4.2 - Alkanes

Question 1

Type of bond alkanes have

Answer 1

single covalent σ bonds which can rotate freely

Question 2

Define sigma bond

Answer 2

overlap of 2 orbitals, 1 from each bonding atom, positioned on line directly between bonding atoms

Question 3

Bond angle around each carbon in alkanes and shape

Answer 3

109.5°
tetrahedral

Question 4

Why would an alkane have a higher BP?

Answer 4

• increased chain length
• more points of surface contact
• stronger London forces
• more energy needed to overcome LFs

Question 5

Why would an alkane have a lower BP?

Answer 5

• more branching
• less points of surface contact
• weaker London forces
• less energy needed to overcome LFs

Question 6

Chemical properties of alkanes

Answer 6

• not very reactive due to strong and non-polar C-C/C-H bonds
• used as fuels because readily available, easy to transport, burn to release no toxic products

Question 7

Name and describe with equations the mechanism for ethane reacting with chlorine

Answer 7

• radical substitution
  initiation: Cl2 -> 2Cl* (requires UV)
  propagation: C2H6 + Cl* -> C2H5* + HCl
  C2H5* + Cl2 -> C2H5Cl + Cl*
  termination: 2Cl* -> Cl2
  2C2H5* -> C4H10
  C2H5 + Cl* -> C2H5Cl

Question 8

What are the limitations of radical substitution?

Answer 8

• further substitutions can occur
• substitution at different positions in C chain
  (forming a mixture of organic products)

Question 9

Why are alkanes not very reactive?

Answer 9

high bond enthalpy and very low polarity of σ-bonds

Question 10

What are the potential dangers of CO?

Answer 10

toxic

Question 11

State the general equation for complete combustion of an alkane

Answer 11

alkane + O2 -> CO2 + H2O

Question 12

State the general equation for incomplete combustion of an alkane

Answer 12

alkane + O2 -> CO + C + H2O