Alkanes - Organic Chem I

Question 1

Hydrocarbon define:

Answer 1

Compounds made up of only hydrogen and carbon

Question 2

What is the general formula for alkanes?

Answer 2

CnH2n+2

Question 3

Define isomer:

Answer 3

Same molecular formula, different structural formula

Question 4

What is each member of the homologous series represented by?

Answer 4

• General formula
• Adjacent members differ from each other by a CH2 group and they share the same functional group
  Therefore have similar chemical properties
• Show gradual trend in physical properties

Question 5

What happens as the length of alkane chain increase?

Answer 5

Boiling point and melting point increases

Question 6

What forces hold alkane molecules together?

Answer 6

Weak intermolecular forces as they are simple covalent molecules.

Question 7

What happens to the IMF when the molecules increase in length?

Answer 7

IMF gets stronger therefore more energy is needed to overcome the IMF between the longer chains

Question 8

How can London forces get stronger?

Answer 8

The greater the number of electrons per molecules, the stronger the London forces

Question 8

How can London forces get stronger?

Answer 8

The greater the number of electrons per molecules, the stronger the London forces

Question 9

What helps to reduce the bpt/mpt and how?

Answer 9

Branched alkanes because fewer IMF can develop between the branched chain molecule therefore less energy needed to overcome them

Question 10

Why are alkanes less dense than water?

Answer 10

Water is also a simple covalent molecule, however, the intermolecular forces between water molecules are stronger than the IMF between alkane molecules. The stronger the forces the closer the molecules will pack to each other

Question 11

Are alkanes soluble?

Answer 11

Alkanes are insoluble in water but soluble in all other hydrocarbons

Question 12

Why are alkanes pretty unreactive molecules?

Answer 12

All the C-C bonds and C-H bonds are strong, sigma bonds.

Question 13

Why do alkanes make great fuel?

Answer 13

They burn well

Question 14

Complete combustion of alkanes:

Answer 14

Plentiful supply of oxygen
Eg/ C2H8(g) + 5O2(g)→ 3CO2(g) + 4H2O(l)

Question 15

Complete combustion of alkanes:

Answer 15

Plentiful supply of oxygen
Eg/ C2H8(g) + 5O2(g)→ 3CO2(g) + 4H2O(l)

Question 16

Incomplete combustion of alkanes:

Answer 16

Lack of oxygen
Eg/ C3H8(g) + 7/2O2(g) → 3CO(g) + 4H2O(l)

Question 16

Incomplete combustion of alkanes:

Answer 16

Lack of oxygen
Eg/ C3H8(g) + 7/2O2(g) → 3CO(g) + 4H2O(l)

Question 17

What is the initiation step?

Answer 17

The frequency of UV light has the right amount of energy to break the Cl-Cl bond homolytically. The highly reactive chlorine radicals then start attacking the methane molecules

Question 18

What is the propagation step?

Answer 18

CH3CH3 + Cl · –> ·CH2CH3 + HCl

·CH2CH3 + Cl2 –> CH3CH2Cl + Cl ·

Question 19

What is the termination step?

Answer 19

·CH2CH3 + ·Cl –> ClCH2CH3

Question 20

What are some limitations of free radial substitution?

Answer 20

• Further substitution occurs, producing a variety of products
• Substitution can occur at different positions in the carbon chain

Question 21

What is homolytic fission?

Answer 21

As the bond breaks, one electron from the shared pair goes on to each of the atoms involved in the bond. Highly reactive free radicals are produced. Homolytic fission occurs in non-polar bonds i.e. when the atoms have similar electronegativities

Question 22

What s heterolytic fission?

Answer 22

As the bond breaks, both electrons go on to one atom and ions are produced. In this case the X-Y bond is polar i.e. there is a difference in electronegativity between X and Y.

Question 23

How are deposits of crude oil and natural gas form?

Answer 23

Formed by the slow decay of marine animals and plants, under heat and pressure in the absence of air

Question 24

What is the composition of crude oil?

Answer 24

Complex mixture consisting mainly of alkanes

Question 25

How is crude oil separated?

Answer 25

Fractional distillation - compounds have different boiling points

Question 26

What are the bonds in crude oil like?

Answer 26

C-H bonds are non-polar, so there are only weak London forces between the molecules. As the alkane chain gets longer, the number of electrons per molecule increases so the strength of the London forces increases resulting in an increase in the melting and boiling points

Question 27

How does fractional distillation work?

Answer 27

Crude oil is vaporised at high temperature and passed into a column, which is hot at the bottom and cold at the top. As the vapour rises up the fractionating column it condenses at different levels. The fractions are collected at the different levels.

Question 28

What is a fraction?

Answer 28

A fraction is a mixture of hydrocarbons with similar boiling points

Question 29

As the C chain gets longer, the hydrocarbons:

Answer 29

• Become more viscous (thick)
• Harder to ignite
• Less volatile
• Higher boiling points

Question 30

What are some fractions contaminated with?

Answer 30

Sulphur containing impurities

Question 31

What do these impurities burn to form?

Answer 31

To form SO2 and SO3, toxic gases soluble in rainwater, leading to the formation of sulphurous acid (H2SO3) and sulfuric acid (H2SO4).

Question 32

Which fractions are more in demand?

Answer 32

The fractions with shorter chains (eg/petrol and naphtha) are more in demand than larger fractions

Question 33

What can the excess larger hydrocarbons be used for?

Answer 33

The longer hydrocarbons are cracked
Long chain alkane → shorter chain alkane(s) + alkene(s) (+ hydrogen)

Question 34

Conditions needed for catalytic cracking:

Answer 34

Requires a catalyst (eg/Al2O3) and is carried out at a temperature of approximately 500

Question 35

Conditions needed for thermal cracking:

Answer 35

Uses heat energy (approx. 800) to break C-C and C-H bonds. As the C-C bonds are weaker than the C-H bonds, they are more readily broken.

Question 36

What is reforming?

Answer 36

The process which straight chain hydrocarbons are converted into the more desirable, branched chain hydrocarbons.

Question 37

What are the conditions needed for reforming:

Answer 37

Heated to 500 in the presence of a platinum catalyst until they fragment. When the fragments join together, they reform as branched molecules

Question 38

What is the effect of increasing the branching in a molecule?

Answer 38

Increases the efficiency with which they can burn in an engine

Question 39

Why is CO2 not a pollutant?

Answer 39

It occurs naturally in the atmosphere

Question 40

How is CO formed and what are ways to reduce it?

Answer 40

Formed: incomplete combustion of fuels
Problem: toxic
Ways to reduce: ensure a good supply of O2 when burning fuels

Question 41

How is C (soot) formed and what are ways to reduce it?

Answer 41

Formed: Incomplete combustion of fuels
Problem: Blackens buildings, can cause respiratory problems, global dimming
Ways to reduce: Ensure a good supply of O2 when burning fuels

Question 42

How is SO2 formed and what are ways to reduce it?

Answer 42

Formed: Combustion of S containing compounds in fuel (eg/fossil fuels)
Problem: Acid rain (SO2 + H2O → H2SO3)
Ways to reduce: Remove the sulphur from the fuel before burning, flue gas de-sulphurisation

Question 43

How is NOx formed and what are ways to reduce it?

Answer 43

Formed: Reaction of N2 in the air with O2 in the air at very high temperature
Problem: Acid rain (2NO + H2O + 1/2O2 → 2HNO3)
Ways to reduce: Use catalytic converters in cars (NO2 + CO → N2 + CO2)