[12.1] properties of alkanes

Question 1

what are alkanes?

Answer 1

• a homologous series of saturated hydrocarbons
• its general formula is CₙH₂ₙ₊₂

Question 2

what are common uses of the first twenty members of the alkane homologous series?

Answer 2

• gas, used in domestic fuel (C1,3,4)
• petrol, used in cars (C5-C9)
• kerosene, used in aircraft (C10-C16)
• diesel, used in cars and lorries (C12-C20)

Question 3

why are alkanes used as fuels?

Answer 3

• readily available
• easy to transport
• burn in a plentiful supply of oxygen without releasing toxic products

Question 4

what is a sigma bond? (σ-bond)

Answer 4

• it is the result of a direct overlap of two orbitals, one from each bonding atom
• each overlapping orbital contains one electron, so the σ-bond has two electrons that are shared between the bonding atoms
• (eg.) methane has four sigma bonds

Question 5

what is the shape of alkanes?

Answer 5

• each carbon atom is surrounded by four electron pairs in four σ-bonds
• there is a tetrahedral arrangement and each bond angle is 109.5°
• the σ-bonds act as axes around which the atoms can rotate freely, so these shapes are not rigid

Question 6

what is the process of fractional distillation?

Answer 6

• oil refineries separate crude oil into fractions (hydrocarbons with similar boiling points) by fractional distillation in a distillation tower

1. crude oil is vaporised at high temperatures, and passed into a tower, which is hot at the bottom and cold at the top
2. as it rises up the tower it condenses at different levels

Question 7

how is fractional distillation possible?

Answer 7

because the boiling points of the alkanes are different, increasing as their chain length increases

Question 8

what happens to hydrocarbons as the C chain gets longer?

Answer 8

• more viscous
• harder to ignite
• less volatile
• have higher boiling points

Question 9

what is the purpose of cracking?

Answer 9

• petroleum fractions with shorter C chains (eg. petrol) are in more demand than larger fractions
• longer hydrocarbons are therefore cracked by breaking the C-C bonds
• the products of cracking are more valuable than the starting materials

Question 10

what are the conditions and products of thermal cracking?

Answer 10

• temperature: 900°c
• pressure: 70 atm
• catalyst: none
• products: alkenes

Question 11

what are the conditions and products of catalytic cracking?

Answer 11

• temperature: 450°c
• pressure: 1-2 atm
• catalyst: zeolites
• products: motor fuels, aromatics, cycloalkanes, branched alkanes

Question 12

what is the effect of chain length on the boiling point of alkanes?

Answer 12

• london forces act between molecules that are in close surface contact
• as chain length increases, the molecules have a larger surface area, so more surface contact is possible between molecules
• london forces between the molecules will be greater and so more energy is required to overcome the forces, resulting in a higher boiling point

Question 13

what is the effect of branching on the boiling point of alkanes?

Answer 13

• branched alkanes have fewer surface points of contact between molecules so there are fewer london forces, so have a lower boiling point than a straight chain alkane
• branches of branched molecules also get in the way and prevent branched molecules getting as close together as straight chain molecules, decreasing the intermolecular forces further
• less branching allows closer contact between molecules and so stronger london forces

> more branching = less surface area of contact = weaker london forces