IB Chemistry Sub-topic R1.3 Notes

Energy from fuels

We use fuels everywhere such as to power our cars and our homes. Fuels, which are typically hydrocarbons, such as coal, oil or gas release energy to provide us with the necessary heat and power via complete combustion. This is a  highly exothermic reaction that produces heat and requires oxygen. Complete combustion of fuels occurs with excess oxygen and produces carbon dioxide and water. For example, pentane (C₅H₁₂) combusts as follows:

C₅H₁₂ + 8O₂ → 5CO₂ + 6H₂O

Alcohols can also undergo complete combustion under the same conditions to form Carbon Dioxide and water. For example, Ethanol (C₂H₅OH) combusts as follows:

C­₂H₅OH + 3O₂ → 2CO₂ + 3H₂O

When oxygen is limited, combustion is called incomplete. The products of incomplete combustion are Carbon (soot), carbon monoxide and water. The IB wants you to determine equations for the incomplete combustion of alcohols and hydrocarbons. The products of the reaction are identical, only the balancing has to be practiced.

Methane:

3CH₄ + 4O₂ → C + 2CO + 6H₂O

Ethanol:

C_2​H₅​OH + 2O₂​ → 2CO + 3C (soot) + 3H₂​O

Carbon monoxide is an incredibly poisonous gas and incomplete combustion can therefore be detrimental to health. Carbon monoxide can bind to haemoglobin and limit oxygen supply to organs.

Fossil fuels

Most fuels that we burn to create energy are fossil fuels. These include gas, oil and coal. These fuels have various environmental concerns including the production of CO₂ through combustion, which is a greenhouse gas and contributes to global warming.

To determine the amount of carbon dioxide added to the atmosphere when different fuels burn we can use stoichiometric values of the combustion reactions. The fossil fuels the IB requires you to know are coal, crude oil and natural gas.

1. Coal - this is primarily made of carbon with variable levels of sulphur, oxygen or other impurities. Its high carbon content makes it very prone to incomplete combustion if oxygen levels are not high enough. Complete combustion looks as follows:

   C_(s) + O_{2 (g)} → CO_{2 (g)}

2. Crude oil - this is composed of a variety of hydrocarbons such as alkanes and alkenes with other elements such as sulphur and nitrogen. The presence of impurities can contribute to its likelihood to undergo incomplete combustion.
3. Natural gas - this is primarily composed of methane with varying levels of propane and ethane. Combustion of natural gas is generally efficient but incomplete combustion can still produce carbon monoxide and other harmful substances. Natural gas has the highest energy density.

Biofuels

Biofuels are plant-based fuel sources. These are being harnessed as an alternative fuel source than fossil fuel sources. Biofuels include ethanol and biodiesel.

They are produced by the biological fixation of carbon in a short time frame through photosynthesis.

The equation for photosynthesis is:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Ethanol is produced from glucose via fermentation:

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

Ethanol as a biofuel is carbon neutral as the CO₂ produced during fermentation is also used up in the photosynthesis process.

The advantages of ethanol are:

1. Sustainable source of energy through consistent use of carbon dioxide emitted
2. Widely distributed all over the world

Disadvantages of ethanol are:

1. Ethanol fuel itself is toxic which requires care in use
2. The fuel efficiency in high ethanol percentage blends is lower than regular fuels
3. Biofuels also require more agricultural development, which could strain land and water resources.