alkanes

What is an alkane?

A saturated hydrocarbons containing C-H bonds only

What is the general formula of an alkane?

Cn H2n+2

Are their bonds polar? Why/why not?

Nonpolar- carbon and hydrogen have similar electronegativities

Intermolecular forces? Why?

Only van der Waals forces of attraction - bonds are non-polar

Solubility 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?

Mixture of fractions (hydrocarbons with similar boiling points and properties)

How is it formed? Is it renewable? Why?

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

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

temp: 450 degrees

pressure 70atm (high pressure)

no catalyst

prodyucts : alkene

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?

450C

1-2 atm

Zeolite catalyst (SiO2 and Al2 O3 ) with a honeycomb structure to give a large surface area

motor fuels ( cyclic alkanes, branched alkanes aromatics

What are the main products of catalytic cracking?

Cycloalkanes, aromatic hydrocarbons, branched alkanes

Write an equation for the combustion of propane

C3 H8 + 5O2 → 3CO2 + 4H2 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) + 2H2 O (l) + SO2 (g) + ½O2 (g) → CaSO4 .2H2 O (s) Or

CaCO3 (s) + SO2 (g) + ½O2 (g) → CaSO4 + CO2 (g)

What are catalytic converters made up of?

Ceramic honeycomb coated with platinum, palladium and rhodium (Pt, Pd and Rh) metals

What do they catalyse (equations)? catalytic converters

They catalyse these reactions of products from car exhausts:

2CO (g) + 2NO (g) → N2 (g) + 2CO2 (g)

Hydrocarbons + NO → N2 + CO2 + H2 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 forme

What are the conditions needed for the formation of a free radical chlorine atom?

Presence of UV light

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 (O2 )

2O3 → 3O2

Write free radical substitution equations to show how Cl free radicals catalyse the decomposition of O3

Cl2 → 2Cl• (in presence of UV light)

Cl• + O3 → ClO• + O

2 ClO• + O3 → 2O2 + Cl•

: 2O3 → 3O2