Alkanes

Question 1

what are alkanes

Answer 1

• saturated hydrocarbons (each carbon is bonded 4x with maximum no. of H)
• CnH2n+2

Question 2

what is the shape of an alkane

Answer 2

• tetrahedral around the carbon atom
• bond angle of 109.5
• the bonds repel each other equally
• electrons AFAP

Question 3

size of molecule related to forces

Answer 3

• the bigger the molecule the more induced dipole-dipole forces as you have larger electron clouds

Question 4

when boiling, what is being broken

Answer 4

• when you boil a liquid the weak induced dipole dipole forces are broken
• NOT the covalent bonds

Question 5

effect of chain length on BP

Answer 5

• chain length increases
• BP increases
• molecules have larger SA
• more surface contact
• greater London forces
• more energy needed to overcome forces

Question 6

effect of branching on BP

Answer 6

• lower BP
• fewer surface points of contact
• fewer London forces
• molecules cant get as close because branches are in the way

Question 7

Describe the complete combustion of alkanes

Answer 7

• alkanes burn in oxygen completely to form co2 and water

Question 8

why are alkanes good fuels?

Answer 8

• burn readily to produce large amounts of energy
• readily available
• easy to transport
• burn in plentiful o2 without producing any toxic products.

Question 9

describe the incomplete combustion of alkanes

Answer 9

• limited oxygen = H-atoms in alkane are oxidised to water

- CO or C(soot) formed due to incomplete combustion of C

Question 10

describe CO and soot

Answer 10

• poisonous, colourless and odourless gas
• binds to haemoglobin and prevents oxygen bonding
  CO can be removed using a catalytic converter
• soot can cause breathing problems
• dirty building
• clog up engines

Question 11

Describe the reaction of alkanes with halogens

Answer 11

initiation:
- the covalent bond in the Halogen(Br2) is broken by homolytic fission
- energy for this bond fission is provided by UV
- each bromine takes an electrons from the shared pair forming 2 Br radicals:
Br-Br = Br• + Br•

propagation:
- the reaction propagates through 2 propagation steps - a chain reaction :
CH ₄ + Br• = •CH₃ + HBr
•CH₃ + Br₂ = CH₃Br + Br•

Termination:
- when 2 radicals collide, forming a molecule with all electrons paired
Br• + Br• = Br₂
•CH₃ + •Br = CH₃Br

Question 12

why do alkanes lack reactivity

Answer 12

• C-C and C-H bonds are strong
• C-C bonds are non-polar
• EN of C+H is so similar that the C-H bond can be considered non-polar.

Question 13

limitations of radical substitution

Answer 13

• further substitution occurs producing a variety of products (which have to be separated due to being impurities)
• substitution can occur at different points on the carbon chain (isomers)

Question 14

how to reduce the further substitution in radical

Answer 14

• adding excess organic compound (methane)

- there is a greater chance of the halogen (chlorine) reacting with the methane.