I recently had the opportunity to attend a conference of the Associated Chemistry Teachers of Texas (ACT2). I had  great time interacting with and learning from a whole bunch of wonderful chemical educators from the great state of Texas. One of the most interesting things I learned was in reference to the classic “whoosh bottle” experiment, which is powered by the combustion of isopropyl alcohol:

2 C₃H₇OH(l) + 9 O₂(g) → 6 CO₂(g) + 8 H₂O(g)        Equation 1

Interestingly, the reaction in this experiment yields different results when conducted at different temperatures (Video 1).

Video 1: Effect of temperature on a combustion reaction, pchemstud on TikTok. June 20, 2022.

That’s quite a significant difference! When I shared this video online, I had a few people comment that the difference observed might not be due to a faster reaction rate at the higher temperature. Instead, it was argued that the difference was because more alcohol vapor is contained in the warm bottle as compared to the cold one, on account of the higher vapor pressure of alcohol at higher temperatures. We can see this quantitatively by first noting the vapor pressures of isopropyl alcohol at 5.0^oC (12 mmHg = 0.016 atm) and 32 ^oC and 20^oC  (32 mm Hg = 0.042 atm).¹ Because the volume of the bottle is known (5 gallons = 19 L), we can then use the idea gas law (as n = PV/RT) to calculate the moles of isopropyl alcohol present in each bottle:

That’s about 2.5 times more alcohol vapor in the warm bottle over the cool one. Because reaction rates increase with higher concentration of reactants, perhaps the effect observed really is due to the presence of more fuel in the warm bottle.

Or is it…?

I decided to look into this a bit further, attempting to use the Arrhenius Equation to compare the kinetic rates of the reaction at the different temperatures:

Where k is the rate constant for the reaction, A is the pre-exponential factor, E_a is the activation energy for the reaction, T is temperature, and R = 8.314 J mol^-1 K^-1. The ratio of the values of the rate constants at 20^oC (293 K) and 5^oC (278 K) would therefore be:

Substitution of 125 kJ mol^-1 as an estimate for the activation energy of the combustion of isopropanol^2-4 into Equation 3 yields:

Therefore, this analysis suggests combustion in the warm bottle should proceed about 16 times faster than in the cold bottle.

So which is it?

Well, 16 (kinetic temperature effect) is over 5 times bigger than 2.5 (amount of fuel effect), so perhaps these analyses should lead me to conclude that combustion in the hot “whoosh bottle” goes faster because of the effect of temperature on reaction rates – not from the presence of more fuel in the hot bottle as compared to the cool one. But honestly, I’m not sure. I’m not entirely comfortable with my estimation of the activation energy of isopropanol combustion used in the Arrhenius analysis. I’m also not sure that the Arrhenius  analysis I did is entirely appropriate. Furthermore, what I am confident about is that multiple types of combustion took place in the warm bottle but not in the cool bottle. I say this because I saw soot had formed on the outside of the warm bottle, but not the cool one after the reaction subsided (look for this carefully in Video 1). The formation of soot indicates incomplete combustion, in which carbon monoxide (Equation 4) and soot (Equation 5) are formed:

C₃H₇OH(l) + 3 O₂(g) → 3 CO(g) + 4 H₂O(g)                        Equation 4

2 C₃H₇OH(l) + 3 O₂(g) → 6 C (s, soot) + 8 H₂O(g)               Equation 5

If multiple reactions occurred in the warm bottle but only complete combustion in the cool bottle, this complicates matters further. In the end, if I had to guess, I’d venture that both effects play a role.

But I’d like to know what you think. Why does the “whoosh bottle” go faster at higher temperatures? Is it simply because of the effect of temperature on reaction kinetics? Or is it because more alcohol vapor is present in the warmer bottle due to the higher vapor pressure of alcohol at increased temperature? Might both impact the reaction rate? What thoughts or criticisms do you have on my analysis presented here? Do you have any suggestions for experiments I might try to test which of these two possibilities better describes the observations? I look forward to hearing from you, and to carrying out more experiments on the effect of temperature on the “whoosh bottle.”

Happy Experimenting!

Acknowledgement:

Thanks to Dr. Bob Shelton from Texas A&M University in San Antonio for showing me this demonstration.

References:

1. Parks, G. S.; Barton, B. J. Am. Chem. Soc. 1928, 50, 24-26.
2. Vandenabeele, H.; Corbeels, R.; van Tiggelen, A. Combustion and Flame, 1960, 4, 253-260.
3. Frassoldati, A.;  Cuoci, A.;  Faravelli, T.; Niemann, U.; Ranzi, E.; , Seiser, R.; Seshadri, K.  Combustion and Flame, 1960, 4, 253-260.
4. I could not find a literature value for experimentally measured values for the activation energy of the combustion of isopropanol. However, reference 3 does mention a value of less than 35 kcal mol^-1 (146 kJ mol^-1) based on a combination of measurements and theoretical calculation. Further, reference 2 cites 36.5 kcal mol^-1 (153 kJ mol^-1) as the activation energy for methane combustion. These considerations suggest 125 kJ mol^-1 is a fair estimate.