VCE Chemistry

What a fuel is

A fuel is a substance that reacts to release useful energy, most often as heat, and in practice nearly always by combustion with oxygen. The fuels met in VCE Chemistry are carbon-based and fall into two groups, fossil fuels and biofuels, which are separated by where the carbon comes from and by how quickly the source is replaced.

A resource is renewable when natural processes can replace it within a relatively short time, short enough to be set against the rate at which people use it. A resource that takes millions of years to form is treated as non-renewable, since it is used up far faster than it is made.

Fossil fuels

Fossil fuels formed from the remains of living things that were buried and changed by heat and pressure over millions of years. They are non-renewable.

• Coal is mostly carbon, together with smaller amounts of hydrogen, oxygen, sulfur and nitrogen compounds, and is burned for heat and electricity.
• Natural gas is mainly methane, $\ce{CH4}$, with smaller amounts of other light hydrocarbons such as ethane and propane.
• Petrol is a liquid mixture of hydrocarbons, largely containing molecules of roughly five to twelve carbon atoms, obtained from crude oil by fractional distillation and further processing. Diesel and kerosene are heavier liquid fractions of the same kind.

Burning any of them releases carbon dioxide that was locked away for a very long time, so the amount of carbon dioxide in the atmosphere rises. Sulfur in coal and some oils also produces sulfur dioxide, and incomplete combustion produces carbon monoxide and soot.

Biofuels

Biofuels are made from material that was living recently, so the carbon they release was taken from the air by plants within years or decades rather than millions of years. Three are required.

Biogas is a mixture of methane and carbon dioxide, with small amounts of other gases such as hydrogen sulfide and water vapour. Only the methane burns, so a sample with more methane releases more energy per gram, and its energy content varies with the source.

Bioethanol production

Bioethanol is made by the fermentation of glucose:

The fermentation is carried out by yeast in the absence of oxygen. The solution that results is a dilute mixture of ethanol in water, because the yeast stops working once the ethanol concentration gets too high, so distillation is used to concentrate the ethanol into a fuel. Ethanol boils at about 78 °C and water at 100 °C, so the ethanol leaves the mixture first. Distillation needs a lot of energy, and the energy used to grow the crop, transport it and run the distillation must be counted when judging how much useful energy a biofuel really supplies.

Renewable does not mean free of emissions

Biofuels release carbon dioxide when they burn, but the crop took the same gas from the air as it grew, so the carbon is part of a short cycle. This is the reason biofuels are described as more sustainable than fossil fuels. The argument is fair only when the whole process is counted: fertiliser, machinery, transport and distillation often use fossil energy, and clearing land for a crop releases carbon that was stored in the soil and vegetation. Growing fuel crops can also compete with food crops for land and water.

Fossil natural gas and biogas are chemically almost the same fuel, since both are mainly methane. The difference that matters for renewability is the source: biogas comes from waste that is continually produced, natural gas from a deposit that is being used up.

Comparing fuels by energy released

The energy released in complete combustion can be stated per mole ($\pu{kJ mol-1}$), per gram ($\pu{kJ g-1}$) or per millilitre ($\pu{kJ mL-1}$). Which is the right measure depends on the question. Per gram matters when the fuel has to be carried and mass is limited, per millilitre matters when storage volume is the limit, and per mole is used in stoichiometric calculations.

The typical values below are for complete combustion to carbon dioxide gas and liquid water at 25 °C and 100 kPa. A fuel that is a mixture has a range of compositions, so the figures are typical rather than exact.

The natural gas figure per millilitre looks tiny because a millilitre of gas contains very little fuel. This is why gases are compressed or liquefied to be stored and moved.

Reading the values

Petrol, diesel and kerosene give about 45 to 46 $\pu{kJ g-1}$, whereas bioethanol gives about 30. The hydrocarbons contain only carbon and hydrogen, while ethanol already contains an oxygen atom bonded to carbon, so ethanol is partly oxidised before it burns and has less left to release. Biodiesel is an ester, which also has oxygen in the molecule, and it sits between ethanol and the hydrocarbons.

To find the mass of fuel that would release $1.00 \times 10^{3}\ \pu{kJ}$:

So about half as much again of bioethanol is needed for the same heat. A fuel with a lower energy per gram can still be the sensible choice where its source is renewable, where it burns more cleanly, or where an engine is already built for it.

Comparing molar values

The molar enthalpy of combustion is the energy released when one mole of a substance burns completely. The value for one mole rises as the carbon chain lengthens, since each extra $\ce{CH2}$ unit adds more bonds to break and more carbon dioxide and water to form. Methane releases $\pu{890 kJ mol-1}$, ethane $\pu{1560 kJ mol-1}$, propane $\pu{2220 kJ mol-1}$ and octane $\pu{5470 kJ mol-1}$. Per gram the trend is much flatter, because the larger molecules are also heavier: methane gives $890/16.0 = 55.6\ \pu{kJ g-1}$ and octane gives $5470/114.0 = 48.0\ \pu{kJ g-1}$, with a molar mass of 114.0 $\pu{g mol-1}$ for $\ce{C8H18}$.

Alcohols release less per mole than the alkane with the same number of carbons, for the same reason as ethanol compared with petrol: methanol gives $\pu{726 kJ mol-1}$ against $\pu{890 kJ mol-1}$ for methane, and ethanol gives $\pu{1367 kJ mol-1}$ against $\pu{1560 kJ mol-1}$ for ethane.