Very pure hydrogen (99.9) can be made by which of the following process ?

(A)Electrolysis of water (B)Reaction of salt like hydrides with water.

As per my knowledge the answer should be (B) but I am a bit confused and not exactly sure about the answer.

Any explanation for the correct answer would be appreciated

  • $\begingroup$ Hi there! welcome to Chem.SE! We require you to show your efforts on this problem. Share your thoughts on why it should be (B). Please add this to your question. Thanks! $\endgroup$ – MollyCooL Mar 18 '18 at 12:19
  • $\begingroup$ @MollyCooL I read the answer in the book once.However I am not aware of the reason behind it.That’s why I asked this question.Can you please tell me the reason for -1 vote.Where did I go wrong? $\endgroup$ – Calculus Programmer Mar 19 '18 at 14:13
  • $\begingroup$ It must’ve been because your question didn’t fit into the rules. The one who downvoted should have left a comment. But as stated above it has been put on hold clearly stating that reason. $\endgroup$ – MollyCooL Mar 19 '18 at 14:21

It really depends on how you implement it.

(A): If you use deionised water (There are also different levels of purity),add sulphuric acid to it(to make it electrically conducting) and use appropriate electrodes you will get very pure hydrogen. You just have to isolate the two electrodes, otherwise you will get a mixture of hydrogen and oxygen, which is definitively not pure and also extremely dangerous. Just take a look at the Hofmann voltameter on how to do the spacial isolation.

(B): The reaction of hydrides with water generates also pure hydrogen, but you still have to use deionised water and (sometimes pure) hydrides (they need not to be pure if the impurity doesn't liberate a gas in any way in any reaction). You should take into account that hydrides are very reactive and thus dangerous.

I would prefer (A) because deionised water is cheaper than hydrides ( pure or not) and this way is safer. The purity of the two means is (if correctly implemented) very high, there is no major difference in purity.

So which way gives the most pure hydrogen depends on some parameters you didn't specify in your question.

  • $\begingroup$ What about water vapour which may be formed due to the heat of reaction? $\endgroup$ – Eashaan Godbole Mar 18 '18 at 14:03
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    $\begingroup$ Good point! I think it can be prevented, by only adding small amounts of hydride at once or cooling the whole thing. $\endgroup$ – Legolas Mar 18 '18 at 15:28
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    $\begingroup$ You could add a cold trap or several cold traps. If you used liquid helium or liquid hydrogen to cool it with several cold traps in a series you would freeze or condense every other gas first before the hydrogen liquifies as well. Perhaps, as hydrogen adsorbes to some metals you could filter everything else of by temporarily binding it to some metal sponge and the releasing it again. $\endgroup$ – Justanotherchemist Mar 19 '18 at 8:27
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    $\begingroup$ @EashaanGodbole It would be present in both cases and the gas should be dried before use. IRL pure hydrogen is usually produced with electrolysis because it is cheaper (and dirty hydrogen is produced via methane partial oxidation) $\endgroup$ – permeakra Mar 19 '18 at 8:28

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