Alkanes Flashcards
What is the general formula for alkanes?
CnH2n+2
What is a hydrocarbon?
A compound made up of carbon and hydrogen only.
Why are alkanes described as being saturated?
The carbon atoms in alkanes are joined together by single covalent bonds.
Describe and explain the arrangement of bonds around each carbon atom in an alkane.
Each carbon atom in an alkane forms four single bonds, either with hydrogen atoms (C-H) or other carbon atoms (C-C). These bonds are arranged in a tetrahedral geometry around the central carbon atom, with bond angles of ≈109.5°.
The tetrahedral arrangement of the bonds is due to the equal repulsion between the bonding pairs of electrons.
State and explain the effect of increasing chain length on the boiling point of alkanes.
As the chain length of alkanes increases, boiling point increases.
This is because the surface contact between molecules increases which leads to stronger induced dipole-dipole forces between molecules which require greater energy to overcome.
State and explain the effect of branching on the boiling point of alkanes.
Branched alkanes generally have lower boiling points compared to straight-chain isomers.
This is because branched alkanes cannot pack as closely together which means there are fewer surface points of contact. This leads to weaker induced dipole-dipole forces which require less energy to overcome.
What is the general formula of cycloalkanes?
CnH2n
What is crude oil?
Crude oil is a mixture of hydrocarbons, primarily alkanes.
What is the name of the process we use to separate the components of crude oil?
Fractional distillation
What physical property of hydrocarbons allows them to be separated by fractional distillation?
The different boiling points of hydrocarbons.
Outline the process of fractional distillation.
- Crude oil is heated to vaporise it into a gas and then passed into the fractionating column.
- The fractionating column is cooler at the top compared to the bottom. As the gas rises up the column, it gradually cools down.
- When the individual hydrocarbon components in the gas mixture reach a height where the temperature is lower than their respective boiling points, they condense into liquids.
- These liquids are collected in trays placed at specific intervals along the column and are drawn off as separate fractions.
What are the names and uses of crude oil fractions?
- Petrol is used as a fuel for vehicles.
- Naptha is used as a petrochemical feedstock.
- Kerosene is used as jet fuel.
- Diesel is used as a fuel for vehicles.
- Mineral oil is used as a lubricant.
- Bitumen is used in road surfacing.
What is meant by the term fraction, in the context of fractional distillation?
A fraction is a mixture of hydrocarbons with similar boiling points
How do the boiling points of hydrocarbons change as you move down the fractionating tower?
The boiling points of hydrocarbons increase as you move down the fractionating tower.
What happens to the temperature as you move higher up the fractionating tower?
The temperature decreases as you move higher up the fractionating column.
Give 3 physical properties of short-chain hydrocarbons.
- Low boiling points (they are typically gases or volatile liquids).
- High flammability.
- Low viscosity.
Give 3 physical properties of long-chain hydrocarbons.
- High boiling points.
- Low flammability.
- High viscosity (they are often in the form of thick residues like tar or bitumen).
Are hydrocarbons with long or short chains collected at the top of the fractionating column?
shorter chains
Do hydrocarbons with long or short chains have a lower boiling point?
shorter chains have lower boiling points.
Why do short-chain hydrocarbons make good fuels?
They are very flammable and volatile, so are easy to ignite and combust for energy.
Why are long-chain hydrocarbons bad fuels?
They are not very flammable or volatile. This means they are difficult to ignite and combust for energy.
What is cracking?
Cracking is the process by which longer chain hydrocarbons are split into shorter, more useful hydrocarbons.
Why are the products of cracking useful?
- They can be used as chemical feedstocks for the petrochemical industry.
- They make more efficient fuels (than longer-chain hydrocarbons).
What type of reaction is cracking an example of?
Thermal decomposition