14.1 alkanes

State the ways in which alkanes can be produced

Addition of hydrogen to an alkene

Cracking of a longer chain alkane

Hydrogenation reaction

Addition of hydrogen to alkene to make alkane

Ni catalyst and heat

Cracking

Heat with Al2O3

What are the steps of free radical substitution

Initiation, propagation, termination

Describe complete combustion of alkanes

When alkanes are burnt in excess of oxygen, complete combustion and co2 and H2O produced

Describe incomplete combustion of alkanes

• When alkanes are burnt in only a limited supply of oxygen, incomplete combustion will take place and not all the carbon is fully oxidised

• Some carbon is only partially oxidised to form carbon monoxide

• Carbon monoxide is a toxic, odourless gas as it will irreversible bind to haemoglobin preventing supply of oxygen to blood

• Just C (soot) also produced

Why is cracking important?

Explain why alkanes are so unreactive

Due to lack of difference in electronegativity alkanes are nonpolar molecules and have no partial positive or negative charges.

This means they do not react with polar molecules

• They have no electron-deficient areas to attract nucleophiles

• They also lack electron-rich areas to attract electrophiles

What are the environmental consequences of carbon monoxide

toxic and odourless gas that binds irreversibly to haemoglobin in the blood.

• This prevents haemoglobin from carrying oxygen

What are the environmental consequences of oxides of nitrogen

• The oxides of nitrogen are released into the atmosphere

• Car exhaust fumes also contain unburnt hydrocarbons from fuels and their oxides (VOCs)

• In the air, the nitrogen oxides can react with these VOCs to form PAN which is the main pollutant found in photochemical smog

  • Nitrogen oxides can also dissolve and react in water with oxygen to form nitric acid which is a cause of acid rain

How is carbon monoxide removed

• cars are fitted with catalytic converters

• Precious metals (such as platinum) are coated on a honeycomb to provide a large surface area

  • Oxidation of CO to CO₂:

2CO + O₂ → 2CO₂

How are nitrogen oxides removed

Catalytic converter

Reduction of NO/NO₂ to N₂:

2CO + 2NO → 2CO₂ + N₂

Summarise PAN

• Formation:

  • From the photochemical reaction of VOCs and nitrogen oxides in the atmosphere

• Environmental consequence:

  • Contributes to photochemical smog

• Catalytic removal:

  • Oxidise unburnt hydrocarbons

  • Reduce nitrogen oxides to prevent PAN formation

Summarise VOCs

• Formation:

  • Unburnt hydrocarbons from fuels

  • Oxides of these hydrocarbons formed in car engines

• Environmental consequence:

  • React with oxides of nitrogen in the atmosphere to form PAN

• Catalytic removal:

  • Oxidised to CO₂ and H₂O

  • General formula reaction: