Alkanes

Question 1

What is an alkane?

Answer 1

A saturated hydrocarbon containing only C-H bonds

Question 2

What is the general formula of an alkane?

Answer 2

CnH2n+2

Question 3

Explain why their bonds are non-polar

Answer 3

Carbon and hydrogen have similar electronegativities
Non-polar bonds means Van der Waals forces

Question 4

What is the solubility and reactivity of alkanes?

Answer 4

Insoluble in water
Very reactive

Question 5

How does fractional distillation work?

Answer 5

1. Crude oil heated
2. Passed into column that’s cooler at the top
3. Vapours rise up the column and condense at their boiling points
4. Shorter hydrocarbons condense closer to the top

Question 6

What are the pro and cons of cracking?

Answer 6

+ reduces imported gas and electricity
+ gas supply for many years
- lots of traffic to local areas
- pollutes water supplies

Question 7

Why are alkanes cracked?

Answer 7

To turn longer chain alkanes in shorter ones
More economically valuable

Question 8

What are the conditions for thermal cracking?

Answer 8

Temperature = 1200K
Pressure = up to 700kPa

Question 9

What are the conditions for catalytic cracking?

Answer 9

Temperature = 720K
Lower pressure
Zeolite catalyst

Question 10

What are the main products of catalytic cracking?

Answer 10

Cycloalkanes
Aromatic hydrocarbons
Branched alkanes

Question 11

What is a fuel?

Answer 11

Something which releases heat energy when combusted

Question 12

What is incomplete combustion?

Answer 12

Combustion in a limited supply of oxygen
Produces carbon monoxide (toxic)
Typically longer chain hydrocarbons

Question 13

What are the environmental impacts of some of these compounds?

Answer 13

Nitrogen oxides —> acid rain
Sulfur dioxide —> acid rain
Carbon / Soot —> global dimming
Unburnt hydrocarbons —> photochemical smog
Carbon dioxide —> global warming

Question 14

Write an equation that desulfurises flue gas

Answer 14

CaCO3 + SO2 + 1/2 O2 —> CaSO4 + CO2

Question 15

How do greenhouse gases contribute to global warming?

Answer 15

They trap infrared radiation in the atmosphere
Atmosphere heats up = global warming

Question 16

Define carbon neutral activities

Answer 16

Activities that produce no net carbon dioxide emissions

Question 17

Explain the 3 stages of free radical substitution

Answer 17

1. Initiation = breaking halogen bond to form free radicals
2. Propagation = products formed but free radicals remain
3. Termination = free radicals removed, stable products formed

Question 18

What conditions are needed for a free radical chlorine atom?

Answer 18

Presence of UV light

Question 19

Write the free radical substitution of methane with chlorine to make CH3Cl

Answer 19

Initiation:
Cl2 —> 2 Cl* (presence of UV light)
Propagation:
Cl* + CH4 —> HCl + CH3
CH3 + Cl2 —> CH3Cl + Cl
Termination:
CH3 + Cl —> CH3Cl
2Cl —> Cl2
*CH3 + *CH3 —> CH3CH3

Question 20

How do chlorine free radicals catalyse the decomposition of O3?

Answer 20

Cl2 —> 2 Cl* (presence of UV light)
Cl* + O3 —> ClO* + O2
ClO* + O3 —> Cl* + 2 O2
Overall : 2 O3 —> 3 O2