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Alkanes as Fossil Fuels

Alkanes as fossil fuels

Fossil fuels are fuels formed by the natural process of the decomposition of organisms under heat and pressure. They contain a high percentage of carbon and include fuels such as coal, petrol, and natural gases. They are also a non-renewable.

Definition: Hydrocarbon cracking

Hydrocarbon cracking is the process of breaking carbon-carbon bonds in long-chain hydrocarbons to form simpler, shorter-chain hydrocarbons.

Hydrocarbon cracking is an important industrial process. Through this process long, bulky alkanes are broken up into smaller compounds. These compounds include shorter alkanes and alkenes. A few examples of cracking hydrocarbons are given in the figure below.

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The cracking of various hydrocarbons to produce alkanes and alkenes. 

There are two types of hydrocarbon cracking. Thermal cracking occurs under high pressures and temperatures without a catalyst, catalytic cracking occurs at lower pressures and temperatures in the presence of a catalyst. This process is a common source of shorter (more useful) alkanes as well as unsaturated alkenes. The alkanes are then used in combustion processes.

It is possible to separate the products of hydrocarbon cracking and obtain specific products from a crude oil mix through a process called fractional distillation. This is done using a fractionating column. Crude oil evaporates when heated to \(\text{700}\)\(\text{°C}\). The gas bubbles through a tray that is kept at a certain temperature. The alkanes and alkenes that condense at that temperature will then condense in the tray. For example if the tray is kept at \(\text{170}\)\(\text{°C}\) the product will be paraffin oil (see figure below).

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One layer of a fractionating column. Image by Duncan Watson.

A fractionating column has a series of these trays (see figure below), each at a constant temperature. This means that many compounds can be separated from the crude oil mix. The crude oil is heated to \(\text{700}\)\(\text{°C}\) and the gas of the crude oil is passed through the column. Bitumen for tar roads is collected at the bottom of the fractionating column. These are all compounds with more than \(\text{70}\) carbon atoms. The temperature decreases as you move up the column. As the gases rise, compounds with different length carbon chains condense until only the chains with \(\text{1}\)\(\text{4}\) carbon atoms are collected at the top of the column. These are used for liquid petroleum gas.

An example of a fractionating column.

Optional Video: Fractional Distillation

For more information on the process of fractional distillation have a look at this GCSE Science Revision – Fractional Distillation video:

Note that this fits with what we learned in the previous section. The more carbon atoms in the chain, the greater the intermolecular forces and therefore the higher the boiling point. That means that these molecules will condense at higher temperatures.

Combustion of alkanes

Alkanes are our most important fossil fuels. The combustion (burning) of alkanes (also known as oxidation) is highly exothermic.

Definition: Combustion

In a combustion reaction a substance reacts with an oxidising agent (e.g. oxygen), and heat and light are released.

In the complete combustion reaction of alkanes, carbon dioxide (\(\text{CO}_{2}\)) and water (\(\text{H}_{2}\text{O}\)) are released along with energy. Fossil fuels are burnt for the energy they release. The general reaction for the combustion of an alkane as a fossil fuel is given in the figure below.

Fact:

The complete combustion of alkanes produces only \(\text{CO}_{2}\) and \(\text{H}_{2}\text{O}\). Not all combustion process are complete though. An incomplete combustion will also produce carbon monoxide (\(\text{CO}\)).

Tip:

Remember that an exothermic reaction releases energy (\(\Delta \text{H} < \text{0}\)), while an endothermic reaction absorbs energy (\(\Delta \text{H} > \text{0}\)). The fact that energy is released in a combustion reaction implies that \(\Delta \text{H} < \text{0}\) and the reaction is exothermic.

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The complete combustion of an alkane.

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The complete combustion reactions of (a) methane, (b) propane, (c) hexane and (d) octane. 

Example: Balancing Equations

Question

Balance the following equation:

\(\text{C}_{4}\text{H}_{10}(\text{g}) + \text{O}_{2}(\text{g})\) \(\to\) \(\text{CO}_{2}(\text{g}) + \text{H}_{2}\text{O}(\text{g})\)

Balance the carbon atoms

There are 4 carbon atoms on the left. There is 1 carbon atom on the right. Add a 4 in front of the \(\text{CO}_{2}\) molecule on the right:

\(\text{C}_{4}\text{H}_{10}(\text{g}) + \text{O}_{2}(\text{g})\) \(\to\) \(4\text{CO}_{2}(\text{g}) + \text{H}_{2}\text{O}(\text{g})\)

Balance the hydrogen atoms

There are 10 hydrogen atoms on the left. There are 2 hydrogen atoms on the right. Add a 5 in front of the \(\text{H}_{2}\text{O}\) molecule on the right:

\(\text{C}_{4}\text{H}_{10}(\text{g}) + \text{O}_{2}(\text{g})\) \(\to\) \(4\text{CO}_{2}(\text{g}) + 5\text{H}_{2}\text{O}(\text{g})\)

Balance the oxygen atoms

There are 2 oxygen atoms on the left. There are 13 oxygen atoms on the right (\(\text{4}\) x \(\text{2}\) in the \(\text{CO}_{2}\) and 5 in the \(\text{H}_{2}\text{O}\)). Divide the number of \(\text{O}\) atoms on the right by \(\text{2}\) to get \(\frac{13}{2}\), this is the number of \(\text{O}_{2}\) molecules required on the left:

\(\text{C}_{4}\text{H}_{10}(\text{g}) +\)\(\frac{13}{2}\)\(\text{O}_{2}(\text{g})\) \(\to\) \(4\text{CO}_{2}(\text{g}) + 5\text{H}_{2}\text{O}(\text{g})\)

This is acceptable but it is better for all numbers to be whole numbers.

Make sure all numbers are whole numbers

There is \(\frac{13}{2}\) in front of the \(\text{O}_{2}\) while all other numbers are whole numbers. So multiply the entire equation by 2:

\(2\text{C}_{4}\text{H}_{10}(\text{g}) + 13\text{O}_{2}(\text{g})\) \(\to\) \(8\text{CO}_{2}(\text{g}) + 10\text{H}_{2}\text{O}(\text{g})\)

Example: Balancing Equations

Question

Balance the equation for the complete combustion of heptane

Write the unbalanced equation

The molecular formula for heptane is \(\text{C}_{7}\text{H}_{16}\). Combustion always involves oxygen (\(\text{O}_{2}\)). The complete combustion of an alkane always produces carbon dioxide (\(\text{CO}_{2}\)) and water (\(\text{H}_{2}\text{O}\)):

\(\text{C}_{7}\text{H}_{16}(\text{l}) + \text{O}_{2}(\text{g})\) \(\to\) \(\text{CO}_{2}(\text{g}) + \text{H}_{2}\text{O}(\text{g})\)

Balance the carbon atoms

There are 7 carbon atoms on the left. There is 1 carbon atom on the right. Add a 7 in front of the \(\text{CO}_{2}\) molecule on the right:

\(\text{C}_{7}\text{H}_{16}(\text{l}) + \text{O}_{2}(\text{g})\) \(\to\) \(7\text{CO}_{2}(\text{g}) + \text{H}_{2}\text{O}(\text{g})\)

Balance the hydrogen atoms

There are 16 hydrogen atoms on the left. There are 2 hydrogen atoms on the right. Add an 8 in front of the \(\text{H}_{2}\text{O}\) molecule on the right:

\(\text{C}_{7}\text{H}_{16}(\text{l}) + \text{O}_{2}(\text{g})\) \(\to\) \(7\text{CO}_{2}(\text{g}) + 8\text{H}_{2}\text{O}(\text{g})\)

Balance the oxygen atoms

There are 2 oxygen atoms on the left. There are 22 oxygen atoms on the right (\(\text{7}\) x \(\text{2}\) in the \(\text{CO}_{2}\) and 8 in the \(\text{H}_{2}\text{O}\)). Divide the number of \(\text{O}\) atoms on the right by \(\text{2}\) to get \(\text{11}\), this is the number of \(\text{O}_{2}\) molecules required on the left:

\(\text{C}_{7}\text{H}_{16}(\text{l}) + 11\text{O}_{2}(\text{g})\) \(\to\) \(7\text{CO}_{2}(\text{g}) + 8\text{H}_{2}\text{O}(\text{g})\)

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