(PMT) Alkanes Flashcards by Alice Sikora

What is an alkane?

A saturated hydrocarbon

containing C-H bonds only

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What is the general formula

of an alkane?

CnH2n+2

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Are their bonds polar?

Why/why not?

Nonpolar- carbon and
hydrogen have similar
electronegativities

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Which intermolecular forces

do they have? Why?

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

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Are they soluble in water?

why?

Insoluble because hydrogen bonds in
water are stronger than alkanes’ van der
Waals forces of attraction

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How reactive are alkanes?

Very unreactive

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Which reactions will alkanes

undergo?

Combustion and reaction with

halogens

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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

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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

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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

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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)

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What are the conditions for

thermal cracking?

700-1200 K temperature (about 1000C)

Up to 70 atm pressure (up to 7000 kPa)

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What is the intermediate for

the reaction?

Free radicals

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What are the main products

of thermal cracking?

alkenes

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What are the conditions for

catalytic cracking?

Lower temp (720K)
Lower pressure (but above atmospheric)
Zeolite catalyst (SiO2
 and Al2O3) with a
honeycomb structure to give a large surface area

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What are the main products

of catalytic cracking?

Cycloalkanes, aromatic hydrocarbons,

branched alkanes

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Write an equation for the (complete) combustion of propane

C3H8 + 5O2 → 3CO2+ 4H2O

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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.

1. CaO (s) + 2H2O (l) + SO2(g) + ½O2(g) → CaSO4.2H2O (s) | 2. 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 catalytic converters | catalyse (equations)?

They catalyse these reactions of products from car exhausts: 2CO (g) + 2NO (g) → N2(g) + 2CO2(g) Hydrocarbons + NO → N2+ CO2+ H2O

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 CH4 with Cl2 to form CH3Cl

Initiation: Cl2→ 2Cl• (in presence of UV light) Propagation: Cl• + CH4→ HCl + •CH3 •CH3 + Cl2 → CH3Cl + Cl• Termination: •CH3 + Cl• → CH3Cl 2Cl• → Cl2 •CH3+ •CH3→ CH3CH3

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 break down of O3 ```

Cl2 → 2Cl• (in presence of UV light) Cl• + O3→ ClO• + O2 ClO• + O3 → 2O2 + Cl• Overall: 2O3→ 3O2