3.2 Alkanes Flashcards

1
Q

What is an alkane ?

A

A saturated hydrocarbon containing only carbon-hydrogen bonds.

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2
Q

What is the general formula of an alkane?

A

CnH2n+2.

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3
Q

Do alkenes have polar bonds ? Why / why not ?

A

They don’t, this is because they have similar electronegativities.

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4
Q

What intermolecular forces do alkanes have ? Why ?

A

They only have van der waals, they are non polar.

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5
Q

Are alkanes soluble in water ?

A

They are insoluble because the hydrogen bonds in water are stronger than alkanes’ van der waals forces.

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6
Q

How reactive are alkanes ?

A

Very unreactive.

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7
Q

Which reactions will alkanes undergo ?

A

Combustion and reactions with halogens.

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8
Q

What is crude oil? Is it renewable?

A

•It is a mixture of fractions (hydrocarbons with different boiling points)
•They are formed from biological matter at high temperatures of pressure over millions of years - unrenewable.

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9
Q

What is fractional distillation and how does it work with crude oil ?

A

•Crude oil is heated until mostly vaporised
•It is then passed into a fractionating column (cooler at top)
•Vapours rise up the column and condense when temperature < their bp
•Shorter chains will condense at the top as their bp is lowest.

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10
Q

Why are alkanes cracked ?

A

Long chain alkenes can be cracked into a shorter chain alkane and shorter chain alkene, which are both more valuable.

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11
Q

What are the conditions for thermal cracking ?

A

700-1200K
Up to 7000kPa.

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12
Q

What is the intermediate for the thermal cracking ?

A

Free radicals.

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13
Q

What are the main products of thermal cracking ?

A

Shorter chain alkenes.

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14
Q

What are the conditions for catalytic cracking?

A

Lower temp (720K)
Lower pressure but higher than atmospheric
Zeolite catalyst with a honeycomb structure for high SA.

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15
Q

What are the main products of catalytic cracking ?

A

Cycloalkanes, aromatic hydrocarbons and branched alkanes.

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16
Q

Write an equation for the complete combustion of propane

A

C3H8 + 5O2 -> 3CO2 + 4H2O

17
Q

What is a fuel ?

A

Something that releases heat energy when combusted.

18
Q

What are the 5 main alkane fuels ?

A

Methane, butane, propane, diesel (~C8), paraffin (C10-18).

19
Q

What are the products for the incomplete combustion of alkanes ? What are their harmful effects ?

A

CO - poisonous
C - soot, carbon dimming.

20
Q

What type of hydrocarbons are more likely to undergo incomplete combustion ?

A

Longer chain alkanes.

21
Q

What is the environmental impacts of CO being released ?

A

It is toxic / poisonous.

22
Q

What is the environmental impact of nitrous oxides being released ?

A

They form nitric acid -> acid rain.

23
Q

What is the environmental impact of sulfur impurities being released ?

A

Form sulphuric acid -> acid rain.

24
Q

What is the environmental impact of C being released into the atmosphere ?

A

Asthma, cancer, global dimming.

25
Q

What is the environmental impact of CO2 being released into the atmosphere?

A

Greenhouse gas -> global warming.

26
Q

What are flue gasses ?

A

The waste gasses released by power stations - often contain sulfur impurities.

27
Q

What are catalytic converters made up of ?

A

Ceramic honeycomb coated with platinum, palladium or rhodium.

28
Q

What reactions do catalytic converters catalyse ?

A

2CO (g) +2NO (g) -> N2 (g) + 2CO2 (g)
Hydrocarbons + NO -> N2 + CO2 + H2O

29
Q

What are greenhouse gasses ?

A

Gasses which trap IR, increasing the temp of the earths atmosphere.

30
Q

What is the greenhouse effect and how does it contribute to global warming ?

A

Greenhouse gasses trap infrared radiation in the atmosphere, atmosphere heats up -> global warming.

31
Q

Define carbon neutral activities

A

Activities that produce no net / overall CO2 emissions.

32
Q

What reaction produces halogenoalkanes from alkanes ?

A

Free radical substitution.

33
Q

What are the stages of free radical substitution ?

A

Initiation - breaking halogen covalent bond to form free radicals
Propagation - chain part of the reaction where products are made but free radical remains
Termination - free radicals removed, stable products formed.

34
Q

What are the conditions needed to form free radicals ?

A

Presence of UV light.

35
Q

Write all stages for the free radical reaction of CH4 with Cl2

A

I •Cl2 -> 2Cl•
P1 •CH4 + Cl• -> C•H3 +HCl
P2 •C•H3 + Cl2 -> CClH3 + Cl•
T • C•H3 + C•H3 -> C2H6.

36
Q

How is the ozone layer important for the environment ?

A

It protects the earth from exposure to too many harmful UV rays.

37
Q

How do CFCs break the ozone layer down ?

A

Free radical substitution.

38
Q

Write an overall equation for the decomposition of ozone into oxygen

A

2O3 -> 3O2.

39
Q

Write the free radical substitution equation to show how CFCs convert ozone into oxygen

A

Cl2 -> 2Cl•
Cl• + O3 -> ClO• + O2
ClO• + O3 -> 2O2 + Cl•.