Alkanes

Question 1

what are alkanes?

Answer 1

-naturally occurring hydrocarbons formed from crude oil

Question 2

what are alkanes formed by?

Answer 2

The conversion of animal and vegetable matter, which existed under pressure for millions of years

Question 3

why are alkanes saturated compounds?

Answer 3

since they contain single c-c and c-h covalent bonds only

Question 4

what is the shape of an Alkane?

Answer 4

-Each carbon bond makes four bonds
-Around each atom, there are 4 bonding pairs of electrons which repel.
-Thus each carbon atom is at the centre of a tetrahedral structure, with either carbon or hydrogen atoms at the apices of the tetrahedron

Question 5

What are the bond angles of alkanes?

Answer 5

109°

Question 6

What are all bonds between carbon atoms or between a carbon and hydrogen atom?

Answer 6

-Sigma bonds

Question 7

what are all sigma bonds able to do?

Answer 7

freely rotate

Question 8

what are sigma bonds formed by?

Answer 8

Sigma bonds are formed by the direct overlap of orbitals directly between the bonding atoms

Question 9

what does alkanes being saturated hydrocarbons mean?

Answer 9

They have no double or triple bonds

Question 10

What are the three distinctive types of alkanes?

Answer 10

-straight chained alkanes
-branched chained alkanes
-ring or cycloalkanes

Question 11

What is an example of a straight chained alkane?

Answer 11

Butane

Question 12

What is an example of a branched chained alkane?

Answer 12

2-methylpropane

Question 13

What is an example of a ring or cyclo alkane?

Answer 13

cyclobutane

Question 14

What type of forces are between the many (successive) molecules of an alkanes?

Answer 14

Induced dipole dipole interactions

Question 15

What happens to boiling point as the length of the carbon chain increases?

Answer 15

-Boiling point increases
-Larger molecules have greater surface contact and therefore greater induced dipole-dipole interactions
-more energy is required to overcome the greater induced dipole-dipole forces and separate the molecules so the boiling point is higher

Question 16

why are the attractive forces between branched-chained molecules less than the forces between straight-chained molecules?

Answer 16

-because the branched chained molecules cannot get as close together as the straight chain ones, so the induced dipole dipole forces are weaker and easier to break
-The surface contact of branched-chaine isomers is less than that between straight-chain isomers

Question 17

why is the boiling point, melting point and density of the straight chain alkane higher corresponding branched isomer?

Answer 17

more energy is needed to break the induced dipole dipole forces in a straight chained alkane

Question 18

Why do all show little reaction?

Answer 18

-they have strong single bonds (both C-H and C-C have high bond enthalpies )
-They have no polar bonds

Question 19

when is a polar bond formed?

Answer 19

when two elements of differing electronegativities are bonded together

Question 20

Why is the bond between carbon and hydrogen only very slightly polar?

Answer 20

-carbon and hydrogen have similar electronegativities
-Consequently, common chemical reagents do not readily attack alkanes

Question 21

when do alkanes burn completely to produce carbon dioxide and water?

Answer 21

in a plentiful supply of oxygen

Question 22

What is oxidation of alkenes the basis for?

Answer 22

-their use as fuels

Question 23

what is natural gas used for? (burning of alkanes)

Answer 23

-heating purposes

Question 24

what is diesel used for? (alkanes)

Answer 24

power

Question 25

What happens to alkanes in a limited supply of oxygen?

Answer 25

alkanes burn to form carbon monoxide and water
-CH4 (g) + 1/2 O2 (g) —> CO (g) 2H2O (g)

Question 26

what indicates in incomplete combustion?

Answer 26

-The black substance (soot) produced by a smoky flame (which is largely carbon)

Question 27

What are the dangerous arising from carbon monoxide?

Answer 27

-O2 carried around body in rbc’s by haemoglobin
-The bonding between O2 and Hb is quite weak to allow transfer for cell respiration
-Bonding between O2 and CO is stronger and irreversible so O2 is replaced by CO and blocks normal cell respiration
-The consequences are reduced blood oxygen concentration leading to unconsciousness and eventually death

Question 28

when do alkanes react with halogens?
(halogenation)

Answer 28

in the presence of ultraviolet light, they undergo a series of substitution reactions

Question 29

Why is hydrogenation an example of a photo chemical reaction?

Answer 29

It’s only occurs in the presence of light

Question 30

what must happen to the covalent bonds for halogenation to take place?

Answer 30

-they have to be broken homiletic to form free radicals
-In these reactions, a hydrogen atom alkane is substituted by a halogen atom

Question 31

what happens when an alkane and halogen react?

Answer 31

Haloalkane and H (X)

Question 32

What is the mechanism for free radical substitution of alkanes?

Answer 32

-initiation
-Propagation
-Termination

Question 33

What is a mechanism?

Answer 33

A sequence of steps leading to the formation of a product

Question 34

What are the conditions for radical substitution? (of chlorine and methane)

Answer 34

Reagents- Methane and Chlorine
-Conditions- Ultra Violet Light