12 - Alkanes

Question 1

What are alkanes and cycloalkanes?

Answer 1

• saturated hydrocarbons

Question 2

What are sigma bonds in alkanes?

Answer 2

• it is the head on overlapping of two orbitals (one from each bonding atom)
• found in between bonding atoms
• ** each C atom in alkanes have 4 sigma bonds **
• rotation can occur around the sigma bond as there are no double bonds

Question 3

What is the shape around the carbon atom?

Answer 3

• tetrahedral (4 bonded regions with no lone pairs)
• bond angle = 109.5 degrees

Question 4

How does boiling point vary in alkanes?

Answer 4

• the increase in the no. of electrons in the larger molecules increases the size of the London forces (induced dipole-dipole interactions)
  
  • this means that more energy is required to break the intermolecular forces
• the shape can also affect the boiling point
  
  • long, unbranched chains of alkanes have a larger SA of contact between molecules
  • so they have stronger induced dipole-dipole interactions and higher boiling points

Question 5

What explains the low reactivity of alkanes?

Answer 5

• C-C and C-H have strong sigma bonds (high bond enthalpies)
• C-C bonds are non-polar (do not have a +ve/-ve end
• C-H is also considered non-polar due to their similar electronegativities

Question 6

What are the products of complete/incomplete combustion?

Answer 6

• complete = CO2 + H2O
• incomplete = CO/C + H2O

Question 7

What makes CO harmful?

Answer 7

• build up in an enclosed space can cause death
• can form a strong bond with haemoglobin and prevents oxygen from attaching to it instead

Question 8

Why are alkanes used as fuels?

Answer 8

• they are readily available, easy to transport, and burn in oxygen without releasing toxic products

Question 9

What is the reaction of alkanes with halogens?

Answer 9

• substitution reaction (CH4 + Br2 –> CH3Br + HBr)
• UV light provides enough energy for this reaction to occur

Question 10

What are the limitations of radical substitution?

Answer 10

• further substitution could occur if another ___ radical collides with the product of propagation
• longer carbon chains will produce a mixture of monosubstituted isomers (substitution will occur at different positions)v