Chapter 12 - Alkanes

Question 1

what are alkanes

Answer 1

a holologous series of saturated hydrocarbons with the general formula CnH2n+2
they are very unreactive, aside from burning and reacting with halogens

Question 2

describe and explain the trend in boiling points as the carbon chain increases in length
comment on branched isomers

Answer 2

they generally increase as the chain increases because the strength of van der waals forces in between molecules increase as the total number of electrons increases.
for chained isomers, the more branches there are, the lower the boiling point because the molecules cannot pack as close togehter resulting in lower van der waals forces

Question 3

what is crude oil and how is it formed

Answer 3

crude oils are fossil fuels containing a mixture of hydrocarbons
they are formed during the slow decay of marine organisms over millions of years in high heat, high pressure, and anaerobic conditions

Question 4

on what basis are compound separated in fractional distilation

Answer 4

different length chains have different boiling points so we can separate these using distillation

Question 5

how is fractional distillation carries out

Answer 5

crude oil is vaporided
the vapour is passes into a tower which is cold at the top and hot at the bottom
as the vapour rises, it cools
different length molecules will condense at different heights (shorter at top, longer at bottom)

Question 6

what is cracking

Answer 6

the breaking of c-c bonds in long hydrocarbons to make shorter hydrocarbons which are more in demand
it is the thermal decomposition of alkanes

Question 7

what are the conditions of thermal cracking

Answer 7

900 Celsius
70atm
no catalyst
produces alkenes used for polymers for example

Question 8

what are the conditions of catalytic cracking

Answer 8

450 Celsius
1-2 atm
zeolite catalysts
produces motor fuels (aromatics, cyclic alkanes, branched alkanes)

Question 9

what is reforming and why is it used

Answer 9

a process where straight chain hydrocarbons are converted into branched chain alkanes and cyclic
alkanes. Both these products burn more efficiently and are used in petrol for cars.

Question 10

why are alkanes useful as fuels

Answer 10

because when burned in the presence of oxygen, the reaction is highly exothermic

Question 11

what are the products of complete alkane combustion

Answer 11

carbon dioxide and water

Question 12

what are the products of incomplete alkane combustion

Answer 12

either carbon monoxide and water
or carbon and water

Question 13

when does incomplete combustion occur

Answer 13

when there is not enough oxygen

Question 14

how is CO2 formed, what problem does it cause, and how can you reduce this

Answer 14

complete combustion of carbon based fuels
it is a greenhouse gas
burn less fossil fuels

Question 15

how is CO formed, what problem does it cause, and how can you reduce this

Answer 15

incomplete combustion of carbon based fuels
it is highly toxic
ensure a good supply of oxygen when burning fuels

Question 16

how is soot formed, what problem does it cause, and how can you reduce this

Answer 16

incomplete combustion of carbon based fuels
global dimming, respiratory problems, blackens buildings
ensure a good supply of oxygen when burning fuels

Question 17

how is H2O formed, what problem does it cause, and how can you reduce this

Answer 17

combustion of fuels containing hydrogen
it isnt a major problem

Question 18

how is SO2 formed, what problem does it cause, and how can you reduce this

Answer 18

combustion of sulfur compounds in fuels
acid rain
remove sulfur from fuel before burning, use flue gas desulfurisation

Question 19

how are nitrate oxides (NOx) formed, what problem does it cause, and how can you reduce this

Answer 19

reaction of N2 in the air with O2 in the air at very high temperatures
acid rain
use catalytic converter (in cars)

Question 20

how are unburned hydrocarbons formed, what problem does it cause, and how can you reduce this

Answer 20

some fuel just doesnt burn/react
this is a waste of fuel
ensure all engines are well tuned and have a good oxygen supply

Question 21

how can you remove SO2 from waste gases produced in fuel combustion

Answer 21

using flue gas desulfurisation
this involves passing the gases through a scrubber containing calcium oxide or calcium carbonate which reacts with the SO2 to form CaSO3 (gypsum) which can be used to make plasterboard

Question 22

what do catalytic converters do

Answer 22

they remove CO2, NOx and unburned hydrocarbons, turning them into CO2, NOx, and H2O

Question 23

how do greenhouse gases work

Answer 23

They absorb infrared radiation emitted by the Earth – this is due to bond vibrations (these vibrations lead to changes in dipoles that lead to the absorption of infrared).