4.1.2 - alkanes

Question 1

what is the general formula of alkanes?

Answer 1

C(n)H(2n+1)

Question 2

why are alkanes non-polar?

Answer 2

there’s no significant difference between the electronegativities of carbon and hydrogen

Question 3

give 1 use of alkanes

Answer 3

fuels

Question 4

explain the shape and bond angles in an alkane

Answer 4

there are 4 bonding pairs and 0 lone pairs around each carbon. all electron pairs repel. lone pairs repel more than bonding pairs. bong angle is 109.5.

Question 5

explain how sigma bonds are formed in alkanes

Answer 5

the end on end overlap of bonding orbitals directly between atoms. how well they overlap determines the strength of the bond.

Question 6

bonds can __________ in an alkane

Answer 6

rotate

Question 7

explain the relative melting and boiling points of 2 alkanes with different chain lengths.

Answer 7

as chain length increases, boiling/melting point increases, as they have a greater surface area so have more points of contact between molecules. this means strength of induced dipole-dipole interactions increases, which require more energy to overcome.

Question 8

explain the relative melting and boiling points of branched isomers

Answer 8

the more branches there are in an isomer molecule, the lower the boiling point. the branches cause fewer points of surface contact between molecules so induced dipole-dipole interactions are weaker and require less energy to overcome.

Question 9

explain why production of carbon monoxide in the combustion of alkanes is bad.

Answer 9

carbon monoxide is poisonous. leads to oxygen deprivation, which causes fatigues, headaches and nausea.

Question 10

are straight chain or branched chain isomers more efficient in combustion reactions?

Answer 10

branched

Question 11

what is a radical?

Answer 11

highly reactive species with an unpaired electron.

Question 12

what is homolytic fission?

Answer 12

covalent bond breaks and one electron goes to each atom forming 2 radicals.

Question 13

what is heterolytic fission?

Answer 13

covalent bond breaks and both electrons go to the same atom forming 2 ions

Question 14

what does a curly arrow represent?

Answer 14

movement of an electron pair

Question 15

what are the 3 steps for free radical substitution and what conditions are required

Answer 15

1- initiation (radicals are made)
2 - propagation (radicals are used up and created)
3 - termination (radicals combine to make a neutral molecule)
conditions - UV and high temperature

Question 16

what are the drawbacks of free radical substitution?

Answer 16

multi-substitution occurs (can overcome this by using excess alkane). substitution can occur anywhere along the chain, so a mixture of isomers forms

Question 17

explain the importance of ozone

Answer 17

absorbs UV radiation. reduced the amount of UV at the earths surface. UV radiation causes sunburn and skin cancer.

Question 18

explain the natural formation of ozone

Answer 18

ozone absorbs UV radiation
UV at Earth’s surface is reduced
(UV) O2 –> 2O.
O2 + O. –> O3

Question 19

what is a chlorofluorocarbon?

Answer 19

contain chlorine, fluorine and carbon only - all hydrogens have been substituted.

Question 20

give 3 properties of chlorofluorocarbons

Answer 20

volatile, non-flammable and non-toxic

Question 21

give 4 (past) uses of chlorofluorocarbons

Answer 21

aerosol cans, dry cleaning, air conditioning and fridges

Question 22

why are chlorofluorocarbons no longer used?

Answer 22

they were destroying the ozone layer. they catalyse the breakdown of ozone

Question 23

what is the overall equation for the breakdown of ozone?

Answer 23

O3 + O –> 202 (with Cl. or NO. catalyst)

Question 24

NO. radical destroy the ozone layer. how are nitrogen oxides formed?

Answer 24

produces by lighting in thunderstorms and in car/aircraft engine

Question 25

what are hydrochlorofluorocarbons/hydrofluorocarbons and why are they more widely used than chlorofluorocarbons?

Answer 25

HCFCs and HFCs are used as an alternative to CFCs until safer products are developed. HCFCs are broken down in the atmosphere much more quickly (10-20 years whereas CFCs take 90 years). HFCs done contain Cl so done affect the oxone layer. HCFCs and HFCs are greenhouse gases

Question 26

give 3 alternatives to CFCs (not inc HCFCs or HFCs)

Answer 26

aerosols with pump sprays/nitrogen propellants fridges/freezers using ammonia as the coolant gas CO2 is used to make expanded polymers.