Alkanes

Question 1

what is a saturated hydrocarbon?

Answer 1

molecule where all carbon-carbon bonds are single bonds, and all carbons are bonded to the max number of hydrogen’s possible.

Question 2

what is the general formula of alkanes?

Answer 2

CnH2n+2

Question 3

explain how chain length affects boiling and melting point of alkanes?

Answer 3

increases as the number of carbon atoms increases within the molecule (increase in chain length).

Question 4

explain how branching affects boiling and melting point of alkanes?

Answer 4

decreases as branching reduces the surface area available for intermolecular interactions.

Question 5

what significance do intermolecular forces have upon boiling point and melting point of alkanes?

Answer 5

alkanes are non-polar, they only contain london dispersion forces (van der waal forces). as the size of the molecule increases, the number of electrons increases, so the strength of the london forces increases as well. the stronger the force, the harder it is to overcome the bond, increase in boiling and melting point.

Question 6

what is crude oil?

Answer 6

naturally occurring,unrefined petroleum mixture which is composed of mainly alkanes and saturated hydrocarbons and organic compounds

Question 7

through what processes may crude oil be separated?

Answer 7

fractional distillation and cracking

Question 8

explain what is fractional distillation?

Answer 8

process used to separate crude oil into various fractions, each containing hydrocarbon compounds with similar but varying boiling points.

Question 9

explain the temperature gradient in a fractionating column?

Answer 9

fractional distillation is carried out in a fractionating column which is very hot at the bottom and cool at the top

Question 10

what is the trend in viscosity in a fractionating column?

Answer 10

viscosity generally decreases as you move up the column. This is because at higher levels of the column, temperatures are cooler, which increases viscosity.

Question 11

explain the process.

Answer 11

Crude oil enters the fractionating column and is heated so vapours rise.
Vapours of hydrocarbons with very high boiling points will immediately condense into liquid at the higher temperatures lower down and are tapped off at the bottom of the column.The fractions containing smaller hydrocarbons are collected at the top of the fractionating column as gases. The fractions containing bigger hydrocarbons are collected at the lower sections of the fractionating column

Question 12

what is cracking?

Answer 12

process used in fractional distillation to break down larger, heavier hydrocarbons into smaller, lighter ones. this is done to increase the yield of more valuable products such as gasoline and diesel, which are higher in demand.

Question 13

what are the 2 man types of cracking?

Answer 13

thermal cracking and catalytic cracking.

Question 14

what does thermal cracking involve?

Answer 14

heating hydrocarbons
temps of 450-900°C
absence of oxygen
70 atm of pressure
breaks long hyrdocarbons into smaller ones.
forms alkenes, alkanes.

Question 15

what does catalytic cracking involve?

Answer 15

uses a zeolite catalist or alumina-silica catalyst. (lowers activation energy)
temps of 450-550°C
lowered activation energy leads to a more efficient conversion of heavy hydrocarbons into lighter ones.
1-2 atm of pressure
forms branched alkanes, cyclic alkanes and aromatic alkanes

Question 16

what is a benefit of thermal cracking?

Answer 16

forms a large number of alkenes.

Question 17

explain the differences between complete and incomplete combustion.

Answer 17

complete:
occurs with sufficient O2
carbon is oxidised to CO2
hydrogen is oxidised to H2O
these 2 are the only byproducts.
considered as the most desirable form of combustion is terms of minimising air pollution.
incomplete:
occurs with limited O2
carbon is not fully oxidised, forms CO (carbon monoxide)
hydrogen is not fully oxidised, forms water vapour (H2O)
releases less energy than complete.
produces harmful byproducts.

Question 18

what is the issue with car exhausts?

Answer 18

Car exhaust fumes include toxic gases such as carbon monoxide (CO), oxides of nitrogen (NO/NO2) and volatile organic compounds (VOCs)

Question 19

what is carbon monoxide (CO)?

Answer 19

CO is a toxic and odourless gas which can cause dizziness, loss of consciousness and eventually death.
The CO binds well to haemoglobin which therefore cannot bind oxygen and carbon dioxide.
Oxygen is transported to organs
Carbon dioxide is removed as waste material from organs.

Question 20

what are the formulas you must remember for oxides of nitrogen?

Answer 20

N2 + O2 → 2NO
N2 + 2O2→ 2NO2
2NO + O2 → 2NO2
2NO2 + H2O → HNO2 + HNO3

Question 21

why is catalytic removal used?

Answer 21

to reduce the amount of pollutants released in car exhaust fumes, many cars are now fitted with catalytic converters.
precious metals (such as platinum) are coated on a honeycomb to provide a large surface area.

Question 22

what is free-radical substitution of alkanes?

Answer 22

a reaction that occurs when a hydrogen atom in an alkane is replaced by a halogen atom. involves the formation of free radicals.

Question 23

what is a free-radical?

Answer 23

a highly reactive species with an unpaired electron.

Question 24

what is initiation?

Answer 24

a free radical is formed by breaking a weak bond between a halogen-halogen using UV light.
Cl2 –-U.V–> 2Cl∙

Question 25

what is propagation?

Answer 25

the free radical reacts with an alkane molecule, abstracting a hydrogen atom and forming a new alkyl radical.
prop 1-
Cl∙ + CH4 –> ∙CH3 + HCl
prop 2-
∙CH3 + Cl2 –> CH3Cl + Cl∙

Question 26

what is termination?

Answer 26

two free radicals combine to form a stable molecule, effectively ending the reaction.
Cl∙ + ∙CH3 –> CH3Cl
∙CH3 + ∙CH3 –> C2H6