6.2 - alkanes & alkenes

Question 1

what is the general formula of alkanes?

Answer 1

CnH2n+2

Question 2

which 2 types of hydrocarbons are saturated?

Answer 2

alkanes & cycloalkanes

Question 3

which 3 ways are alkane fuels obtained from?

Answer 3

fractional distillation, cracking, reforming of crude oil

Question 4

what does reforming do?

Answer 4

convert straight-chained hydrocarbons into branched / cyclic for efficient combustion

Question 5

how does fractional distillation work?

Answer 5

• crude oil heated in a furnace & flows into fractioning tower
• vapour passes up and condenses at different regions

Question 6

in which direction does the fractioning column get cooler? what hydrocarbons are harvested there?

Answer 6

upwards, hydrocarbons with low chain length / boiling point

Question 7

what are the products of complete combustion of alkanes?

Answer 7

CO2 + H2O

Question 8

what are the products of incomplete combustion?

Answer 8

CO + H2O

Question 9

why is CO toxic to the human body?

Answer 9

binds to haemoglobin, less O2 transported

Question 10

what other pollutants are produced during combustion? what are their impacts?

Answer 10

• nitrogen oxides -> reacts with oxygen & forms acid rain
• sulfur oxides -> acid rain & damages catalytic converters
• carbon particulates -> coughing (heart disease & lung cancer)

Question 11

what are some examples of alternative fuels from renewable sources?

Answer 11

• alcohols
• biodiesel

Question 12

what is an radical?

Answer 12

species with unpaired e-

Question 13

describe the 3 steps of free-radical substitution with halogens.

Answer 13

1) initiation
- produces free radical from molecules

2) propagation
- forms products & more radicals

3) termination
- removes free radicals

Question 14

under what conditions does alkanes react with halogens?

Answer 14

under UV light / on heat

Question 15

what are the limitations of using free-radical substitution?

Answer 15

• mixture of products
• further substitution products

Question 16

what is the general formula of alkenes?

Answer 16

CnH2n

Question 17

what types of organic molecules have the general formula of CnH2n?

Answer 17

alkenes & cycloalkanes

Question 18

when does e/z isomerism occur?

Answer 18

when there is restricted rotation about a bond with 2 different groups attached

Question 19

which type of hydrocarbon produces a cleaner flame when combusted?

Answer 19

alkanes

Question 20

what reaction conditions are involved in the electrophilic addition of H2 to alkenes? what is the product?

Answer 20

Ni catalyst, 150C, 5 atm
forms alkanes

Question 21

what reaction conditions are involved in the electrophilic addition of steam to alkenes? what is the product?

Answer 21

conc. H3PO4 catalyst, 300-600C, 70 atm
produce alcohols

Question 22

what reaction conditions are involved in the oxidation of alkenes? what is the product?

Answer 22

acidified KMnO4 + steam
produce diols

Question 23

what will be the colour change observed when alkenes are oxidised by potassium manganate?

Answer 23

pink -> colourless