Confused about lab results: combustion of ethyne (full test tube/half full test tube/1 cm full test tube)?

Question

Can anyone explain what happened in this lab? I'm totally confused about the results.

So we did a lab in class today where we collected three test tubes of ethyne gas. One tube was full, one was half full of ethyne gas, and one tube with 1 cm of gas.

We collected the gas by reacting CaC2 in water to collect the ethyne gas, meaning that nothing else should be in the test tubes except for the gas.

When we held a flame near mouth of the FULL tube, there was a small pop and a little bit of residue near the top of the test tube, meaning that it was incomplete combustion.

The half-full tube resulted in ALOT of residue, meaning alot of incomplete combustion.

The 1 cm tube resulted in a very loud pop and no residue (complete combustion).

I was assured that the results are correct, but I can't figure out why the half-full tube left alot of residue, yet the full tube only left a little bit of residue.

Also, how would I write the combustion equations for the half tube and full tube?

Physand.. ,

I now understand what happened to the 1 cm and full tube, but I'm still a bit unsure about the 1/2 tube. Here's my interpretation of what happened with the 1/2 tube:

As I released my thumb on the mouth of the test tube, oxygen was able to seep into the test tube, unlike the full tube. When I brought the flame near the mouth of the test tube, the gas (that was already mixed with oxygen after I released my thumb) started a "chain reaction" down the test tube. This was observed when we saw a black blob/streak of residue run down the test tube. While most of the gas was able to react, there wasn't enough oxygen to COMPLETELY react, thus resulting in large amounts of residue.

Is this correct?


P.S. The other thing I forgot to add was that we vigorously shook the 1 cm and half full tubes test tubes to evenly "spread" the gas throughout the test tube.

Answer 1

Wait, the above answer is entirely incorrect!! Here's what happened:

As you released my thumb, oxygen seeped into the test tube yielding half reactions only. When the chain reaction ensued, you observed the black blog... this is just the "tar" of the reaction.. of course it's much more than that, but you can decant it and proceed with the rest of the rxn, avoiding these errors!!!.

Answer 2

The half tube didn't have sufficient oxygen although the entire mixture was heated.

The combustion reaction is the usual equation for ethyne.