Topic 3.2 - Alkanes

What is an alkane?

A saturated hydrocarbons

containing C-H bonds only

What is the general formula

of an alkane?

C n H 2n+2

Are their bonds polar?

Why/why not?

Nonpolar- carbon and

hydrogen have similar

electronegativities

Which intermolecular forces

do they have? Why?

Only van der Waals forces of

attraction - bonds are

non-polar

Are they soluble in water?

why?

Insoluble because hydrogen bonds in

water are stronger than alkanes’ van der

Waals forces of attraction

How reactive are alkanes?

Very unreactive

Which reactions will alkanes

undergo?

Combustion and reaction with

halogens

What is crude oil? How is it

formed? Is it renewable?

Why?

Mixture of fractions (hydrocarbons with similar

boiling points and properties)

Formed at high temperatures and pressures

deep below earth’s surface over millions of years

→ therefore non-renewable

Name the fractions from

high to low boiling point.

Gases - fuel on site

Gasoline/petrol/naphtha - cars

Kerosene/paraffin - jet fuel, lighting

Diesel oil - lorries/taxis

Lubricating oil/waxes - candles, engine oil

Fuel oil - ships, power stations

Tar/bitumen - roads/roofing

What is fractional

distillation/how does it

work?

Crude oil heated until mostly vapourised

Passed into a fractionating tower that is cooler at the top than the bottom

Liquid fractions are piped off at the bottom

Vapours rise up the column and - via trays and bubble caps - condense

when temperature < their boiling point

Shortest chain hydrocarbons condense at the top as they have the lowest

boiling points

What is fracking and how is

it done?

Natural gas held within shale rock

Drill into shale, force pressurised water and sand into rock to

fracture it, Collect gas

HCl and methanol added to break up shale and prevent

corrosion

Pros/cons of fracking?

Advantages - gas supply for many years, reduces imported

gas and electricity

Disadvantages - lots of traffic to local area, concern about

amount of water used, chemical additives can pollute water

supplies, can cause small earthquakes, combust CH 4 → CO 2

→ global warming

Why are alkanes cracked?

To turn a long chain alkane, with is not very

economically valuable, into a shorter chain

alkane (more economically valuable as can be

used as a fuel) and an alkene (more reactive,

starting point for many products)

What are the conditions for

thermal cracking?

700-1200 K temperature

Up to 7000 kPa pressure

What is the intermediate for

the reaction?

Free radicals

What are the main products

of thermal cracking?

alkenes

What are the conditions for

catalytic cracking?

Lower temp (720K)

Lower pressure (but above atmospheric)

Zeolite catalyst (SiO 2 and Al 2

O 3

) with a

honeycomb structure to give a large surface area

What are the main products

of catalytic cracking?

Cycloalkanes, aromatic hydrocarbons,

branched alkanes

Write an equation for the

combustion of propane

C 3 H 8 + 5O 2 → 3CO 2 + 4H 2 O

What is a fuel?

Something which releases heat energy

when combusted

What are the five main fuels

comprising of alkanes?

Methane, butane, propane, petrol (about

C8), paraffin (C10 - C18)

What is incomplete

combustion and what

products are formed in the

case of alkanes?

Combustion in a limited supply of oxygen

CO - carbon monoxide - poisonous

C - carbon - particulates - soot - global dimming

Which type of hydrocarbons

are most likely to undergo

incomplete combustion?

Longer chains

What is the environmental

impact of carbon monoxide?

It is toxic/poisonous

What is the environmental

impact of nitrogen oxides?

form nitric acid → acid rain,

photochemical smog

What is the environmental

impact of sulfur

impurities/sulfur dioxide?

form sulphuric acid → acid rain

What is the environmental

impact of soot (carbon)?

asthma, cancer, global dimming

What is the environmental

impact of unburnt

hydrocarbons?

Photochemical smog

What is the environmental

impact of carbon dioxide?

greenhouse gas → global warming,

increases global temperatures,

speeds up climate change

What is the environmental

impact of water vapour?

greenhouse gas → global warming,

increases global temperatures,

speeds up climate change

What are flue gases?

Gases given out by power stations

Write two equations for two

different ways of

desulfurising flue gases.

CaO (s) + 2H 2 O (l) + SO 2 (g) + ½O 2 (g) → CaSO 4 .2H 2 O (s)

Or CaCO 3 (s) + SO 2 (g) + ½O 2 (g) → CaSO 4 + CO 2 (g)

What are catalytic

converters made up of?

Ceramic honeycomb coated with

platinum, palladium and rhodium (Pt,

Pd and Rh) metals

What do catalytic converters

catalyse (equations)?

They catalyse these reactions of products from

car exhausts:

2CO (g) + 2NO (g) → N 2 (g) + 2CO 2 (g)

Hydrocarbons + NO → N 2 + CO 2 + H 2

O

What are greenhouse

gases?

Gases which trap infrared radiation,

making the earth act like a greenhouse

What is the greenhouse

effect and how does it

contribute to global

warming?

Greenhouse gases trap infrared radiation in the

atmosphere, atmosphere heats up → global

warming

Define carbon neutral

activities

Activities that produce no net / overall

carbon dioxide emissions

How are halogenoalkanes

formed from alkanes?

Free radical substitution reaction

What are the three stages of

free radical substitution?

Initiation - breaking halogen bond to form free radicals

Propagation - chain part of the reaction where products are

formed but free radical remains

Termination - free radicals removed, stable products formed

What are the conditions

needed for the formation of

a free radical chlorine atom?

Presence of UV light

Write equations for the

reaction of CH 4 with Cl 2 to

form CH 3 Cl

Initiation: Cl 2 → 2Cl• (in presence of UV light)

Propagation: Cl• + CH 4 → HCl + •CH 3

•CH3 + Cl 2 → CH 3

Cl + Cl•

Termination:

•CH3 + Cl• → CH 3

Cl

2Cl• → Cl 2

•CH3

+ •CH 3

→ CH 3

CH3

What is the ozone layer’s

function?

protects the earth from harmful exposure to too many UV rays

How do CFCs break the

ozone layer down?

Free radical substitution

Write an equation for the

overall decomposition of

ozone into oxygen (O 2 )

2O 3 → 3O

Write free radical

substitution equations to

show how Cl free radicals

catalyse the break down of

O

Cl 2 → 2Cl• (in presence of UV light)

Cl• + O 3 → ClO• + O 2

ClO• + O 3 → 2O 2 + Cl•

Overall: 2O 3 → 3O 2