alkanes (organic chem)

Question 1

define ‘alkanes’

Answer 1

saturated hydrocarbons

Question 2

define ‘petroleum fraction’

Answer 2

mixture of hydrocarbons with similar chain length & b.p range

Question 3

gen. formula of alkanes

Answer 3

CnH(2n+2)

Question 4

reactivity of alkanes

(and why?)

Answer 4

unreactive apart from combustion –> strong, non-polar C-C & C-H bonds

Question 5

process of fractional distillation of crude oil

Answer 5

> the petroleum fractions condense at diff. heights
the temp. of the column decreases upwards
smaller molecules condense at top –> lower temp = lower b.p –> smaller size –> smaller VDWs
larger molecules condense at bottom –> higher temp = higher b.p –> larger size –> larger VDws

Question 6

smaller molecule characteristics

Answer 6

• lighter colour
• very flammable
• low viscosity

Question 7

larger molecule characteristics

Answer 7

• darker colour
• less flammable
• high viscosity

Question 8

vacuum distillation

Answer 8

• heavy residue from fractionating column distilled under vac.
• lowering the pressure over a liquid = lower b.p

Question 9

why might vacuum distillation be used?

Answer 9

• allows heavier fractions to be further seperated
• no need for high temps –> could break them down

Question 10

define ‘cracking’

Answer 10

conversion of long hydrocarbons to smaller, more useful hydrocarbon molecules by breakage of C-C bonds

Question 11

high Mr alkanes –>

Answer 11

smaller Mr alkanes + alkenes + (hydrogen)

Question 12

name a solid material made from alkanes

Answer 12

polymer

Question 13

why does cracking req. high temps?

Answer 13

involves splitting of strong cov. bonds (C-C)

Question 14

list the economic reasons for cracking

Answer 14

1. petroleum fractions with shorter C chains (eg. petrol) are higher in demand
2. products of cracking = more valuable than starting materials

Question 15

conditions for thermal cracking

Answer 15

• high pressure (7000kPa)
• high temp. (400-900 degrees)

Question 16

products of thermal cracking

Answer 16

• mostly alkenes
• sometimes prod. hydrogen

Question 17

process of thermal cracking

Answer 17

• proceeds via free radical mechanism
• 1st step is homolytic fission of C-C bond

Question 18

conditions for catalytic cracking

Answer 18

• slight pressure
• high temp (450 degrees)
• zeolite catalyst

Question 19

products of catalytic cracking

Answer 19

• branched and cyclic alkanes
• aromatic hydrocarbons

Question 20

why is catalytic cracking used?

Answer 20

• branched/cyclic hydrocarbons burn more smoothly than unbranched ones –> used to give fuels higher octane no.
• cheaper than t. cracking –> uses less energy –> lower temp n pressure

Question 21

why are alkanes used as fuels?

Answer 21

• they burn readily in presence of oxygen
• this combustion of alkanes = highly exo.
• can be used as fuels (fuels release heat energy when burnt)

Question 22

complete combustion

Answer 22

• fuel + O2 –> CO2 + H2O
• excess O2

Question 23

incomplete combustion

Answer 23

• fuel + O2 –> C or CO + steam
  –> CO = colourless, odourless, toxic gas which bonds to haemoglobin
  –> C = causes global dimming, respiratory issues
• insufficient O2

Question 24

types of pollution from combustion

Answer 24

• sulphur impurities
• nitrogen oxides
• greenhouse gases

Question 25

sulphur impurities --> SO2 | (type of pollution)

Answer 25

* found on coal * SO2 produced via **incomplete combustion** when sulfur is burnt * SO2 dissolves in water --> **acid rain**

Question 26

removing SO2

Answer 26

* removed from waste gases in furnace via **flue gas desulphurisation** * gases pass through scrubber with basic CaO --> reacts with acidic SO2 in neutralisation reaction

Question 27

what is the product of flue gas desulphurisation? | (& it's use)

Answer 27

calcium sulphite --> can be further made into c**alcium sulphate** for plasterboard

Question 28

N + O2 --> 2NO

Answer 28

* **high temp** and spark in car engine provides **sufficient** emergy to **break** strong N2 bond * reacts with oxygen --> prod. nitrogen oxides * dissolves in water --> **acid rain**

Question 29

what do catalytic converters do?

Answer 29

* remove carbon monoxide, nitrogen oxides & unburned hydrocarbons --> convert them into **harmless** gases * has ceramic honeycomb coated with **thin** layer of **catalyst metals** --> gives **large** S.A

Question 30

identify a catalyst in the converter

Answer 30

palladium, platinum

Question 31

why is UV light needed for the synthesis of chloroalkanes?

Answer 31

UV light supplies **energy** to **break** the Cl-Cl bond

Question 32

what is the process of breaking the bonds during initiation called?

Answer 32

homolytic fission

Question 33

free radical mechanism | (steps)

Answer 33

1. initiation -> split halogen into free radicals 2. propagation -> extract H from alkane 3. propagation -> replace H in free radical alkane with halogen from diatomic molecule 4. termination -> two radicals prod. stable molecule