Alkanes

Question 1

what is the general formula for alkanes

Answer 1

CnH2n+2

Question 2

what are unbrached chains called

Answer 2

straight chains. but the c-c-c angle is 109.5 degrees so the chains are not actually straight.

Question 3

what happens in an unbranched alkane

Answer 3

each carbon atom has two hydrogen atoms except the end carbon which have one extra

Question 4

what is the general formula for ring carbons

Answer 4

ring alkanes have the general molecular formula CnH2n because the end hydrogens are not required

Question 5

Explain why Alkanes are almost non-polar

Answer 5

Alkanes are almost non-polar because the electronegative of carbon (2.5) and hydrogen (2.1) are so similar.

as a result, the only intermolecular forces between their molecules are weak van der waals forces, and the larger the molecule, the stronger the van der Waals forces

Question 6

why do Boiling points of alkanes increase as the chain length increases

Answer 6

because the intermolecular forces increase as the chain gets longer

Question 7

why do Alkanes with branched chains have lower melting points than unbranched alkanes with the same number of carbon atoms

Answer 7

this is because branched chains pack together as closely as unbranched chains an so van der waals forces are not so effective

Question 8

why are alkanes insoluble in water

Answer 8

alkanes are insoluble in water.

this is because water molecules are held together by hydrogen bonds which are much stronger than the van der Waal’s forces that act between alkane molecules

however, alkanes do mix with other relatively non-polar liquids

Question 9

why are alkanes relatively unreactive

Answer 9

they have strong carbon-carbon and carbon-hydrogen bonds

they do not react with acids, bases, oxidising agents, and reducing agents

Question 10

in what way do alkanes react

Answer 10

they burn and they will react with halogens under suitable circumstances

they burn in a plentiful supply of oxygen to form carbon dioxide and water

in a restricted supply of oxygen they form carbon monoxide or carbon

Question 11

describe the process of fractional distillation

Answer 11

the crude oil is first heated in a furnace

a mixture of liquid and vapour passes into a twoer that is cooler at the top than at the bottom

the vapour pass up the tower via a series of trays containing bubble caps until they arrive at a tray that is sufficiently cool.
then they condense to liquid

the mixture of liquids that condenses n each tray is piped off

the shorter chain hydrocarbons condense in the trays nearer to the top of the tower, where it is cooler, because they have lower boiling points

the thick residue that collects at the bottom of the base of the tower is called tar.

Question 12

what can tar be used for

Answer 12

road surfacing, as supply often exceeds demand this fraction is often further processed to give more valuable products

Question 13

state each fraction in order (crude oil)

Answer 13

natural gases

petrol

naphtha

kerosene

gas oil

mineral oil

bitumen

Question 14

since fractional distillation is a physical process what happens

Answer 14

fractional distillation is a physical process so no covalent bonds within the molecules are broken.

it is the van der waals forces between the molecules that are broken during vaporisation and reform on condensation

Question 15

what type of hydrocarbon is economically more valuable

Answer 15

shorter chain hydrocarbons

Question 16

what are the benefits of cracking

Answer 16

shorter, more useful chains are produced, especially petrol

some of the products are alkenes which are more reactive than alkanes

Question 17

what are alkenes used as

Answer 17

chemical feedstock ( so they supply industries with the starting materials to make different products

they are converted into polymers and a variety of products from paints to drugs

Question 18

what is the most important Alkene

Answer 18

ethene

this is because it is the starting material for polyethene and a wide range of other everyday materials

Question 19

what temperatures are required for thermal cracking

Answer 19

this reaction involves heating alkanes to a high temperature, 700-1200k, under high pressure, up to 7000kPa.

Question 20

explain how the carbon-carbon double bonds break

Answer 20

carbon-carbon double bonds break in such a way that one electron from the pair in the covalent bond goes to each carbon atom

so initially two shorter chains are produced, each ending in a carbon atom with an unpaired electron

Question 21

define free radicals

Answer 21

highly reactive intermediates and react in a number of ways to form a variety of shorter chain molecules

Question 22

what is produced during thermal cracking

Answer 22

hydrogen may be produced

a high proportion of alkenes

Question 23

how do we avoid too much decomposition

Answer 23

the alkanes are kept in these conditions for a very short time, typically one second

Question 24

describe catalytic cracking

Answer 24

takes place at a lower temperature ( 720 k)

and a lower pressure ( but more than atmospheric)

using a zeolite catalyst, consisting of silicon dioxide and aluminium ocide

Question 25

describe zeolites

Answer 25

zeolites have a honeycomb structure with an enormous surface area.

they are also acidic

Question 26

what does catalytic cracking produce

Answer 26

motor fuels

the products are mostly branched alkanes, cycloalkanes, and aromatic compounds

Question 27

what happens to the products obtained from cracking

Answer 27

they are separated by fractional distillation