C.9 Flashcards
What is a hydrocarbon?
A compound made up of hydrogen and carbon atoms ONLY.
What is the general formula for an alkane?
CnH2n+2
What are the important features of alkanes?
- Carbon atoms are connected by single covalent bonds
- They are saturated molecules as they are fully bonded to the hydrogen atoms
What is viscosity and why is it relevant?
Viscosity is the thickness of a substance and it increases as the size of the hydrocarbon chain increases.
What is flammability, and again, why is it relevant?
Flammability is how easily a hydrocarbon combusts. Short chain hydrocarbons are extremely flammable and long chain hydrocarbons aren’t very flammable. (Alcohol is a short chain hydrocarbon becuase it is flammable).
Why is the boiling point relevant?
Hydrocarbon boiling point increases with chain length - links to fractional distillation. Methane, ethane, propane and butane are all gases at room temperature.
How does fractional distillation of crude oils work?
The crude oil enters the fractionating column at the BOTTOM where it is hottest. The longer hydrocarbon chains with the highest boiling points cool and condense first and leave the column at the bottom. The shortest hydrocarbon chains evaporate and then condense at the top of the column because it is coolest and they have the lowest boiling points.
The change in temperature is known as a temperature gradient.
Why are hydrocarbons combusted?
Combustion of hydrocarbon fuels is needed as it releases energy. During complete combustion, the hydrogen and carbon reacts with the oxygen to become oxidised, and it forms products of CO2 and water.
What are fractions?
Fractions contain hydrocarbons with a similar number of carbon atoms and are used for fuels such as petrol, diesel, kerosene and heavy fuel oils.
Other fractions are used as FEEDSTOCK (chemicals used to make other chemicals) for the petrochemical industry.
What is feedstock?!
Feedstock is chemicals used to make other chemicals such as solvents, lubricants, detergents and polymers.
Why are long chain hydrocarbons unuseful?
Long chain hydrocarbons are poor fuels due to their low flammability and high boiling points. This increases the demand for short chain hydrocarbons which are a finite resource.
How is demand for short chain hydrocarbons solved?
Cracking. Cracking is where long chain hydrocarbons are broken down into smaller, more useful chains.
What are the products of cracking?
A short chain alkane and an alkene
What are the two types of cracking?
- Catalytic cracking - using a high temperature and a catalyst
- Steam cracking - using a high temperature and steam
How are alkanes different to alkenes?
Alkenes have a double covalent bond between the carbon atoms but alkanes only have a single bond.
