12 Alkanes

Question 1

What is the general formula for all alkanes?

Answer 1

CnH2n+2

Question 2

What are the bond angels of unbranched alkanes ?

Answer 2

109.5

Question 3

Why are alkanes almost non-polar?

Answer 3

•hydrogen and carbon have similar electronegativities

Question 4

What are the intermolecular forces in alkanes?

Answer 4

•Van der Waals
•bigger the molecule the stronger the force

Question 5

What is the trend of boiling point in alkanes?

Answer 5

•BP increases with chain length
•Beaqcuse of increasing intermolecular forces

Question 6

Do straight chain or branched chain have lower melting points?

Answer 6

•branched chains
•can’t pack together as close
•van der Waals forces less effective

Question 7

Why are alkanes insoluble in water?

Answer 7

•water molecules have hydrogen bonds which are stronger then van der Waals

Question 8

What forms acd rain?

Answer 8

-sulfur impurities in crude oil
-forms sulfur dioxide when burnt
-reaction with water in atmosphere to form sulfuric acid

Question 9

How does fractional distillation happen?

Answer 9

-crude oil heated in furnace
-liquid and vapour mixture passes into tower that is cooler the top and hot at bottom
-vapour passes up the fractions and arrive at a temp lower than their bp
-mixture then condenses at each fraction
-shorter chain condense at the top (due to low bp)

Question 10

What can products of fractional distillation be used for?

Answer 10

-5/10 chain is petrol
-11/16 is kerosene
-thick residue collected at base is tar/ bitumen which is used for road surfacing
-long chains can be cracked to meet demands of short chain

Question 11

How are alkanes a chemical feedstock?

Answer 11

•they supply industries with starting materials to make products

Question 12

What is thermal cracking?

Answer 12

•heating alkanes to high temperatures (700-1200K)
•high pressure 7000kPa

Question 13

What are products of thermal cracking?

Answer 13

•forms 2 shorter chains with an unpaired electron (free radicals which are highly reactive)
•alkane+alkene

Question 14

What is catalytic cracking?

Answer 14

•lower temp (720K)
•lower pressure
•zeolite catalyst (silicon dioxide and aluminium oxide)

Question 15

What are the products of combustion ?

Answer 15

•carbon dioxide
•water

Question 16

What are products of incomplete combustion ?

Answer 16

•Carbon monoxide (poison gas)
•soot (carbon)

Question 17

What are the pollutants formed when crude oil is burnt?

Answer 17

•Carbon monoxide
•carbon dioxide
•Water vapour
•nitrogen oxides (petrol engine react with oxygen in air to form nitric acid which contributes to acid rain)
•sulphur dioxide (acid rain)
•carbon particles /particulates
•unburnt hydrocarbons (greenhouse gases)

Question 18

What is flue gas desulfurisation?

Answer 18

•lime (calcium oxide ) and water sprayed into flue gas
•forms calcium sulfite
•can be oxides to form calcium sulfate (gypsum which is saleable)

Question 19

What do catalytic converters do ?

Answer 19

•in exhaust systems to reduce output of CO, nitrogen oxides ,unburnt hydrocarbons
•honeycomb shape for large SA
•coated with platinum and rhodium (catalyst )
•polluting gases pass over catalyst and react to form less harmful products

Question 20

What is needed for a alkane to react with a halogen?

Answer 20

•ultra violet component of light
Or sometimes high temps

Question 21

What are the 3 stages of a chain reaction?

Answer 21

•initiation
•propagation
•termination

Question 22

What happens at initiation stage of chain reactions?

Answer 22

-UV light provides energy (single quantum which is stronger then halogen bonds) for halogen bonds to break
-forms two halogen free radicals (2Br•) which are very reactive
-CH bonds don’t break

Question 23

What happens in the propagation stage of a chain reaction?

Answer 23

-halogen free radical takes hydrogen from alkane
-forming alkane free radical (e.g. •CH3)
-alkane radical is reactive so it reacts with a halogen molecule forming another halogen free radical + stable compound
-steps are repeated till termination

Question 24

What happens at the termination stage of a chain reaction

Answer 24

-free radicals are removed by:
•two halogens free radicals react
•two alkane free radicals react
•halogen and alkane free radicals react
Forming stable compound weird NO free electrons

Question 25

Why are chain reactions important for the ozone layer

Answer 25

•CFCs(chloroflurocarbons) destroy ozone layer •ozone is made of 3 oxygen molecules and too much ozone at ground level causes lung irritation and degradation but protects earth from UV light •chlorine free radicals form from CFSs beacuse C—CL bond breaks homolytically in UV •ozone molecules ate attacked by chlorine free radicals (Cl• +O3->ClO• +O2) then (ClO•+O3->2O2+ Cl•) So chlorine is a catalyst ,as it’s not destroyed , in breakdown of ozone to oxygen

Question 26

What is Ozone depletion?

Answer 26

•Ozone depletion is the thinning of the ozone layer in the stratosphere •chemicals that release chlorine or bromine atoms into the stratosphere cause it

Question 27

What is a nucleophile?

Answer 27

•ion with a negative charge (or partially ) •takes part in an organic reaction by attacking electron deficient area in a reactant

Question 28

What is electronegativity?

Answer 28

•tendency for an atom to attract a pair of electrons in a covalent bond

Question 29

How do halogens act as a catalyst in ozone depletion?

Answer 29

-they don’t get used up, as it’s regenerated -e.g. ( Br•+O3->BrO•+O2) (BrO•+O3->Br•+2O2)