Carbon Cycle & Fossils Fuels

The Carbon Cycle

The carbon cycle shows how carbon moves from the atmosphere, through animals and plants, then back into the atmosphere again.

All cells contain carbon compounds such as proteins, fats and carbohydrates. Carbon is passed from the atmosphere, as carbon dioxide, to living things. It is passed from one organism to the next in complex molecules, and returned to the atmosphere as carbon dioxide again.

The video below shows a brief description of the carbon cycle:-

In the atmosphere, carbon is attached to some oxygen in a gas called Carbon Dioxide.

Plants use Carbon Dioxide and the energy from sunlight to make their own food and grow. This is called Photosynthesis.

The Carbon becomes part of the plant. Plants that die and are buried may turn into fossil fuels made of carbon like Coal over millions of years.

When humans burn fossil fuels most of the carbon quickly enters the atmosphere as the gas Carbon Dioxide.

The carbon cycle shows where the carbon which you use comes from, and also where the carbon that you produce goes.

Fossil Fuels

Many of the fuels we use in everyday life are obtained from fuels called fossil fuels. These fuels are mostly hydrocarbons – compounds that contain the elements carbon and hydrogen only but contain some impurities which can lead to pollution when we burn them.

Fossil fuels like coal, crude oil and natural gas have been formed over millions of years from dead plant and animal remains which have been buried under many layers sediment.

The video below shows how the process of sedimentation can be seen in the lab:-

The video below shows how "coal" in the form of Carbon can be formed from "plant materials" in the form of sugar:-

Combustion

When a substance burns, it reacts with oxygen. This is known as combustion.

All combustion reactions are exothermic because they release energy, e.g. heat energy is given out when methane is burned in a Bunsen burner.

In order for combustion to happen, three things are needed:-

If any of the three are removed, the combustion will stop. This is how firefighters put out fires, by either:-

  1. Removing the heat - for example, using water hoses to cool burning timber.

  2. Removing the Oxygen - for example, using a Carbon Dioxide fire extinguisher on an electrical fire.

  3. Removing the fuel - for example, firefighters cutting a firebreak to stop a forest fire.


Fire Extinguishers

Fire extinguishers can be very useful in an emergency, but they must be used correctly or they can make the situation worse!

The safest option if you discover a fire is to leave the area and call 999 immediately.

Each fire extinguisher is designed to work with a specific type of fire:-

Combustion - Hydrocarbons

Crude oil is a mixture of hydrocarbons. Energy is released during burning/oxidation and respiration. The most common form of oxidation is the direct reaction of a fuel with oxygen through combustion. Combustion is the reaction of burning a fuel in oxygen. Burning natural gas means reacting with the Oxygen in the air. There will be products from the reaction, these products can be collected and identified.

Combustion can take two forms:

  1. Complete combustion - where the fuel was burned in a plentiful supply of oxygen to produce carbon dioxide and water.

  2. Incomplete combustion - where the hydrocarbons burn in a limited supply to produce carbon monoxide (a poisonous gas) and carbon.

Fossil Fuels

Fossil fuels release carbon dioxide gas when they burn which adds to the greenhouse effect and increases global warming. Of the three fossil fuels, for a given amount of energy released, coal produces the most carbon dioxide and natural gas produces the least.

Coal and oil contain sulphur impurities. When these fuels burn, the sulphur burns too which releases sulphur dioxide (SO2) gas. Sulphur dioxide causes breathing problems for living creatures and contributes to acid rain:-

Use of catalytic converters to reduce carbon monoxide emissions. In engines, catalytic converters can be used to minimise the output of carbon monoxide. Reducing carbon emissions. Ways to reduce carbon dioxide emissions are explored (including methods of carbon capture). Impact on the environment of burning fossil fuels including the effect on the carbon cycle. Combustion of fossil fuels impacts on the environment and contributes to the carbon cycle.

Conservation of mass during combustion

No atoms are created or destroyed in a chemical reaction. Instead, they just join together in a different way than they were before the reaction, and form products.

The total mass stays the same during a chemical reaction. This is the law of conservation of mass.


Example 1 -

Burning methane in oxygen :

Reactants Products

Methane + oxygen carbon dioxide + water

16g + 64g 44g + 36g

Total = 80g Total = 80g


The total mass of the reactants equal the sum of the mass of the products.

States of Matter

The kinetic particle theory explains the properties of the different states of matter. The particles in solids, liquids and gases have different amounts of energy. They are arranged differently and move in different ways:-

Solids

Liquids

Gases

Extracting materials from Crude Oil

Hydrocarbons are chemical compounds that contain the elements carbon and hydrogen only. They are compounds that are obtained from the fossil fuel crude oil by a process called fractional distillation. When oil is removed from the earth it is found as a mixture of different sized liquid molecules.

Fractional distillation separates a mixture into a number of different parts, called fractions. A tall fractionating column is fitted above the mixture, with several condensers coming off at different heights. The column is hot at the bottom and cool at the top. Substances with high boiling points condense at the bottom and substances with lower boiling points condense on the way to the top:-

The video below shows how different materials can be separated, including by fractional distillation:-

Alkanes, Alkenes and Cycloalkanes

Hydrocarbons are compounds made from hydrogen and carbon. They are found as a mixture in crude oil when it is extracted from the ground.

There are three types of hydrocarbon: Alkanes, alkenes and cycloalkanes.


A General Formula for Alkanes

The general formula is a short way of working out the formula for any hydrocarbon. Each family of hydrocarbons has its own general formula.


For alkanes it is;

CnH2n+2

(n is the number of carbon atoms)


For example : Heptane has 7 carbons so it has (2 x 7)+2 hydrogens i.e. C7H16


A General Formula for Alkenes

The alkenes are another homologous series. The alkenes have a double carbon to carbon bond.

For alkenes the general formula is

CnH2n

( n is the number of carbon atoms)


For example : Heptene has 7 carbons so it has (2 x 7) hydrogens i.e. C7H14


A General Formula for Cycloalkanes

The cycloalkanes are another homologous series and their carbon atoms join up in a ring. For cycloalkanes the general formula is

CnH2n

( n is the number of carbon atoms)


For example : Cyclo-Heptane has 7 carbons so it has (2 x 7) hydrogens i.e. C7H14


Drawing Hydrocarbons

The following diagrams show what the three hydrocarbon types look like in symbol form:-

Naming Hydrocarbons

Whatever the type of hydrocarbon, they all follow the same naming rules based on how many carbon atoms are in the molecule:-

Cracking Hydrocarbons

Crude oil often contains too many large hydrocarbon molecules and not enough small hydrocarbon molecules. There is more demand for shorter alkane molecules and alkenes than for many of the longer chains formed during fractional distillation. This is where cracking comes in. Cracking allows large hydrocarbon molecules to be broken down into smaller, more useful hydrocarbon molecules. Fractions containing large hydrocarbon molecules are heated to vaporise them. Cracking produces a mixture of smaller alkanes and alkenes. This helps to meet the demand for the more useful fractions and to increase profit:-

Alternatives to Fossil Fuels - Biofuels

Biofuels are a wide range of fuels which are in some way made from biological (living) materials. The term covers solid biomass, liquid fuels and various biogases. Biofuels are gaining increased public and scientific attention, driven by oil prices and the need for increased energy security.

Bioethanol is an alcohol made by fermenting the sugar components of plant materials and it is made mostly from sugar and starch crops. Ethanol can be used as a fuel for vehicles in its pure form, but it is usually used as a petrol additive. Bioethanol is widely used in the USA and in Brazil.

Biodiesel is made from vegetable oils, animal fats or recycled greases. The oils can be pure or heavily used so you can find raw material for making bio-diesel anywhere you have oil (or fat ) used in cooking. Biodiesel can be used as a fuel for vehicles in its pure form, but it is usually used as a diesel additive. Biodiesel is most common biofuel in Europe.


The video below shows an introduction to biofuels:-