Why didn't my hydrogen/oxygen mixture produce a bang?

[glassgiant]

Every year, my kids and I destroy our Halloween pumpkins in a fun way. This year, we made a hydrogen/oxygen mixture from stainless steel plates and a battery charger in a baking soda/water mixture. A funnel captured all the bubbles (not just from the anode or cathode) in an inverted plastic water bottle, which was initially filled with water. Once the bubbles displaced all the water in the bottle, I capped it.

An hour later, to ignite the bottles, we lit some sawdust and lighter fluid in the bottom of the pumpkin.  Once the kids were far away, wearing safety glasses, I tossed the bottle in.  We'd done something similar on a smaller scale with dish soap and water, lighting small amounts of foam with a match.  This produced a satisfying, sharp bang. I had expected the same, only on a larger scale, from the bottles.  Instead, we got a lethargic pop after a moment, then a secondary pop a few seconds later.  I am wondering a few things:

1. Why 2 bangs? If the first was just the pressure of the heated bottle escaping when it finally melted through (as would happen with normal air in the bottle), I would have expected the second bang to be instantly on the tail of the first.
2. Is it possible the oxygen and hydrogen settle in the bottle, with the first bang releasing the oxygen from the bottom, and the second being when the hydrogen going off with a scarcity of oxygen?
3. What else can I do to make this more entertaining?

[scooke]

Dear Shawn,

That does sound like an exciting event!  I would first caution that even (or especially) “fun” explosive demonstrations should really be avoided with kids.  At a minimum a serious discussion of the safety issues (which may dampen the event, somewhat) as well as the use of safety equipment and barricades must be used.  The concern with a range of ages is that some of them may try similar things on their own without really understanding the dangers.  Well, that may happen anyway, but at least give them a good “talking to” before your demonstrations.

Without a close look at your system I can only speculate on a few items that would be weak points in the system.  The generation of the hydrogen and oxygen is pretty straightforward, and your collection sounds fine, so we may assume that you have a stoichiometric ratio of the two in the bottle.

Now, the time delay could be an issue, as hydrogen is quite permeable in many materials, and especially through a simple screwed-cap closure.  So, it is quite likely that a substantial portion of the hydrogen in the bottle escaped in the hour delay from capture to ignition.

Requiring a burn-through of the bottle before igniting the gases instead of an instantaneous (often electric) ignition could also reduce the effect dramatically.  As you noted, there may simply have been a “pop” from the expansion of the gases and softening of the bottle before any ignition – including a possible ignition of escaping gases.  Both would reduce the available energy for a shockwave generated by the final ignition of the remaining bottle contents.

I enjoy energetic displays as much as the next person (I love to do “volcano” demonstrations – but all with fairly benign materials), but if the definition of “entertaining” is a spectacular destruction of a pumpkin I’d be a little careful about putting “up close and personal” next to “explosion”!  But then, if that is what you desire, don’t get too crazy with chemistry (especially if that is not the lesson being taught by the demo).  Just put a couple commercial cherry bombs inside.

Best regards,

Steven Cooke