O2, but it's less stable than a water- O2 combination, so you shouldn't get much. You'd have to look up some redox potentials or ask somebody more competent whether in the presence of high Cl- concentrations (e.g. from NaCl) you might get Cl2 rather than O2. If you use a basic solution, with the main negative ion being OH-, that won't be a problem. At the other electrode, you should only get H2 gas, because most of the typical positive ions that might be around (Na+, etc ) would plate out as solids, not gases. At any rate, the common experience with reasonably pure water is that you get H2 at one electrode and O2 at the other.

Mike W.

Sodium plating on an electrode immersed in water won't happen because metallic sodium reacts instantly with the water to form sodium ions, hydroxide ions (which stay in solution), and hydrogen gas, which may bubble off (it's the same electrode the hydrogen bubbles off of anyway so you wouldn't notice a difference with the salt present on that side. I can't say much about the chlorine -- it might dissolve in the water). Hydrogen peroxide also is fairly soluble in water, so I wouldn't imagine much of that would come off as a gas.

Tom

(published on 10/22/2007)

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