Alkanes chp 12

Question 1

what are alkanes

Answer 1

alkanes are the main components of natural gas and crude oil, they are amongst the most stable organic compounds which allows crude oil deposits to remain in the earth for many millions of years

Question 2

what are the characteristcs of bonding in alkanes

Answer 2

alkanes are saturated hydrocarbons, containing only carbon and hydrogen atoms joined by single covalent bonds (σ bonds)

Question 3

what is the shape of an alkane

Answer 3

each carbon atom is surrounded by 4 electron pairs in four σ-bonds.
Repulsion between these electrons pairs results in a 3D tetrahedral arrangement around each carbon atom, each bond angle is 109.5 degrees

Question 4

what are some common uses for alkanes

Answer 4

Question 5

what are the varitations in the alkanes

Answer 5

Question 6

why do longer chains have higher boiling points

Answer 6

As the chain length increases, the molecules have a larger surface area and so the London forces between molecules will be greater and so more energy is required to overcome the forces.

Question 7

what is the effect of branching of the boiling point of alkanes

Answer 7

• branches get in way, prevent molecules getting as close together as unbranched molecules
  ^decreasing number of intermolecular forces
• branching means less surface area of contact
• less energy required to break london forces

Question 8

how reactive are alkanes and what causes this

Answer 8

• alkanes do no react with most common reagents
• covalent (σ-Bond) sigma bonds are strong
• bonds are non-polar

Question 9

Can alkanes be involved in combustion reactions

Answer 9

despite their low reactivity, all alkanes react with a plentiful supply of oxygen to produce carbon dioxide and water

Question 10

why are alkanes used as fuels

Answer 10

alkanes are used as fuels because they are readily available, easy to transport, and burn in a plentiful supply of oxygen without releasing toxic products

Question 11

what is the global warming potential of methane
what does it mean

Answer 11

• methane is a greenhouse gas with a global warming potential of 22
• ^meaning that it has 22 times the warming effect of carbon dioxide in the atmosphere

Question 12

what are the 2 different products that can be formed from the incomplete combustion of alkanes

Answer 12

CO (carbon monoxide) and C (carbon in the form of soot)

Question 13

what conditions are needed for alkanes to react with halogens

Answer 13

in the presence of sunlight, alkanes react with halogens.
this is because the high-energy ultraviolet raditation provides the energy needed for the reaction to take place

Question 14

what is the mechanisms for the halogenation of alkanes called

Answer 14

free-radical substitution

Question 15

what occurs during the initiation stage of free-radical substitution

Answer 15

the reaction is started by homolytic fission (each atom gets a single unpaired electron from the pair) and 2 highly reactive halogen radicals form.

Question 16

what occurs during the proporgation steps of free-radical substitution

Answer 16

-in this stage, the reaction propagates through 2 steps (a chain reaction)
-A free-radical reacts with a non-radical molecule to create a radical molecule and a non-radical molecule
-in theory the propagation steps could continue until all the reactants have been used up
-it is estimated that up to a 1,000,000 propagation cycles take place before a termination step stops the reaction

Question 17

what occurs during the termination steps of free-radical substitution

Answer 17

• 2 radicals react, forming normal molecules
• ^both radicals are removed from the reaction mixture

Question 18

what are the limitations of free-radical substituition in organic synthesis

Answer 18

• hard to stop further substitution reactions taking place
• hard to direct exactly where radical substitue onto carbon chain