What makes hydrocarbon a good fuel source

As one would expect, gas-state fossil fuels like methane and propane have small, short molecules and molecule chains. Heavy fossil fuels like diesel and bunker fuel contain large, long-chain hydrocarbon molecules. Gasoline is a medium weight of fossil fuel. Additionally, the number of hydrogen atoms attached to the carbon molecules in the backbone of a hydrocarbon also plays a role in weight and energy density.

Carbon molecules are heavier than hydrogen molecules — every element on the periodic table is heavier than hydrogen. So, the higher the number of carbon atoms in relation to hydrogen atoms in a hydrocarbon, the heavier the hydrocarbon. So, the heaviest, most energy-rich hydrocarbons are those that have size and density. The largest and longest and hydrocarbon molecule chains have the greatest weight and density.

And, those that have the highest carbon-to-hydrogen ratios have the greatest weight and density. Gasoline has greater weight and density that gas-state fossil fuels like natural gas — methane — and propane. Fossil fuels like diesel and kerosene have greater weight and density than gasoline.

With respect to molecule chain size and length and carbon-to-hydrogen ratio, gasoline is somewhere near the middle of the spectrum of fossil fuels. There are hundreds of hydrocarbons in gasoline. But, each type of hydrocarbon falls into one of two categories: saturated or unsaturated. Saturated hydrocarbons are the most stable.

Saturated hydrocarbons are those with carbon backbones with no space to take on more hydrogen or carbon atoms. There are three types of saturated hydrocarbons. They can be linear, branched, or loops.

Branched saturated hydrocarbons that are looped have the name cycloalkanes. Like saturated hydrocarbons, unsaturated hydrocarbons can be linear, branched, or loop. But, unsaturated hydrocarbons can easily take on additional hydrogen atoms. Unsaturated hydrocarbons, as a result, are unstable. Because of their stability, saturated hydrocarbons burn with a clear, clean flame.

Unsaturated hydrocarbons burn with a smoky flame and can be toxic. There are three types of saturated hydrocarbons in gasoline: alkanes, isos, and cyclic hydrocarbons. Alkanes are saturated hydrocarbons with a continuous, linear chain of carbon atoms that does not branch. Up to three hydrogen atoms can attach to each carbon atom. Iso hydrocarbons are saturated hydrocarbon chains with branches.

Along the linear chain of carbon atoms in a hydrocarbon, up to three carbon atoms can attach to each carbon atom in the chain. And to the carbon atoms attached to the carbon atoms in the chain, hydrogen atoms can attach. The third kind of saturated hydrocarbons in gasoline are cyclic.

A cyclic saturated hydrocarbon is one in which the last two carbon atoms at the ends of a hydrocarbon chain bond making a loop.

For example, cyclohexane is a looped saturated hydrocarbon chain containing six carbon atoms. The two categories of hydrocarbons — saturated and unsaturated — are comprised of two classes each. Aromatics and olefins are classified as unsaturated hydrocarbons. They contain carbon to carbon double bonds or aromatic bonds that can be converted to single bonds by adding hydrogen atoms to the adjacent carbons.

There are between and 1, types of hydrocarbons in every fossil fuel. Alkanes type compounds, either straight chain or branched compounds are present in the greatest amounts. Smaller amounts of alkane cyclic and aromatic compounds are also present.

It is the ratio of one type of hydrocarbon to another that determines fossil fuel type. The most prevalent hydrocarbons in gasoline, alkanes are saturated hydrocarbons with large reserves of energy. Each carbon atom forms 4 bonds either C-H or C-C bonds. Each hydrogen atom is connected to a single carbon atom, by an H-C bond. Again, alkanes are extremely stable because they have no space for the addition of more carbon or hydrogen atoms. All alkanes molecule chains have the same basic structure.

The carbon atoms at the end of an alkane chain have three hydrogen bonds and one carbon bond. The carbon atoms in the middle of an alkane molecule chain have two carbon atom bonds and two hydrogen atom bonds. Some alkane hydrocarbons have branches or loops. But, every carbon atom in all alkanes has four bonds. Every carbon atom either has one carbon bond and three hydrogen bonds or two carbon bonds and two hydrogen bonds. So, the only difference between different alkanes is the number of carbon-to-carbon bonds.

N-butane, for example, has four carbon atoms. Two of the carbon atoms in n-butane — those two at each end — have one carbon bond and three hydrogen bonds. The two carbon atoms in the middle have two carbon bonds and two hydrogen bonds.

Alkanes constitute roughly 55 percent of the hydrocarbons in gasoline. This rock put a lot of pressure on the dead animals and plants. Rocks around them also heated them up.

Together the heat and the pressure turned the remains into crude oil. It was important that no air or oxygen was present. Fossil fuels are finite non-renewable energy resources. Their supply is limited and they will eventually run out.

Fossil fuels do not renew themselves, while fuels such as wood can be renewed as trees capture energy from the sun in chemical reactions. This is the energy that is released when they burn. Fossil fuels release carbon dioxide gas when they burn which adds to the greenhouse effect and increases global warming. Under intense heat and pressure over the eons, these carbon-based molecules were transformed into more uniform molecules, called hydrocarbons — long chains of carbon, coated with hydrogen.

The plants that were buried deep at sea were converted to oil and gas, and those buried in swamps became coal. The only difference between the oil, gas and coal is the length of the hydrocarbon molecules that make them up.

The shortest chain hydrocarbons are gases up to four carbon atoms long , petroleum is made of medium-length chains seven to 12 carbons and oils and grease are longer still up to 20 carbons. Coal is a bit of a different story. It's mostly made of rings of carbon, not chains, and it's a bit lighter on with the hydrogen coating.

Fossil fuels store their energy in the chemical bonds that hold the hydrocarbon molecules together. That energy can stay trapped like that right up until the moment the hydrocarbons meet both heat and oxygen O2.

Heat from a match or spark breaks the hydrocarbon and oxygen molecules apart, and then oxygen atoms react with the freed carbon and hydrogen atoms to give carbon dioxide CO2 and water H2O , and loads of energy as heat along the way. That extra heat is the key to fossil fuel's success. More heat energy is released when the new bonds form in CO2 and H2O than it took to break the original bonds in oxygen and the fuel.

So once combustion starts, it's a runaway chain reaction giving off excess heat. We've used the heat directly for cooking and warming, and indirectly to boil water for steam-powered turbines for electricity. The gases given off from burning fuels have driven pistons in internal combustion engines for more than a century.

Not only can we start a fire with a single spark, we can stop it by cutting the supply of fuel, air or heat, so it's a source of energy we can turn on and off as needed. A controllable source of energy that has only two major downsides — finite supply and being the source of the CO2 that's driving climate change.

No wonder fossil fuels have been such a hard habit for industrialised countries to kick.