alkanes

Question 1

source of alkane

Answer 1

crude oil

Question 2

are alkanes un/saturated

Answer 2

saturated

Question 3

both C-C and C-H bonds are made up of a ______ bond

Answer 3

sigma bond

Question 4

describe the bonding in alkanes

Answer 4

alkanes are saturated hydrocarbons
both C-C and C-H bonds are made up of a sigma bond
the sigma bond is formed between two carbon atoms by the direct single overlap of orbitals directly between bonding atoms
this allows free rotation of the sigma bond

Question 5

explain the shape and bond angle around each carbon atom in alkanes in terms of electron pair repulsion

Answer 5

tetrahedral shape with bond angle of 109.5
four bond pairs of electrons around C atom repl each other equally

Question 6

carbon chain length

Answer 6

as the C chain length increases the boiling point increases
because there is more surface contact between molecules (and more electrons)
so there are more induced dipole-dipole interactions between the molecules
which need more energy to overcome

Question 7

branching

Answer 7

a branched isomer has a lower boiling point than the unbranched isomer
in a more branched alkane, there is less surface contact between molecules leading to fewer induced dipole-dipole interactions between molecules
which need less energy to break the weaker induced dipole-dipole interactions between molecules

Question 8

alkanes are relatively unreactive because…

Answer 8

the C-C and C-H sigma bonds are
non polar
strong

Question 9

alkanes have high bond enthalpy which…

Answer 9

gives them a low reactivity

Question 10

combustion

Answer 10

combustion of alkanes is an exothermic reaction
because heat energy is released to the surroundings
alkanes are useful as fuels
in a plentiful supply of oxygen alkanes burn completely to produce CO2 and H2O
in a limited supply of oxygen alkanes burn incompletely to produce CO and H2O

Question 11

carbon monoxide

Answer 11

a toxic gas that prevents hemoglobin in red blood cells binding with oxygen which can be fatal

Question 12

radical substitution facts

Answer 12

reagents - halogen (cl2 or br2 or i2) and excess alkane
conditions - uv radiation

Question 13

radical substitution reaction

Answer 13

substitution type reaction because a H atom in the alkane molecule is replaced by a halogen atom

Question 14

during the reaction the…

Answer 14

Cl-Cl (or Br-Br or I-I) and C-H bonds break by homolytic fission to form radicals so this reaction is known as radical substitution

Question 15

define radical

Answer 15

a species with an unpaired electron

Question 16

further substitution can occur when…

Answer 16

another H atom is replaced by another halogen atom

Question 17

limitations of radical substitution

Answer 17

further substitution means that a mixture of haloalkanes (chloromethane, dichloromethane, trichloromethane and tetrachloromethane) is produced until all of the hydrogen atoms have been replaced
there may be reactions at different positions in a carbon chain creating structural isomers (1-bromopentane, 2-bromopentane, 3-bromopentane

Question 18

define reaction mechanism

Answer 18

shows the sequence of steps which make up the overall reaction

Question 19

order of steps in radical substitution mechanism

Answer 19

1. initiation
2. propagation
3. termination

Question 20

describe the steps of radical substitution mechanism for methane and chlorine

Answer 20

1. the reaction initiates by breaking the Cl-Cl bond by homolytic fission to make two chlorine radicals
   the uv radiation provides the energy needed to break the Cl-Cl bond
2. a very reactive chlorine radical attacks a methane molecule in a chain reaction
   the chlorine radical is used up in the first propagation step but is regenerated in the second propagation step (acts like a catalyst). The chlorine radical is now able to attack a new methane molecule and the reaction continues - propagates
3. the reaction terminates when any two radicals combine to form a new molecule.

Question 21

a chlorine radical reacts with a methane molecule, removing a hydrogen to create a methyl radical and hydrogen chloride

Answer 21

propagation (1st step)

Question 22

two methyl radicals react to form ethane

Answer 22

termination

Question 23

uv light is absorbed by a chlorine molecule providing energy to break the bond. One electron goes to each chlorine creating two free radicals. his is homolytic fission

Answer 23

initiation

Question 24

two chlorine radicals react to form chlorine

Answer 24

termination

Question 25

a chlorine and methyl radical react to form chloromethane

Answer 25

termination

Question 26

a methyl radical reacts with a chlorine molecule creating another chlorine radical and chloromethane

Answer 26

propagation (2nd step)

Question 27

suggest why the Br-Br bond breaks in the first step rather than a C-H bond of the alkane

Answer 27

Br-Br bond is weaker and so needs less energy to break than the C-H bond