Alkanes

Question 1

What is the general formula for an alkane?

Answer 1

CnH2n+2 (For non-cylcic alkanes).

Question 2

What is the structure of alkenes? What is the bond angle?

Answer 2

Tetrahedral. 109.5 degrees.

Question 3

Why does boiling point increase as chain length increases?

Answer 3

As in a longer chained alkane, there are more points of contact between molecules leading to more van der Waals’ forces between the molecules.

Question 4

Why do branched isomers have a lower boiling point than unbranched isomers?

Answer 4

In a branched alkane there are fewer points of contact between molecules, leading to fewer van der Waals’ forces between the molecules. Also, branched molecules cannot get as close to each other as unbranched ones, again decreasing the intermolecular forces between the molecules.

Question 5

Why can’t alkanes mix with water? What can they dissolve in?

Answer 5

They are non polar. They can dissolve in most organic solvents.

Question 6

When balancing combustion equations, what must you remember?

Answer 6

The same number of CO2 molecules as carbon atoms. Half the number of H2O molecules as hydrogen atoms.

Question 7

What are the conditions for the halogenation of alkanes?

Answer 7

Must be ultraviolet light present or a temperature of about 300 degrees Celsius.

Question 8

What is the initiation stage for chlorination of methane?

Answer 8

Cl-Cl —->(UV light) 2Cl.

Question 9

What are the two steps for the propagation stage of chlorination of methane?

Answer 9

1. Cl. + CH4 —-> CH3. + HCl

2. Cl2 + CH3. —-> CH3Cl + Cl.

Question 10

What are the possible steps for the termination of chlorination of methane?

Answer 10

1. Cl. + Cl. —-> Cl2
2. Cl. + CH3. —-> CH3Cl
3. CH3. + CH3 —-> C2H6

Question 11

What is isomerisation?

Answer 11

Isomerisation is the conversion of unbranched alkanes into branched alkanes.

Question 12

What is reforming?

Answer 12

Reforming is turning aliphatic hydrocarbons into alicyclic hydrocarbons.

Question 13

Burning hydrocarbons can increase atmospheric pollutants such as…?

Answer 13

Carbon monoxide - internal combustion engines.
Carbon dioxide - greenhouse effect.
Nitrogen oxides - acid rain.
Sulfur dioxide- acid rain.

Question 14

How do catalytic converters work?

Answer 14

They convert CO into CO2. NOx is converted into N2. Unburnt hydrocarbons are converted to H2O and CO2.

Question 15

Write the equation for the reaction between CO and NO in a catalytic converter?

Answer 15

2CO + 2NO —-> 2CO2 + N2

Question 16

Outline the stages that take place in a catalytic converter to allow CO to react with NO?

Answer 16

The CO and NO are adsorbed onto the surface. This lowers the activation energy. A Chemical reaction takes place. CO2 and N2 desorbs from the surface.

Question 17

Where do shorter chained hydrocarbons condense in a fractional column?

Answer 17

Shorter chained have lower boiling points therefore they condense at the top.

Question 18

Where do longer chained hydrocarbons condense in a fractional column?

Answer 18

Longer chained have higher boiling points therefore they condense at the bottom.

Question 19

Conditions for cracking?

Answer 19

Initially, the catalysts Al2O3 and SiO2 were used. However, in modern day industry cracking uses a zeolite catalyst at around 450 degrees Celsius.