[gaidzag]

Hi all,

There is a phenomenon I saw in a video while preparing a lesson for my students that I cannot explain.  Please see the following http://www.middleschoolchemistry.com/multimedia/chapter4/lesson3#sodium_in_water and scroll down to the video "Sodium in Acid".

Here is the crux: the sodium reacts far more violently to weak hydrochloric acid than strong hydrochloric acid.  Please, please, please, can someone explain this to me?  Is it too much interference from chloride ions?  WTF?

Thanks for your time,

G

[Borek]

Can't see the video, but in general this is tricky, and not because of chemistry. First of all, judging how vigorous the bubbling is just by watching is not a best way of telling how fast the reaction is. Second, the reaction speed depends here on the speed of transport of the solution to the sodium surface, bubbles interfere as - during creation - they separate the metal from the solution. So it can happen that while the reaction is in fact faster in the concentrated acid, bubbles interfere with the solution transport so effectively, that the observed bubbling seems to be slower.

[gaidzag]

Can't see the video, but in general this is tricky, and not because of chemistry. First of all, judging how vigorous the bubbling is just by watching is not a best way of telling how fast the reaction is. Second, the reaction speed depends here on the speed of transport of the solution to the sodium surface, bubbles interfere as - during creation - they separate the metal from the solution. So it can happen that while the reaction is in fact faster in the concentrated acid, bubbles interfere with the solution transport so effectively, that the observed bubbling seems to be slower.

It's a shame you couldn't see the video.  There appears to be a white precipitate forming.  Could it be NaCl forming so quickly that it also forms a barrier between the sodium metal and the solution?  That could also explain why the concentrated nitric acid doesn't have the same issue, since none of the sodium nitrate formed would precipitate and form a barrier in the same way.

[AWK]

I think that this phenomenon  comes from poor solubility of NaCl from the surface of sodium in concentrated HCl + isolation by bubbles