Chapter 12

Question 1

What are alkanes?

Answer 1

Alkanes are saturated hydrocarbons, containing only carbon and hydrogen atoms joined together by single covalent bonds.

Question 2

What is a sigma bond?

Answer 2

A sigma bond is an overlap of of orbitals directly between the bonding atoms. Rotation can occur around a sigma bond.

Question 3

What shape and bond angles are alkanes? Why?

Answer 3

They have a tetrahedral shape and have a bond angle of 109.5. This is because there is 4 pairs of electrons around the central carbon to achieve minimum repulsion.

Question 4

Why do alkanes have an increasing boiling point?

Answer 4

The increasing boiling point can be explained by the number of electrons in the bigger molecules causing an increase in London forces between molecules.

Question 5

What effect does the chain length have on boiling point?

Answer 5

Longer chain length has a larger surface area of contact between molecules for London forces to form, which increases the boiling point.

Question 6

What effect does branching have on boiling point?

Answer 6

As branching increases, there are fewer surface points of contact, leading to less London forces being formed. Also, as branching increases, molecules can’t get as close together as straight chains, this leads to weaker London forces. As branching increase, decreases the boiling point.

Question 7

Why do alkanes have low reactivity with many reagents?

Answer 7

This is because of the high enthalpies of c-c and c-h bonds and the very low polarity of the sigma bond.

Question 8

Why are alkanes used as fuels?

Answer 8

Alkanes are used as fuels as they readily undergo combustion and release a lot of energy due to their very exothermic combustion enthalpy changes. (Fuels- release energy when burnt)

Question 9

What is complete combustion?

Answer 9

It occurs when there is excess oxygen and burns with a clean, often blue flame.

Question 10

What products are formed from complete combustion

What are the impacts of these?

Answer 10

Water (H2O) and Carbon dioxide (CO2). These are both greenhouse gases and are known to contribute towards global warming when released into the atmosphere.

Question 11

What is the equation to work out complete combustion?

Answer 11

CxHy + (x+y/4) = xCO2 + y/2H20

Question 12

What is incomplete combustion?

Answer 12

It occurs when there is a limited supply of oxygen and burns directly with a dirty yellow flame.

Question 13

In complete combustion, when another -CH2- gets added how much oxygen is needed?

Answer 13

1.5 O2

Question 14

What products are formed from incomplete combustion?

Answer 14

CO (carbon monoxide) or CO2 and H2O

Question 15

What are the impacts of Carbon monoxide?

Answer 15

It is a colourless, odourless, highly toxic gas and can combine with haemoglobin in the body, blocking the transport of oxygen to cells.

Question 16

What is the mechanism for bromination of alkanes?

Answer 16

Radical substution

Question 17

What is the essential condition for radical substution?

Answer 17

UV- ultraviolet radiation

Question 18

What the three stages of radical substution?

Answer 18

1- Initiation
2- Propagation
3- Termination

Question 19

What happens at initiation?

Answer 19

The UV light supplies the energy to break the bond. The bond is broken by a process called homolytic fission which forms free radicals.

Question 20

What happens at propagation?

Answer 20

The free radicals are used and remade, causing a chain reaction. Propagation can happen 1000s times before termination occurs. Radicals are on both side of the equation. Adding together the propagation steps gives the overall equation.

Question 21

What happens at termination?

Answer 21

Free radicals react together to form a stable molecule,, using them up and stopping the process. There are multiple termination reactions that may occur. Two radicals collide, forming a molecule with all electrons paired. When two radicals collide and react, both radicals are removed from the reaction mixture, stopping the reaction.

Question 22

What are the limitations of radical substitution?

Answer 22

• The substution can occur at different positions in the carbon chain. This results in a mixture of products.
• Further substution- Radical substution reactions are difficult to control, resulting the formation of products.

Question 23

An example of further substutution.

Answer 23

Once the haloalkane is formed in the
propagation step another bromine radical
could attack it to form a dihaloalkane. This can continue until all hydrogen atoms have been substituted with bromine.