Why does a hydrocarbon need a lot of oxygen

Regardless of the type of hydrocarbon, combustion with oxygen produces 3 products: carbon dioxide, water and heat, as shown in the general reaction below.

The energy required to break the bonds in the hydrocarbon molecules is substantially less than the energy released in the formation of the bonds in the CO 2 and H 2 O molecules.

For this reason, the process releases significant amounts of thermal energy heat. This thermal energy can be used directly perhaps to heat a home or else it can be converted to mechanical energy, using a heat engine. However, this is subject to efficiency losses, resulting in necessary significant energy losses as waste heat governed by the second law of thermodynamics.

The resulting useful mechanical energy will be a lot less than the initial thermal energy provided by the hydrocarbon combustion. Note that CO 2 is always produced in hydrocarbon combustion; it doesn't matter what type of hydrocarbon molecule.

Producing CO 2 and H 2 O is actually how useful energy is obtained from fossil fuels. For this reason, it is important to distinguish between carbon dioxide and other "waste" products that arise from impurities in the fuel such as sulfur and nitrogen compounds. Gas fires and boilers must be serviced regularly to ensure they do not produce carbon monoxide. Here are the equations for the incomplete combustion of propane, where carbon is produced rather than carbon monoxide:.

Combustion Fuels are substances that react with oxygen to release useful energy. Complete combustion Complete combustion needs a plentiful supply of air so that the elements in the fuel react fully with oxygen.

Upper panel: absorption spectrum without ignited plasma. Lower panel: absorption spectrum with plasma ignition at 3. The density, rotational and vibrational wagging mode temperature are 2. The calculation follows the same steps, as shown in detail by Klarenaar et al. The lines are then broadened by convoluting them with a normalized line width function f assuming pressure broadening, which is the dominant broadening mechanism in our experiment yielding Lorentzian line widths in the range of 0.

The actual width is extracted directly from the spectra. This fitting is rather robust, because the Gaussian line width from the instrumental broadening and the Lorentzian line width from the pressure broadening lead to a very distinct line shape.

Excellent agreement is found. Both codes agree very well. The measurement and the model show a good agreement and provide a density of 2. The power absorbed in the plasma is varied between 0. This rotational temperature is usually close to the gas temperature and indicates a heating of the gas stream at higher plasma powers. This is not surprising, given the fact that up to 7 W are being absorbed in the plasma mainly by helium ionization and excitation.

The power is at first absorbed by the electrons in the plasma that cause excitation and ionization of helium and of the admixed molecular gases. This electronic excitation of the plasma species is eventually converted into other degrees of freedom of the molecules such as rotation, vibration and translation in super elastic collisions.

The corresponding heating of the different degrees of freedom depends very much on the collision dynamics and on the different relaxation times in between these degrees of freedom. For example, electronic excitation of the molecules causes at first a vibrational excitation, which is then converted into rotational excitation and then into translational energy.

It can be seen that the vibrational temperatures are significantly higher than the rotational temperatures. This has already been observed previously and illustrates the non-equilibrium character of plasma excitation [ 8 , 9 ]. In principle, the rotational and vibrational temperatures can also be deduced from the emission bands of CO, which requires a complete analysis of the excitation and emission processes.

Such experiments are planned in the future. The advantage of the analysis of IR absorption spectra is their straightforward method of quantification that yields temperatures and densities at the same time.

The experiments show an increase in vibrational temperature of CO from to K indicated as a dashed line in Fig. The resulting temperature estimates are shown as dashed lines in Fig. It can be seen that these estimates agree fairly well.

It is striking that this very simple thermodynamic analysis reproduces the typical temperatures rather well and any more intricate excitation and de-excitation model is apparently not needed. Apparently, the frequent collisions among the species assure thermal equilibrium between the different degrees of freedom of the molecules. The CO production does not vary too much, although plasma power and oxygen admixture vary.

This indicates that CO constitutes an intermediate product. At very high plasma powers, the density of CO decreases again. This interpretation is supported by the data for very low oxygen admixture of 0. The conversion rates for 1. This is striking, because the conversion of methane is not improved although more oxygen is admixed.

Apparently, the oxygen atom production reaches an optimum with respect to oxygen admixture between 1. This constitutes a concentration between 0. This observation is consistent with experiments performed on an equivalent radio frequency discharge in helium with varying admixture of oxygen [ 11 , 18 , 19 ]: in a so-called COST-jet an optimum oxygen atom production at an admixture level of 0.

This reaction, however, is more evident in the afterglow of the plasma, since efficient ozone formation requires low electron temperatures [ 18 ]. The plasma chemistry is also analysed by regarding the mass balances of all atoms that are measured by FTIR in the system to identify the occurrence of other species invisible to our diagnostic. Figure 6 shows the carbon a , hydrogen b , and oxygen c balance of the atoms in the individually measured species.

It can be seen that the balances are almost fulfilled. A small decrease in all mass balances with increasing plasma power is observed that can be related to an increase in temperature with increasing plasma power and thus a lowering of the overall gas density. The resulting extrapolation of the mass balances to higher plasma powers is shown as dotted lines in Fig. Apparently, all atoms in the molecules fed into the plasma can be found in the plasma reaction products.

The mass balance concerning oxygen atoms, however, can only properly be extrapolated from the set gas mixture at higher plasma powers and small admixtures of 0. Any atomic or molecular oxygen that is not consumed in the reaction is inherently missing in the oxygen mass balance. This is consistent with the data. At an admixture of 1.

The dashed lines denote the expected density variation based on the ideal gas law taking into account an increase in gas temperature according to measured rotational temperatures and thus a corresponding decrease of density at constant pressure. The gas flow is in the direction of the increasing position coordinate. The efficiency of depletion scales with the power absorbed in the plasma, although at the highest absorbed power of 6 W and 7 W no large differences can be found.

The density of CO goes through a maximum in the center of the gas channel, before it decreases again. This confirms the intermediate character of CO in the reaction sequence, as already discussed above. In any application, such reactions would be avoided by restricting the residence times so that the products leave the plasma volume when their maximum concentration is reached.

This could be explained as follows: at the beginning of the plasma channel, the electron density is high, whereas the oxygen atom density is still low [ 20 ]. This oxidation plays an important role in how we obtain energy from fossil fuels.

Have students re-make their methane models, as described in the first lesson of this series. Review some principles of the first lesson with students by asking them:. Tell students that hydrocarbons go through a special type of oxidation called combustion. When hydrocarbons burn, the reaction produces carbon dioxide and water. Have students construct an oxygen O 2 model using gummy bears and toothpicks. Ask students to react their methane compound with the oxygen to produce carbon dioxide and water by rearranging the marshmallows, gummy bears, raisins, and toothpicks.

Students will realize that they need more than one oxygen to utilize all the hydrogen atoms to produce water. Tell students that they began with a methane molecule and then broke it into carbon dioxide and water pieces.

To help them understand that energy is released in this reaction, ask two students to volunteer. Ask the two students to hold hands and pull back from each other. Tell the class that the joining of hands represents a covalent bond.

To reinforce that energy is released when the Carbon-Hydrogen bond is broken, ask the two students to hold hands again while they stand next to each other, without any pulling. Ask the class again about the energy that will be released if the bond is broken. Less energy is released because there was less stored energy in the bond to begin with. Tell students that knowledge of the amount of energy released from hydrocarbons is important because methane is the main component of natural gas.

When methane goes through the process of combustion i. This is known as complete combustion. Review with students:. Students can answer questions about the reading on the Combustion of Fossil Fuels student sheet.

You can find answers to the questions on the Combusion of Fossil Fuels teacher sheet. Discuss the reading together and clearly review the Combustion Reaction Energy from Bond Energies image. The image can be given to students as a handout or displayed as an overhead.