Chapter 12 - Alkanes Flashcards
Module 4
What are alkanes composed of?
Alkanes are hydrocarbons meaning that they contain carbon and hydrogen only.
Are alkanes saturated or unsaturated?
They are saturated. Molecules classified as alkanes only contain sigma (σ) bonds between C-C and C-H.
What is the general formula for alkanes?
CnH2n+2.
Describe the structure and bonding present in carbon atoms.
They can form a maximum of four bonding pairs with other atoms, so they can form 4 covalent/sigma bonds.
They form a tetrahedral structure when bonded to four other molecules and have a bond angle of 109.5°.
What is the effect of chain length?
As the chain length increases/longer chain alkanes have a greater boiling point. This is because:
⦿ longer chains means that the molecules have a larger surface area.
⦿ this results in greater surface points of contact.
⦿ so there are stronger London forces acting between the molecules.
⦿ hence, more energy is required to overcome and break the intermolecular/London forces, resulting in a higher boiling point.
Explain the effect of branching.
Branched chain alkanes always have a lower boiling point. This is because:
⦿ they contain fewer surface points of contact between the molecules.
⦿ there are fewer London forces acting between the molecules.
⦿ so, less energy is required to overcome and break the intermolecular/London forces, resulting in a lower boiling point.
Give two reasons that explain why alkanes are generally unreactive.
⦿ They contain strong sigma bonds between the C-C and C-H.
⦿ The C-C bonds are non-polar.
What are the two types of combustion?
Complete and incomplete combustion.
State the generic formula for the complete combustion of alkanes.
Alkane + oxygen -> carbon dioxide + water.
State the generic formula for the incomplete combustion of alkanes.
Alkane + oxygen -> carbon monoxide + carbon (soot) + water.
List the risks of carbon monoxide being formed during incomplete combustion.
Carbon monoxide (CO) is a colourless, odourless, and highly toxic gas. It binds permanently/irreversibly with haemoglobin present in red blood cells to form carboxyhaemoglobin. This means that there are less red blood cells to transport oxygen around the body - i.e. to respiring cells and tissues.
State the generic formula for cracking.
Long chain alkane -> shorter chain alkane + alkenes.
What are short chain alkanes from cracking used for?
They can be used as fuel - i.e. in the petrochemical industry.
What can alkenes from cracking be used for?
They are feedstock for polymer products/production and used to make paints/dyes.
What is the main purpose/reason for cracking?
Short chain alkanes are in a higher demand.
How can branched chain alkanes be produced?
Straight chain alkanes are converted into branched alkanes by isomerisation.
E.g. Pentane -> 2,2-dimethylpropane.
How can cyclic hydrocarbons be produced?
Aliphatic hydrocarbons can be converted in (ali)cyclic hydrocarbons by the process reforming.
E.g. Pentane -> Cyclopentane + H2.
What is a mechanism for the substitution of alkanes?
The mechanism is free radical substitution.
What are the stages present in free radical substituition?
Initiation, propagation, termination.
What is the limitation of free radical substituition?
The process can contain further substitution which produces other by-products in a mixture which could be difficult to separate.
What are the two types of cracking?
Thermal and catalytic cracking.
What is thermal cracking?
A reaction that involves heating alkanes to a high temperature (700 to 1200K) and high pressure (up to 700kPa).
What is catalytic cracking?
A reaction that takes place at a lower temperature and pressure involving a zeolite catalyst.
What is a zeolite catalyst composed of?
SiO2 and Al2O3. Also, it has an enormous surface area and is acidic.
