Topic 7: Organic Chemistry Flashcards
What is a hydrocarbon?
Any compound that is formed from carbon and hydrogen atoms only
What is the general formula for alkanes?
CⁿH²ⁿ⁺²
Are alkanes saturated or unsaturated?
Saturated
Why are alkanes said to be saturated?
Each carbon atom forms 4 single covalent bonds
Do alkanes contain single or double bonds?
Single
What is a homologous series?
A group of organic compounds that react in a similar way
What are the first 4 alkanes?
- Methane
- Ethane
- Propane
- Butane
What is the trend as hydrocarbon chains get longer?
i) Viscosity
ii) Boiling point
iii) Flammability
i) Become more viscous (gloopy)
ii) Boiling point increases
iii) Become less flammable
What is the equation for the complete combustion of a hydrocarbon?
Hydrocarbon + oxygen —> carbon dioxide + water
During combustion, what happens to both carbon and hydrogen?
They are oxidised
Why are hydrocarbons used as fuels?
Due to the amount of energy they release when they combust completely
Describe how crude oil was formed [2]
- Formed from the remains of plants and animals, mainly plankton, that died millions of years ago and were buried in mud
- Over millions of years, with high temperatures and pressures, the remains turned to crude oil
Why is crude oil described as a non-renewable fuel?
It is being used up much faster than it is being formed
Why is crude oil described as a finite resource?
One day it will run out
What is crude oil made up of?
A mixture of lots of different hydrocarbons, most of which are alkanes
How are the different compounds in crude oil separated?
Fractional distillation
Describe how different compounds in crude oil are separated by fractional distillation [7]
- Oil is heated until it has turned into a gas
- The gas is fed into a fractionating column
- In the column there is a temperature gradient (it’s hot at the bottom and gets cooler as you go up)
- The longer hydrocarbons have high boiling points. They condense back into liquids and drain out of the column early on, when they’re near the bottom
- The shorter hydrocarbons have lower boiling points. They condense and drain out much later on, near the top of the column where it’s cooler
- You end up with a crude oil mixture separated out into different fractions
- Each fraction contains a mixture of hydrocarbons that all contain a similar number of carbon atoms, so have similar boiling points
What are the long hydrocarbons in fractional distillation used for?
Heavy fuel oil
What can heavy fuel oil be? [3]
- Heating oil
- Fuel oil
- Lubricating oil
What are the medium hydrocarbons in fractional distillation used for? [3]
- Diesel
- Kerosene
- Petrol
What are the short hydrocarbons in fractional distillation used for?
LPG (liquefied petroleum gas)
What does the petrochemical industry use hydrocarbons for?
Feedstock
Why are long-chain hydrocarbons not very useful?
They form thick, gloopy liquids which cannot be used as fuels
In what process are long-chain hydrocarbons made into short-chain hydrocarbons?
Cracking
In cracking, what other hydrocarbon is produced?
Alkenes
What kind of reaction is cracking?
A thermal decomposition reaction
Define ‘thermal decomposition’
Breaking molecules down using heat
What are the 2 types of cracking?
- Catalytic cracking
- Steam cracking
Describe the process of catalytic cracking [3]
- Heat the long-chain hydrocarbons to vaporise them
- Pass the vapour over hot powdered aluminium oxide catalyst
- The long-chain molecules split apart on the surface of the specks of catalyst
Describe the process of steam cracking [3]
- Heat the long-chain hydrocarbons to vaporise them
- Mix them with steam
- Heat them at very high temperatures
When writing an equation for cracking, what is it important to do?
Balance the number of carbon and hydrogen atoms on both sides
What kind of bond does an alkene have?
C=C double bond