Answer to Question #251761 in General Chemistry for Mikay

Question

Based upon ethane vs ethanol, what can you conclude about the introduction of an
oxygen atom into a hydrocarbon fuel?

Answer 1

ethanol has the chemical formula C2H5OH

C2H5OH. Ethanol can be used as a fuel since it can produce heat energy as a result of alcohol combustion. Most ethanol is usually produced in one of two ways, using either petroleum or biomass. Petroleum derived ethanol happens in large chemical plants through a petrochemical process. Raw materials are separated from petroleum through fractional distillation. Ethane(C₂H₆), one of the separated raw materials, goes through cracking to become ethene (C₂H₄), also referred to as ethylene. Ethene is converted to ethanol through a hydration process that requires a catalyst (often phosphoric acid) while the ethene reactant is under high pressure steam in the reaction chamber. High pressure increases the reaction rate, so that more ethanol will be produced. This hydration process has the following exothermic chemical reaction which releases heat energy.

C₂H₄ + H₂O →catalyst→ C₂H₅OH + Heat Energy (Enthalpy)

C₂H₅OH + 3 O₂ → 2 CO₂ + 3 H₂O + Heat Energy (Enthalpy)

CO₂ is always produced in hydrocarbon combustion; it doesn't matter what type of hydrocarbon molecule. Producing CO₂ and H₂O 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.^[1] Wastes that arise from impurities can be eliminated with the right technology; CO₂ cannot be eliminated unless the fossil fuels are not burned in the first place.

Not all fossil fuels have the same composition. Natural gas is composed of over 90% methane (CH₄) which is the smallest hydrocarbon molecule. Oil tends to be composed of medium sized molecules, although composition varies greatly from one grade of crude to the next. In general, the denser the oil, the longer the carbon chains in the molecules. Finally coal contains the largest and most complex hydrocarbon molecules.

Since different hydrocarbons have different ratios of hydrogen to carbon, they produce different ratios of water to carbon dioxide. In general, the longer and more complex the molecule, the greater the ratio of carbon to hydrogen. For this reason, combustion of equal amounts of different hydrocarbons will yield different quantities of carbon dioxide, depending on the ratio of carbon to hydrogen in molecules of each. Since coal contains the longest and most complex hydrocarbon molecules, burning coal releases more CO₂ than burning the same mass of oil or natural gas. This also changes the energy density of each of these fuels

The amount of CO₂ produced when a fuel is burned is a function of the carbon content of the fuel. The heat content, or the amount of energy produced when a fuel is burned, is mainly determined by the carbon (C) and hydrogen (H) content of the fuel. Heat is produced when C and H combine with oxygen (O) during combustion. Natural gas is primarily methane (CH₄), which has a higher energy content relative to other fuels, and thus, it has a relatively lower CO₂-to-energy content. Water and various elements, such as sulfur and noncombustible elements in some fuels, reduce their heating values and increase their CO₂-to-heat contents