Help: Temperature increases but reaction slows down

[cphsing]

My son is working on a science fair project. The subject is - temperature impact when mixing vinegar and baking soda. We observed something that we cannot explain. When temperature raised to certain point, the reaction actually slowed down.

The experiment was done by observing how much gas a balloon captures from the reaction in fixed amount of time - 10 seconds (mix reactants, vinegar and baking soda, in a beaker sealed with a balloon). The variable is the vinegar temperature. The same test was repeated for various temperature, from 8 degree Celsius to 18, 28, 38, 48 and 58 degree. Initially the balloon captured more and more gas in 10 seconds in each test as the temperature went up. It peaked at 38 degree Celsius. The trend reversed after the temperature raised to 48 degree and 58 degree, the balloon captured less and less gas. We made sure there were enough reactants left to continue after 10 seconds for each test. Temperature is the only variable.

So, why does the reaction slows down after 48 degree Celsius (somewhere between 38 and 48)? I thought this is a endothermic reaction, the more heat/energy the better?

Please help us find the answer.

[deleted-71709]

This one had me scratching my head, too.

I am not a chemist, but do remember the basics from my university days. Those recollections did not help answer your question, however, so I went to the internet and have found the answer here:

http://antoine.frostburg.edu/chem/senes ... alpy.shtml

Based on my extensive background as a mechanical engineer in heat transfer and thermodynamics, I concur with the authors conclusions. The rapid cooling of the gas produced initially by the rapid reaction reduced the temperature of the vinegar, thereby slowing the reaction.

Science can really be a mystery sometimes!

I'm glad you and your student are working together on this. I love to see parent involvement.

Continue to have fun with science!

[cphsing]

Thank you so much for your reply. The explanation in your internet research makes sense to me.
I love working with the kids on their projects, while they still allow me... Science creates magic in another way.

[deleted-71588]

the balloon captured more and more gas in 10 seconds in each test as the temperature went up

How are you measuring the amount of gas produced in 10 seconds? There is an ideal gas law that indicates PV = nRT, Pressue, Volume, number of moles, Reynold's constant, Temperature in Kelvin. The elasticity of the balloon is also affected by temperature and previous expansion and contraction. If you aren't weighing the gas, you need to know the pressure, the volume, and the temperature of the gas to be able to make valid comparisoins. This is in addition to the things mentioned in the previous link.

[cphsing]

Craig, all the points you raised are very valid.

The gas captured is measured by allowing the reaction to go for 10 sec and then twist and seal the balloon. The measurement of the expansion is done by measuring the volume (height, width and length) of a virtual box enclosing the balloon. The balloon was pre-expanded to relax the tension.

As you can see, none of the setting is done accurately or precisely by scientific standard. For science project at this stage, it is more of a exploration and illustration. A lot of assumptions in variables were made but not really controlled. To avoid error effect from the setting, larger delta change in temperature was used. In the end the outcome proved the temperature impact so my kid was happy, but the changed trend of reaction speed was a surprise.

What we learned from edneu3's response was one mistake we made that we only measured and record the initial temperature and assume it would remain constant during the reaction. This is actually not true, and was the contributing factors to why we see the change in trend. My kid is going to repeat the test in his further research by recording temperature change before and after so the change in trend can be properly explained. He is also trying to find out if there's a way to maintain the temperature during the process (It won't be easy without serious equipment setup). Overall, he learned a lot from this experiment, not just the outcome, but the process as well (and so did I). Now that you point out more factors, he would have a lot more to think about. This is really what the science fair project about.