Module 4 - Alkanes

Question 1

what are alkanes

Answer 1

-saturated hydrocarbons with the general formula of CnH2n+2

Question 2

how many hydrogens per carbon in cycloalkanes

Answer 2

-2

Question 3

describe the shape of alkanes

Answer 3

-tetrahedral
-109.5
-as bonds repel each other equally
-to keep as far apart as possible

Question 4

what are the different shapes/lengths of alkanes

Answer 4

-unbranched that leads to more surface contact
-branched

Question 5

pattern of increasing boiling points in alkanes

Answer 5

-increasing number of electrons in bigger molecules
-which causes an increase in the size of the london forces between molecules

Question 6

explain how long, straight chain hydrocarbons have higher boiling points

Answer 6

-more london forces
-more energy needed to overcome these forces
-so boiling point increases

Question 7

explain how branched hydrocarbons have lower boiling points

Answer 7

-branches mean less points of contact
-so weaker london forces
-less energy needed to overcome these forces
-so boiling point decreases

Question 8

how is a sigma bond formed in alkanes

Answer 8

-one sp3 orbital from each carbon overlap to form a single C-C bond called a σ bond

Question 9

why are alkanes quite unreactive

Answer 9

-C-C and C-H bonds are strong
-C-C bonds are non polar
-C-H bonds are virtually non polar
-molecules are symmetrical so are non polar

Question 10

what does fuel do

Answer 10

-releases heat energy when burnt

Question 11

describe how alkanes burn in oxygen

Answer 11

-burn completely and readily
-highly exothermic
-form CO2 and H2O
-so used as fuels

Question 12

what is the combustion of alkanes used for

Answer 12

-powering vehicles
-most electricity

Question 13

describe what happens when complete combustion occurs

Answer 13

-in a plentiful supply of oxygen
-complete combustion of an alkane occurs
-producing CO2 and H2O

Question 14

equation for complete combustion

Answer 14

fuel + oxygen -> carbon dioxide + water

Question 15

how to balance complete combustion equations

Answer 15

-half the number of H in the fuel and put this number in front of H2O
-put number of carbons from the fuel in front of the CO2
-work out missing oxygen from reactants side

Question 16

describe what happens when incomplete combustion occurs

Answer 16

-in a limited supply of oxygen
-incomplete combustion of an alkane occurs
-producing CO, C and H2O

Question 17

equation for incomplete combustion

Answer 17

fuel + oxygen -> carbon + carbon monoxide + water

Question 18

difference in energy produced in complete and incomplete combustion

Answer 18

-incomplete combustion produces less energy per mole than complete combustion

Question 19

features of carbon monoxide

Answer 19

-highly toxic
-odourless
-can cause death if builds up in an enclosed space due to a faulty heating appliance
-can form a strong bond with haemoglobin in red blood cells that is stronger than that made with oxygen
-so oxygen is prevented from attaching to haemoglobin
-can be removed using a catalytic converter

Question 20

features of carbon (soot)

Answer 20

-can cause global dimming
-reflection of the suns light

Question 21

what is bond fission

Answer 21

-the breaking of a covalent bond

Question 22

when do alkanes react with halogens

Answer 22

-in the presence of UV light

Question 23

what is the mechanism for the reaction between an alkane and halogen

Answer 23

-free radical substitution

Question 24

what are the steps of a free radical substitution

Answer 24

-initiation
-propagation
-termination

Question 25

describe the initiation step of free radical substitution

Answer 25

-the UV light supplies the energy to break the covalent bond in a diatomic halogen molecule (homolytic fission)
-it is broken in preference to the others as it is the weakest
-the broken bond leads to formation of free radicals

Question 26

what is the initiation step in free radical substitution also called

Answer 26

-homolytic fission

Question 27

define a free radical

Answer 27

-a highly reactive species due to having an unpaired electron

Question 28

features of free radicals

Answer 28

-do not have a charge
-represented by a *

Question 29

describe the propagation step in free radical substitution

Answer 29

-the free radicals are very reactive and remove a H from the alkane
-the alkane free radical reacts with the halogen molecule to produce the main product and another halogen free radical
-producing a new molecule and new free radical

Question 30

features of propagation

Answer 30

-always two steps
-always have a free radical in the reactants and in the products
-as the halogen free radical is regenerated, it can react with several more alkane molecules in a chain reaction

Question 31

describe the termination step in free radical substitution

Answer 31

-two free radicals react to form a new non radical molecule
-this ends the chain reaction

Question 32

where does the dot go on a free radical

Answer 32

-needs to be on the correct carbon in the propagation stages

Question 33

products of free radical reactions

Answer 33

-further substitution can occur with more Hs being replaced by halogens
-substitution can also occur at different points in a carbon chain (isomers formed)
-the reaction produces a mixture of products

Question 34

disadvantage of free radical substitution

Answer 34

-not efficient
-as limited use in synthesis as making a desired product takes multiple steps