Alkanes

Question 1

What is the hybridisation of carbon in alkanes?

Answer 1

sp³ hybridised

Question 2

What is the general formula for alkanes?

Answer 2

C_nH_(2n+2)

for non-cyclic alkanes

Question 3

Why aren’t alkanes very reactive?

Answer 3

They are saturated hydrocarbons and are stable

Question 4

What reactions do alkanes undergo?

Answer 4

1. Oxidation
   CH_{4 (g)} + O_{2 (g)} → CO_{2 (g)} + H₂O _(g)
2. Substitution

Question 5

What is the name for the substitution reactions in alkanes?

Answer 5

Free radical substitution reactions

These are necessary because free radials are very reactive, instigating a reaction in alkanes

Question 6

What can be said about the physical state of alkanes?

Answer 6

The first 3 exist as volatile gases but their state increases with an increase in ‘n’ due to increases van De Waal’s forces

Question 7

How many ‘umbrella’ steps are there in a free radical substitution reaction?

Answer 7

Three: Initiation, Propagation and Termination

Question 8

What happens in the initiation step?

Answer 8

The first radicals are formed

Question 9

What happens in the propagation step?

Answer 9

The first radicals react with compounds and more radicals are formed

Question 10

What happens in the termination step?

Answer 10

Radicals combine with each other to form compounds

Similar/dissimilar radicals can combine

Question 11

What is the catalyst in the reaction of chlorine gas with an alkane?

Answer 11

UV light/radiation (from the sun)

Be sure to specify

Question 12

What is the catalyst in the reaction of chlorine gas with an alkane?

Answer 12

UV light/radiation (from the sun)

Be sure to specify

Question 13

What does the UV catalyst do?

Answer 13

It causes the homolytic fission of the Cl-Cl bond forming two radicals ( ^. Cl)

Question 14

What does the ^. Cl radical take each time it attacks the alkane?

Answer 14

A hydrogen atom

This C-H bond is also broken homolytically, and an alkyl radical is formed, meanwhile, HCl is also formed/

Question 15

What does this alkyl radical attack?

Note the difference, carbocations are electrophiles and are not used here

Answer 15

A neutral chlorine molecule, forming another (single) chlorine radical
For methane,
^.CH₃ + Cl_{2 (g)} → CH₃Cl _(g) + ^. Cl

A similar mechanism to this continues until all the hydrogen atoms are substituted

Question 16

Do alkanes decolourise bromine water?

Answer 16

No

Question 17

Will there be any change to the reacting system

if the mixture containing the alkane and bromine water si exposed to UV light?

Answer 17

Yes

but it will be SLOW

Free radicals would be formed and a substitution reaction will occur, decolourising the solutiion

Question 18

Which has a higher boiling point?

branched or straight-chain alkanes

with the same molecular mass

Answer 18

Straight-chain alkanes

Question 19

Why do straight-chain alkanes have a higher boiling point?

Answer 19

There is a larger surface area

Question 20

What happens when incomplete combustion of alkanes occurs?

Answer 20

Carbon monoxide is formed

with water

as the carbon is not fully oxidised to carbon (IV)

Question 21

What kind of relationship can be see between the no. of carbons in an alkane and the energy released from combustion?

Answer 21

A direct relationship

The higher the molecular number, the more Σ released on combustion