Hydrocarbons

Question 1

What are alkanes

Answer 1

Alkanes are saturated hydrocarbons which are member s of a homologous series
↳ all the carbon-carbon bonding is single
↳ bonds are spaced tetrahedrally around carbon atoms

Question 2

What are the physical properties of alkenes?

Answer 2

• Boiling point increases as they get more carbon atoms in their formula
• More atoms = greater intermolecular van der waalsforces
• Greater intermolecular force= more energy to separate the molecules
• Greater energy required, higher boiling point
• the greater the branching, the lowerthe boiling point
• The straight chain molecules have greater interaction than branched

Question 3

Why are alkanes unreactive?

Answer 3

• Absence of a functional group
• non polar therefore un reactive toward water or polar reagent like HBr
• Alkanes react with non-polar reagents like cl2 or br 2 only in the presence of uv light or sunlight
• c-c and ch bonds have high bond enthalpy and therefore require a lot of energy to overcome

Question 4

Why do alkanes make good fuels?

Answer 4

• Release huge amounts of energy when burnt

- readily available and easy to transport

Question 5

When does complete combustion occur

Answer 5

In excess oxygen

Question 6

Product of complete combustion

Answer 6

Water and carbon dioxide

Question 7

When does incomplete combustion occur

Answer 7

In insufficient oxygen

Question 8

Products of incomplete combustion

Answer 8

Water and various other product and pollutants including carbon particulates, carbon monoxide and carbon dioxide

Question 9

What are the problems with carbon monoxide?

Answer 9

• carbon monoxide is poisonous
• It irreversibly binds to haemoglobin, reducing the amount of oxygen that can be carried in the blood
• Cells may become de oxygenated, leading to cell death
• if too much carbon monoxide is inhaled, this could lead to death

Question 10

What are, carbon particulates?

Answer 10

• Small fragments of unburned hydrocarbon
• unless removed from the waste products in industry, these can cause serious respiratory problems as they pollute the air

Question 11

What is crude oil a source of ?

Answer 11

Aliphatic and aromatic alkanes

Question 12

What is cracking?

Answer 12

The process in which complex organic molecules are broken down into smaller organic molecules such as simple alhanes

Question 13

Why is cracking used?

Answer 13

To obtain more useful and smaller alkanes and alkenes of which there is a greater demand for

Question 14

What’s markovnikov’s rule?

Answer 14

In the electrophillic addition of a hydrogen halide to an asymmetric alkene, the hydrogen atom will bond to the carbon that is already bonded to the most hydrogens.
this leads to the formation of the major product

Question 15

What are the classifications of carbocation

Answer 15

Primary , secondary and tertiary

Question 16

What is a primary carbocation

Answer 16

Carbocation is bonded to one alkyl group

Question 17

What is secondary carbocation

Answer 17

Carbocation is bonded to two alkyl groups

Question 18

What is tertiary carbocation

Answer 18

Carbocation is bonded to three alkyl groups

Question 19

What is the trend in carbocation stability ?

Answer 19

• The more alkyl groups attached to the carbocation the more the positive change is spread out the increasing stability
• stability increases from primary→ secondary→tertiary

Question 20

What is additional polymerisation

Answer 20

The joining together of unsaturated alkene monomers to form a long chain polymer
- no other products are formed so the atom economy is 100%

Question 21

How do alkenes undergo addition polymerisation?

Answer 21

The pi bond breaks and each electron goes towards forming a sigma bond with an adjacent monomer unit

Question 22

Why are polymers difficult to dispose of ?

Answer 22

• They are non-biodegradable

- the combustion of polymers can often form harmful compounds such as Hcl for chlorinated polymers

Question 23

Why are alkanes suitable for use as fuels?

Answer 23

Alkanes have high standard enthalpy changes of combustion so they release a lot of energy upon combustion

Question 24

Why are atmospheric oxides of nitrogen pollutants?

Answer 24

• Nitrogen monoxide can contribute to the production of acid rain by reacting with chemicals in the clouds
• nitrogen dioxide can contribute to photochemical smog

Question 25

What are the environmental consequences of unburnt hydrocarbons?

Answer 25

Hydrocarbons in the atmosphere can absorb infraredradiation, contributing to the greenhouse effect

Question 26

What are the environmental consequences of the emission of greenhouse gases?

Answer 26

The greenhouse effect:

• electromagnetic radiation from the sun passes through the atmosphere. Some radiation is aborted by earth sotemperature increases
• heat is radiated from the earth as infrared radiation which is absorbed by greenhouse gases in the atmosphere. This causes the bunds in these molecules to vibrate before this energy is re-emitted
• as a result, the atmosphere warms up even further contributing to global warming

Question 27

Chemical test for alkene

Answer 27

Add bromine water

If a c=c bond is present, the orange solution will decolorise to form a colorless solution

Question 28

what reactions produce alkanes

Answer 28

hydrogenation and cracking

Question 29

what is the hydrogenation reaction

Answer 29

addition reaction
equation: alkene + hydrogen gas -> alkane
catalyst: heat + platinum/nickel catalyst
(see notes for diagram)

Question 30

what is the cracking reaction

Answer 30

large hydrocarbon fed into steel chamber and heated strongly then passed over Al2O3 catalyst

equation: large alkane -> smaller alkane + alkene