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

The full test tube could only react at the mouth of the tube where the O2 met the gas, so there was little combustion. It was incomplete due to the low amount of O2 present.

The 1/2 full test tube had a small amount of O2 already in the tube when you turned it over to test the reaction. That small amount of O2 was more than the amount that could react with the full test tube - but not enough for complete combustion. More of the incomplete combustion was able to occur.

The last tube had much more O2 moving into the test tube as you turned it over to test it. This higher amount of O2 compared to the ethyne allowed complete combustion to occur.

Answer 2

Combustion Of Ethyne Lab Answers

Answer 3

you need to think about the LEL and the UEL.
Lower Explosive Limit and the Upper Explosive Limit
In our lab we use 5% methane in argon on an instrument with an ECD(Electron Capture Detector) that only inches away has a hydrogen air flame in a FID (Flame Ionization Detector).
There is never an explosion because the LEL of methane is 5.5%.
Look at the MSDS for acetylene and see what the LEL and UEL are.