With my 6yo daughter, I have conducted a simple experiment. We used 1/4 tsp (we don't have sub-gram scales) of bicarbonate of soda for each test. In each test, we used a different amount of vinegar (5% w/w acetic acid), topped to 120ml with water. In each test we left the reaction to (for the purposes of measurement) complete and measured the $CO_2$ volume produced. (the reaction took place in an upturned measuring cylinder).

  • our first hypothesis was that we would get a linear relationship up to the point that the bicarbonate of soda became the limiting reactant.
  • we then modified this thinking that a certain amount of $CO_2$ might get dissolved in the water so the graph line might pass below the origin.

The results we got were surprising. With each increase in vinegar, the volume of gas increased by more. See below

vinegar  |  CO2
  (ml)   |  (ml)
   5     |  8.5
   10    |  22
   15    |  34
   20    |  110

We have repeated the measurements a few times. We have a fair amount of variability but the same pattern. (I appreciate the woolliness of this statement, more statistic next time).

I guess that the increased rate of reaction which came with increased concentration meant the $CO_2$ had less chance to dissolve. Can someone tell me what the correct explanation is?

As a bonus - is there a way to conduct this experiment in such a way that we will be better able to observe the relationships between the reactants and the product? One thought that I had was saturating the vinegar with NaCl so that it won't allow any carbon dioxide. Could this work? Is there a better way

  • $\begingroup$ Was the upturned measuring cylinder then put into some container of water? I'm having trouble visualizing how your "simple experiment" was done. Please explain the experimental procedure in detail. $\endgroup$ – MaxW May 9 '19 at 23:37
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    $\begingroup$ I think your vinegar was the limiting reagent in each case. Vinegar is only 5% in acetic acid. You were using pure sodium bicarbonate. Given that more acid you added, you observed more CO2, supports this point. $\endgroup$ – M. Farooq May 10 '19 at 0:56
  • $\begingroup$ There is another aspect of creating acetate pH buffer. Acetic acid : sodium acetate 1:1 gives pH 4.75. OTOH, pH of water saturated by CO2 Is 3.85 ( derived as pH=0.5(6.35-log 0.034) from chemistry.stackexchange.com/questions/42947/…). So the acid needs some excess. Another aspect is bicarbonate pH buffer with effective pKa 6.35. $\endgroup$ – Poutnik May 10 '19 at 4:05
  • $\begingroup$ @M.Farooq - yes I'm sure the vinegar was the limiting reagent. The intent was to observe the change in product with the increase in vinegar. $\endgroup$ – maninalift May 11 '19 at 23:36
  • $\begingroup$ @MaxW the measuring cylinder was upturned on a jam-jar lid providing a small reservoir of water covering the opening of the cylinder, preventing air from entering it. This makes a very simple was to measure the volume of gas produced. The downside is that vinegar will be displaced as gas is produced but this would have the opposite effect to what we observed (reducing the rate of increase in product with reagent) $\endgroup$ – maninalift May 11 '19 at 23:50

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