Chapter 12

Question 1

Are alkanes saturated or unsaturated hydrocarbons?

Answer 1

Alkanes are saturated hydrocarbons

Question 2

What sort of bonding do alkanes have? Draw a diagram

Answer 2

Alkanes have sigma bonds due to the overlap of the s orbitals of the covalent bonds

Question 3

What is the shape and bond angles in an alkane around each carbon?

Answer 3

Each carbon has four pairs of bonded electrons so it has a tetrahedral shape and due to the repulsion of the electron pairs 109.5 degree bond angles

Question 4

Are alkanes rigid? Why/why not?

Answer 4

Alkanes are not ridged as the sigma bonds act as axis around which the atoms can rotate.

Question 5

Do alkanes have a high or low melting point? What are the trends in melting points in alkanes?

Answer 5

Alkanes have low melting points
The melting point increases as chain length increases due to the increase in surface area for London forces to act on
Branched alkanes have lower boiling points as they increase distance between molecules so the forces are weaker

Question 6

Describe the reactivity of alkanes

Answer 6

Alkanes have a low reactivity because they have a high bond enthalpy, low polarity and strong sigma bonds present. The only reactions they take part in is combustion and free radical substitution

Question 7

Write the equation for the complete combustion of an alkane and state what it is used for

Answer 7

Alkane+ oxygen—> water + carbon dioxide

Alkanes are used as fuel

Question 8

Write the equation for the incomplete combustion of alkanes and the problems with this

Answer 8

Alkanes+ oxygen–> Carbon monoxide +water

Question 9

Write the equations for the complete and incomplete combustion of methane

Answer 9

CH4 + 2O2 –> CO2 + 2H2O

2CH4 + 3O2 –> 2CO + 4H2O

Question 10

Explain the steps of free radical substitution

Answer 10

1) Initiation- the covalent bond in the bromine/ chlorine atoms is broken by homolytic fission to form two radicals
2) Propagation- The halogen radical reacts with a C-H bond forming a methyl radical and a molecule of hydrogen bromide/chloride. The methyl radical then reacts with an atom of chlorine/ bromine forming bromomethane/ chloromethane and a chlorine/bromine radical
3) termination- two radicals collide forming a species with a paired electron.

Question 11

Draw the mechanism for the free radical substitution of methane with bromine

Answer 11

check in textbook

Question 12

What are the conditions for free radical substitution?

Answer 12

UV light