Heat of Combustion of Alkane vs Alcohol
For a particular number of carbon atoms, the reaction enthalpy is more exothermic for an alkane than for an alcohol. In this demo, equal moles of methane and methanol are combusted in a cylinder to launch a foam ball up into the air. The height of the launch shows that the gas expands to a greater degree (more exothermic) for the methane.
- cylinder with spark plug and injection port. Cylinder volume is 600 mL.
- hot plate (this is used to heat the cylinder to ensure complete vaporization of the methanol)
- Tesla coil to spark the spark plug
- glass syringe for methane
- microsyringe for methanol
- temperature probe to monitor the cylinder temperature prior to loading methanol
- bicycle pump (or equivalent) to flush out gas space between the two combustions
- The cylinder apparatus is placed onto the hot plate prior to the demo. The temperature is monitored at the base of the cylinder to hold at 45°C.
- Temperature probe is removed and a foam ball is plugged into the top of the cylinder, careful to make a smooth seal with the walls.
- 0.08 mL of methanol (0.00204 moles methanol) are injected into the port with a fast burst from the syringe to assist in vaporizing it.
- Ask the audience to take careful note of how high the ball goes. Within a minute, the Tesla coil is used to ignite the methanol/air mixture sending the ball about 10 feet in the air with a satisfying popping noise.
- Use air pump to flush products out of cylinder and replenish the air.
- Put the foam ball back into the cylinder.
- Inject 50 mL of natuarl gas (methane) from glass syringe into the port (0.00204 moles methane). Unlike the methanol, you want to inject this less vigorously since it is already a gas. Some will leak out around the ball, and that's okay. The cylinder temperature is less important for the methane, so it is better to do this fuel second, as described here.
- Spark the methane with the Tesla coil. The ball should pop higher into the air, usually about 15 feet.
The height that the ball flies into the air depends on the pressure that builds up behind the ball before it is pushed out of the cylinder. In a simplistic way, this is dependent on the exothermicity of the reaction since the temperature change causes the gas to expand and the pressure to increase. The less simplistic detail, however, is that the kinetics of the reaction are important in obtaining the max pressure before the ball moves out of the tube. In fact, this demo could probably be used in a Pchem course to highlight the interplay of thermodynamics and kinetics. For Gen Chem purposes, the exothermicity is sufficiently demostrated using the setup described. In a sense, the hot plate is serving to not only vaporize the methanol, but to also tune the kinetics of the combustion to give the desired (apparent) thermodynamical result in the classroom. Trials in which equal moles were combusted without heating the cylinder gave only a few feet launch of the ball for methanol, while the methane always went very high. This could be due to a higher activation energy for methanol combustion causing kinetically sluggish combustion, and/or incomplete vaporization of the methanol fuel. It is probably a complex combination of both!
Using the setup as described above in a Gen Chem setting, the following discussion is relevant.
Combustion of 1 mole of methane is written:
CH4(g) + 2 O2(g) --> CO2(g) + 2 H2O(g)
Combusion of 1 mole of methanol is written:
CH3OH(g) + 1.5 O2(g) --> CO2(g) + 2 H2O(g)
You can calcuate the heats of reaction (∆H0rxn) using Hess' Law (heats of formation, Ch. 6 Silberberg) or by summing the heats of breaking all bonds and then reforming all of the new bonds (Ch. 9 Silberberg).
Special Safety Notes
- The Tesla coil uses high voltage and can shock you
- The combustion reaction releases a very small fire ball when it is ignited in the cylinder. This reaction, as described here, is on a very small scale. The ball launching makes a small popping noise rather than a loud explosion.
- The hot plate is somewhat hot.