16:Alkanes

Question 1

Definition of hydrocarbon

Answer 1

compounds that only contain carbon and hydrogen

Question 2

What is a saturated organic compound

Answer 2

Contains only single bonds between carbon atoms

Question 3

What is the prefix of a hydrocarbon with one carbon in the chain

Answer 3

meth-

Question 4

What is the prefix of a hydrocarbon with two carbons in the chain

Answer 4

eth-

Question 5

What is the prefix of a hydrocarbon with three carbons in the chain

Answer 5

prop-

Question 6

What is the prefix of a hydrocarbon with four carbons in the chain

Answer 6

but-

Question 7

What is the general formula for alkanes

Answer 7

Cn H₂n+₂

Question 8

What is a functional group

Answer 8

An atom or group of atoms that gives a compound its characteristic chemical reactivity

Question 9

What is a homologous series

Answer 9

A series of compounds that have the same functional group and the same general formula, and which successive members of the series differ by an extra CH₂ unit
eg. alkanes form a homologous series

Question 10

What is a structural isomer

Answer 10

Compounds that have the same molecular formula but different structural formula

Question 11

What is the general formula for cycloalkanes

Answer 11

Cn H₂n

Question 12

What bonds are broken to melt or boil alkanes

Answer 12

Induced dipole-dipole bonds

Question 13

Why do more carbons in an alkane increase the boiling point

Answer 13

The surface area of contact increases so there are more induced dipole-dipole formed

Question 14

Why do straight chain alkanes have higher boiling points than isomers with more branches

Answer 14

The surface area of contact is higher so there are more induced dipole-dipole formed

Question 15

Are alkanes more or less dense than water

Answer 15

Less

they will float on water

Question 16

Why are alkanes virtually insoluble in water

Answer 16

They are non-polar so cannot form hydrogen bonds

Question 17

What is the equation for complete combustion of alkanes

Answer 17

Complete combustion of propene

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

Question 18

What is the equation for incomplete combustion of alkanes

Answer 18

Incomplete combustion of propene
C₃H₈ + 3 1/2O₂ → 3CO + 4H₂O
or
C₃H₈ + 2O₂ → 3C + 4H₂O

Question 19

What are the products of the complete combustion of alkanes

Answer 19

CO₂ + H₂O

Question 20

What are the products of the incomplete complete combustion of alkanes

Answer 20

CO + H₂O
or
C + H₂O

Question 21

Why is carbon monoxide toxic

Answer 21

It binds to haemoglobin in blood thus limiting the supply of oxygen

Question 22

What are the key condition for substitution reactions

Answer 22

They need exposure to UV light

Question 23

What is a radical

Answer 23

An atom or group of atoms which has an unpaired electron

Question 24

What are the 3 stages of the mechanism of radical substitution

Answer 24

Initiation
Propagation
Termination

Question 25

What is homolytic fission

Answer 25

A process in which a covalent bond breaks so that one of the electrons that made up the bond stays with one of the atoms and the other electron stays with the other atom

Question 26

What happens in the initiation stage of radical substitution

Answer 26

The fission of a halogen molecule

eg. Cl₂(g) → Cl•(g) + Cl•(g)

Question 27

What happens in the propagation stage of radical substitution

Answer 27

1) The halogen radical reacts with the alkane
eg. Cl•(g) + CH₄(g) → HCl(g) + •CH₃(g)

2) The alkane radical reacts with the halogen molecule
eg. •CH₃(g) + Cl₂(g) → CH₃Cl(g) + Cl•(g)

Question 28

What happens in the termination stage of radical substitution

Answer 28

Two radicals collide and combine to form a molecule

eg. 1) Cl• + Cl• → Cl₂
2) •CH₃ + •CH₃ → C₂H₆
3) Cl• + •CH₃ → CH₃Cl