4.1.2: Alkanes

Question 1

What is the general formula of alkanes ?

Answer 1

CnH2n+2

Question 2

What is the general formula of cycloalkanes?

Answer 2

CnH2n

Question 3

What can alkanes be described as ?

Answer 3

saturated hydrocarbons

Question 4

What shape does every alkane form around carbon atoms ?

Answer 4

Tetrahedral (109.5)

Question 5

What bonds do alkanes contain ?

Answer 5

sigma bonds from the head on overlap of atomic s orbitals between the bonding atoms

Question 6

Explain long vs short chain alkanes

Answer 6

A longer chain has more SA points of contact meaning more London forces with more energy needed to overcome the intermolecular forces causing higher bp

Question 7

Explain straight chain vs branched alkanes

Answer 7

Straight chain have more SA points of contact so more London forces and more energy needed to overcome the intermolecular forces causing higher bp

Question 8

What are the products from the complete combustion of alkanes ?

Answer 8

H20 and C02

Question 9

What is the test for H20

Answer 9

Dry cobalt chloride paper blue to pink
or Dry copper sulfate white to blue

Question 10

What is produced from the incomplete combustion of alkanes ?

Answer 10

CO and H20

Question 11

How is CO dangerous and how can it be removed

Answer 11

TOXIC as binds to haemoglobin in blood and stops oxygen binding (respiratory problems)
- removed with catalytic converter

Question 12

How do alkanes react ?

Answer 12

Free radical reactions

Question 13

What are the 3 stages of free radical substitution ?

Answer 13

• Initiation
• Propogation
• Termination

Question 14

What does free radical substitution form ?

Answer 14

Halogenoalkanes

Question 15

What conditions does free radical substitution require ?

Answer 15

UV light

Question 16

What type of bond fission is involved in free radical substitution ?

Answer 16

Homolytic

Question 17

What is an issue with free radical substitution and how can it be mitigated ?

Answer 17

Makes a mixture of further products as chlorine keeps reacting to for di,tri haloalkane so add excess methane for the chlorine to react with

Question 18

Why will isomers often be produced from free radical substitution

Answer 18

As free radical substitution could occur anywhere on the hydrocarbon chain

Question 19

Explain reactivity of alkanes

Answer 19

Low reactivity as high bond enthalpies and low polarity of sigma bonds

Question 20

Explain reactivity of fluoroalkanes

Answer 20

Very unreactive due to strength of C-F bonds which are stronger than C-H bonds and require lots of energy to break

Question 21

Give a problem of using radical substitution in the reaction between chlorine and cyclohexane

Answer 21

• More than one hydrogen atom is replaced(multisubstitution)
• Substitution at different points along the chain

Question 22

Why are alkanes generally unreactive ?

Answer 22

Bonds are non polar so atoms have similar electronegativity and they contain strong covalent bonds

Question 23

Write an equation for the formation of C4Cl10 from butane

Answer 23

C4H10 + 10 Cl2 → C4Cl10 + 10 HCl

Propogation step without radicals

Question 24

What intermolecular forces are present in alkanes ?

Answer 24

London forces

Question 25

How many termination steps are there usually in free radical substitution ?

Answer 25

3

Question 26

Why are branched chain alkanes desired in the petroleum industry

Answer 26

Branched chains have more efficient combustion so make more efficient fuels

Question 27

Name the process where long chain alkanes are converted to short chain alkanes

Answer 27

Cracking

Question 28

What conditions lead to incomplete combustion ?

Answer 28

Insufficient supply of oxygen