Organic chemistry Flashcards
What is crude oil?
Crude oil is:
* formed over millions of years from the fossilised remains of plankton
* found is porous rock in the Earth’s crust.
* a finite (non renewable) resource that is used to produce fuels and other chemicals
What are hydrocarbons?
- Most of the compounds in curde oil are hydrocarbons (molecules made of only hydrogen and carbon atoms).
- Hydrocarbon molecules vary in size, which affects their properties and how they can be used as fuels.
What is the trend in increasing hydrocarbon length?
The larger the hydrocarbon:
* the more viscous it is (i.e. the less easily it flows)
* the higher its boiling point
* the less volatile it is
* the less easily it ignites
How is crude oil separated?
- Crude oil can be separated into different fractions (parts) by fractional distillation.
- Each fraction contains hydrocarbon molecules with similar number of carbon atoms.
- Most of the hydrocarbons obtained are alkanes.
- First, the crude oil is heated until it evaporates.
- The vapour moves up the fractionating column.
- The top of the column is much colder than the bottom.
- Shorter hydrocarbon molecules can reach the top of the fractionating column befpre they condense and are collected.
- Longer hydrocarbon molecules condense at higher temperatures and are colllected lower down the column.
What are alkanes?
- Carbon atoms are linked to four other atoms by single bonds.
- Alkanes only contain single bonds and are described as saturated hydrocarbons (because they contain the maximum number of bonds possible).
- Alkanes are fairly unreactive, but they burn well.
- The general formula for alkanes is double the carbon atoms and add two to get the number of hydrogen atoms.
- The shorter chain alkanes release energy more quickly by burning, so there is greater demand for them as fuels.
What occurs during the combustion (burning) of hydrocarbon fuels?
During the combustion (burning) of hydrocarbon fuels:
* both carbon and hyrdogen are oxidised
* energy is released
* waste products are produced, which are released into the atmosphere.
What occurs during the incomplete combustion of hydrocarbon fuels?
- If combustion is not complete, then carbon monoxide, unburnt fuels and solids particles containg soot (carbon) may be released.
- Carbon monoxide is a colourless, odourless and toxic gas.
- Solid particles in the air, called particulates, can cause global dimming by reducing the amonut of sinlight reaching the Earth’s surface and cause damage to people’s lungs.
- Due to high temperatures reached when fueldd burn, nitrogen in the air can react with oxygen and form nitrogen oxides.
- These gases can cause respiratory problems in people and react with rain water (in the same way as sulfur dioxide) to form acid rain, which can damage plants and buildings.
- Sulfur can be removed from fuels before burning (in motor vehicles) and removed from the wastee gases after combustion (in power stations).
What is cracking?
- Longer chain hydrocarbons can be broken down into shorter, more useful hydrocarbons. This process is called cracking.
- Cracking is an inddustrial process.
- The two main methods of cracking are steam cracking and catalytic cracking.
- The products of cracking incluse alkanes and another type of hydrocarbon called alkenes.
During steam cracking:
* the hydrocarbons are mixed with steam
* they are heated to 850 degrees celsius
During catalytic cracking:
* the hydrocarbons are heated until they vapourise
* the vapour is passed over a hot aluminium oxide catalyst
* a thermal decomposition reaction then takes place
How are the products of cracking used?
- No atoms are made or destroyed during the cracking reaction, so the chemical equation must always balanced.
- The alkanes produced are valuable as fuels.
- There is a high demand for fuels with small chains of carbon atoms because they are easy to ignite and have low boiling points.
- The alkenes produced can be used to make a range of new compounds, including polymers and industrial alcohol.
How can bromine water be used to test for alkenes?
- Alkenes are more reactive than alkanes.
- They react when shaken with bromine water, turning it from orange to colourless.
- This can be used to differentiate between alkanes and alkenes.