reactions

Question 1

reactant, condition and final product for substitution reaction

Answer 1

reactant: alkane + halogen
condition: under light
final product: haloalkane

Question 2

why is light necessary for substitution reaction?

Answer 2

provide sufficient energy to break the Cl-Cl bonds in Cl2 molecules to form Cl radicals (where Cl can be Br or F(unlikely))

Question 3

free radical definition & why it is highly reactive

Answer 3

definition: a particle that has at least one unpaired electron
unstable: it does not have a stable noble gas electronic arrangement/does not obey octet rule

Question 4

what reaction will happen during propagation cycles? what is the definition of the reaction?

Answer 4

chain reaction; a sequence of reactions where a reactive product or by-product cause additional reactions to take place

Question 5

observable changes when Br2 reacts with alkane under light

Answer 5

1. orange bromine solution changes to colorless
2. white fume is observed because of the formation of HBr

Question 6

how is white fume formed during substitution reaction of alkanes with Br or Cl?

Answer 6

HBr/HCl formed dissolves in moisture in air to form acid droplets, which are seen as white fume

Question 7

observable change when Cl2 reacts with alkane

Answer 7

1. chlorine gas changes from yellow-green to colorless
2. white fumes are formed due to the formation of HCl gas

Question 8

what will be observed on a wet litmus paper if it is placed at the mouth of the test tube after substitution reaction of CH3 with Br?

Answer 8

the WET, not dry litmus paper will turn from blue to red

Question 9

what will be observed on a dry litmus paper if it is placed at the mouth of the test tube after substitution reaction of CH3 with Br?

Answer 9

nothing its dry

Question 10

what will be observed on a wet litmus paper if it is placed at the mouth of the test tube after substitution reaction of CH3 with Cl?

Answer 10

changes from blue to red as HCl gas is acidic –> ionizes in water; then changes to white as OCl- is a bleaching agent

Question 11

rate of reaction (F2, Cl2, Br2, I2) with alkanes

Question 12

major product for excess butane with chlorine

Answer 12

Excess alkane is monosubstitution so answer is C4H9Cl

Question 13

when/where does cracking occur in the catalytic cracking set up?

Answer 13

when the vapour of alkane passes through the broken unglazed porcelain

Question 14

catalyst used for catalytic cracking on an industrial scale

Answer 14

aluminium oxide or silicon dioxide

Question 15

catalytic cracking in laboratory:
1. why heat broken unglazed porcelain strongly not only alkane? (2)
2. why broken?
3. why cannot remove delivery tube before heat source off?
4. what can replace broken unglazed porcelain?

Answer 15

1. prevent alkane from vaporizing without reaction (1) if the surface of broken unglazed porcelain is not hot enough (1)
2. increase surface area -> increase rate of reaction
3. to prevent sucking back of water (1) sudden cooling may break boiling tube (1)
4. broken pumice stone/broken porous pot/aluminium oxide (1)

Question 16

importance of cracking in petroleum industry (2 reasons)

Answer 16

1. to obtain smaller fractions which are of higher demand like petrol
2. to obtain more alkenes which are important raw materials for chemicals (e.g. producing petrochemicals, plastics)

Question 17

explain from molecular level why petrol can be obtained from petroleum by fractional distillation (2 marks)

2014 dse q6a

Answer 17

• different hydrocarbons in petroleum have different boiling points (1)
• the longer the carbon chain, the higher the boiling point (1)

Question 18

other than directly obtaining petrol from fractional distillation of petroleum, suggest a way of producing extra petrol

Answer 18

cracking of heavy hydrocarbons

Question 19

explain why CH3Cl is not the only organic product formed in the reaction between CH3 and Cl

Answer 19

CH3 undergoes further substitution/polysubstitution to form CH2Cl2, CHCl3 and CCl4

Question 20

types of addition reactions (5 types)

Answer 20

1. hydrogenation/catalytic hydrogenation
2. reaction with cold acidified potassium permanganate solution
3. halogenation
4. hydrohalogenation
5. addition polymerization

Question 21

definition of catalytic hydrogenation

Answer 21

conversion of unsaturated alkenes to saturated alkanes

Question 22

hydrogenation reaction formula

Answer 22

–C=C– + H2 — (Pt, Pd or Ni as catalyst) and HEAT → -C-C- with two H underneath

Question 23

condition and catalysts required for catalytic hydrogenation + what format are the catalysts in

Answer 23

heat and catalyst (Ni, Pt or Pd) –> finely divided or powdered form

Question 24

why are hydrogenation reactions not common in industry

Answer 24

as alkenes have higher demand then alkanes in industry

Question 24

if no catalyst is used for hydrogenation, how can the reaction occur?

Answer 24

under strong heating

Question 25

Conversion + equation of adding cold mno4 (aq) to alkene

Answer 25

Conversion of alkene to diol. C=C + [O] + H2O → -C-C- with 2 OH underneath

Question 26

observation when cold acidified vs neutral vs alkaline MnO4 is added to alkene + reason

Answer 26

acidified: the solution changes from purple to colorless (oxidized from MnO4- to Mn2+) neutral and alkaline: the solution changes from brown to colorless as MnO2 is formed (manganese (IV) oxide is formed)

Question 27

IF reaction occurs at a high temperature what will happen

Answer 27

side reactions may occur

Question 28

why can acidified K2Cr2O7 not be used in the reaction

Answer 28

it is not strong enough to oxidize alkenes

Question 29

definition of halogenation reaction

Answer 29

converts alkene to dihaloalkane

Question 30

product when alkene react with cold acidified mno4

Answer 30

c-c with two hydroxyl groups underneath

Question 31

observation when halogenation reaction occurs for alkene (+ provide reactant/condition)

Answer 31

react with X2 in organic solvent solution turns from (orange for bromine, yellow-green for chlorine, brown for iodine) to colorless

Question 32

observation when halogenation reaction occurs for alkane (+ provide reactant/condition)

Answer 32

under light: solution turns from (orange for bromine, yellow-green for chlorine, brown for iodine) to colorless

Question 33

is halogenation of alkene and alkane faster

Answer 33

alkene

Question 34

state markovnikov's rule

Answer 34

markovnikov's rule states that when a molecule HX adds to a asymmetrical alkene, the hydrogen atom mainly attaches to the carbon atom that already carries the greater number of hydrogen atoms

Question 35

when will only one haloalkane be formed in hydrohalogenation of alkene?

Answer 35

when the alkene is symmetrical