List 3 examples of fossil fuels.
- Coal
- Crude oil
- Natural gas
How are fossil fuels formed?
Remains of dead organisms are trapped in sediments millions of years ago under conditions of heat, pressure and absence of oxygen.
What is crude oil?
Crude oil is a thick black liquid trapped in sedimentary rocks and a non-renewable energy resource. It made up of a mixture of hydrocarbons that may also contain impurities like sulfur.
What is fractional distillation?
A process used to separate a mixture of liquids that have different boiling points.
How is crude oil separated into fractions?
- Oil is heated to about 450°C and pumped into the bottom of a fractionating column/tower, where it vaporizes.
- The column is very hot at the bottom but much cooler at the top. As the vapours rise, they cool and condense at different levels, according to their boiling temperature.
- Heavy fractions (containing large carbon chain molecules) have a high boiling point and condense near the bottom of the column.
- Lighter fractions (containing small carbon chain molecules) have a lower boiling point and condense further up the column.
- The vapours with the lowest boiling points do not condense at all and leave at the top as a mixture of gases.
What are the different characteristics between fractions containing large and small carbon chain molecules?
Small carbon chain molecules:
1. Low boiling point
2. Very volatile
3. Flows easily
4. Ignites easily
Large carbon chain molecules:
1. High boiling point
2. Not very volatile
3. Does not flow easily
4. Does not ignite easily
What are the different types of fractions in a fractionating column of crude oil and what are they used for (lowest to highest boiling points)?
Check teacher notes diagram SC20b slides 9, 10 and 12.
What features do the members of the alkane homologous series have in common?
- Molecular formulae of neighbouring compounds differ by CH₂
- Have the same general formula
- Show a gradual variation in physical properties (e.g. boiling points)
- Have similar chemical properties
What is the general formula of alkanes?
CₙH₂ₙ₊₂
What are the different types of formulae?
Check teacher notes diagram SC20c slide 20.
What are fuels?
Substances that release heat energy when burned.
What is combustion?
An oxidation reaction where a substance burns and reacts with oxygen in the air, releasing heat and light energy.
What is the test to see whether incomplete or complete combustion occurred on a fuel?
A yellow and/or sooty flame indicates incomplete combustion.
A clean, blue flame indicates complete combustion.
What are some unwanted byproducts of incomplete combustion of a fuel?
- Carbon monoxide
- Carbon
- Soot
- Smoke
What problems does soot cause?
- Blocks pipes carrying away waste gases from an appliance
- Blackens buildings
- Breathing problems if it collects in the lungs
What does carbon monoxide do and how is it harmful/what problems does it cause?
Carbon monoxide is a colourless, odourless, toxic gas that binds irreversibly with haemoglobin. This prevents oxygen combining meaning that less oxygen is carried around the body and leading to sleepiness, unconsciousness or even death.
What causes acid rain?
Oxides of nitrogen (reaction due to high temperatures in car engines) and sulfur (petrol impurity) dissolving in rainwater.
What reactions occur in acid rain?
Sulfur dioxide dissolves in the water inside clouds to form a mixture of acids, including sulfurous acid. Sulfurous acid is then oxidised in the air to form sulfuric acid.
What are the effects caused by acid rain?
- Lakes and soil become more acidic
- Fish die
- Crops cannot grow
- Buildings and statues made of marble (carbonates) dissolve
What are the solutions to acid rain?
- Use of catalytic converters
- Filters to capture acidic gases coming out of factories
What is cracking?
The breaking of large alkane molecules (obtained from crude oil) into smaller alkanes and alkenes.
What is thermal cracking?
An example of a thermal decomposition reaction where large hydrocarbon molecules are broken down into smaller molecules using heat or a catalyst (aluminium oxide).
How do we perform the cracking experiment?
- Paraffin oil is the hydrocarbon to be cracked.
- It is absorbed by the mineral wool to stop it flowing along the boiling tube.
- The broken pot is heated strongly and the paraffin oil is heated.
- Bubbles of gas start to come out of the delivery tube.
- The first few bubbles are allowed to escape then a test tube filled with water is placed over the end of the delivery tube to collect the gas.
Check teacher notes diagram SC20f slide 45.
How does the cracking experiment work?
When the paraffin oil is heated, it evaporates to form a gas and the paraffin gas passes over the hot pieces of broken pot. The heat causes some of the covalent bonds in the paraffin to break so the paraffin is cracked. An alkene with a shorter carbon chain and ethene is formed. It is a gas so it is collected in the test tube.
