Chapter 13- Alkanes

Question 1

What is an alkane? General formula?

Answer 1

Saturated hydrocarbons with single carbon-carbon bonds.

Molecular formula CnH2n+2

Question 2

Molecular formula for cycloalkanes?

Answer 2

CnH2n

Question 3

What is a σ (sigma) bond?

Answer 3

The result of the head on head overlap of two orbitals, one from each atom (sp3)

♾ (overlap) ♾ kinda looks like that

Question 5

Describe and explain the shape of the molecules an alkane.

Answer 5

• Each carbon is bonded to 4 hydrogens
• the 4 bonding regions repel each other equally to be as far apart as possible
• this results in a tetrahedral shape around the carbon atom
• bond angle 109.5°

Question 6

Sigma bonds are the ______ type of covalent bond and because of them the atoms in the molecule can ______ about their ____.

Answer 6

Strongest
Rotate
Axis

Question 7

Describe the trend in boiling points as the alkanes

- get longer

Answer 7

As chain gets longer, RMM increase, more points of contact, more London forces, more energy to overcome
BOILING POINT INCREASES

Question 8

Why can fractional distillation be used to separate alkanes?

Answer 8

They have various boiling points

Question 9

Describe the trend in boiling points as the alkanes

- get more branched

Answer 9

As branching increases, the molecules pack less tightly, and there is less surface contact, weaker London forces, less energy required to overcome
BOILING POINT DECREASES

Question 10

Explain the reactivity of alkanes?

Answer 10

Alkanes are very unreactive due to strong C-H bonds and C-C bonds which are relatively unpolar

Question 11

What are alkanes used as fuels?

Answer 11

Readily available, easy to transport, burn in plentiful oxygen without waste products and release large amounts of energy.

Question 12

What does homolytic fission give rise to?

Answer 12

Free radical substitution

Question 13

What is free radical substitution?

Answer 13

A mechanism which involves a chain reaction where one atom in a molecule is substituted by another in another molecule.

Question 14

steps in free radical substitution?

Answer 14

INITIATION:
halogen –> halogen radical x2
PROPAGATION:
molecule + halogen radical –> molecule-H radical + halegonealkane
molecule-H radical + halogen –> molecule-H halogen + halogen radical
TERMINATION:
all possible radical + radical –> non radical

Question 15

how to calculate the overall equation?

Answer 15

combine propagation equations, add all molecules in propagation together, cancel those on both sides
done (like ionic equation)

Question 16

what is a radical?

Answer 16

a molecule/ion/species with an unpaired electron

Question 17

describe how a fraction is obtained from crude oil

Answer 17

-

Question 18

why does petrol ignite quicker/more easily than tar, even though both contain alkanes?

Answer 18

reactions occur faster at gas state

- petrol shorter chain, small molecule, weaker IMF e.g. London forces, easier to make gas, easier to ignite

Question 19

why do we break down the larger hydrocarbon fractions after fractional distillation?

Answer 19

• There isn’t enough economic use for the larger hydrocarbons.
• Cracking produces molecules like ethene and propene which are very useful in the organic chemical
  industry (smaller hydrocarbons)
• Cracking produces the smaller hydrocarbons which make up petrol (gasoline): useful fuel