12 - Alkanes Flashcards

1
Q

What is the general formula for an alkane?

A

C(n)H(2n+2)

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2
Q

Describe the polarity and intermolecular forces in alkanes.

A

Non-polar
Form only van der Waals

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3
Q

What are alkanes?

A

Saturated hydrocarbons.

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4
Q

What is petroleum?

A

A mixture containing mostly alkane hydrocarbons that can be separated by fractional distillation.

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5
Q

Describe the conditions for thermal cracking.

A

High (700-1200K) temperature, high pressure (up to 7000kPa/70 atm)

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6
Q

Give the conditions for catalytic cracking.

A

High but lower temperature (720K), slight pressure, zeolite catalyst.

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7
Q

What does catalytic cracking produce?

A

Motor fuels and aromatic compounds.

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8
Q

What occurs in thermal cracking?

A

C-C bonds are broken.

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9
Q

What does thermal cracking produce?

A

Mostly alkenes.

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10
Q

Where is the fractionating column hottest?

A

The bottom

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11
Q

Describe the process of fractional distillation. (3)

A

Crude oil vapourised
Gases enter fractionating column
Gases condense at different points due to their different boiling point
Each fraction is a mixture

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12
Q

Why is cracking done?

A

Shorter chains are more useful
Alkenes are more reactive and are used a chemical feedstock

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13
Q

What do alkanes completely combust to?

A

Carbon dioxide + water

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14
Q

What do alkanes incompletely combust to?

A

Carbon monoxide + C (soot) + water

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15
Q

Why are alkanes used as fuels?

A

They release high amounts of energy.

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16
Q

What harmful byproducts are produced by alkane combustion engines?

A

Carbon monoxide, nitrogen oxides

17
Q

What do catalytic convertors do?

A

Remove nitrogen oxides, carbon monoxide and unburnt hydrocarbons (particulates) from exhausts into safer compounds ie CO2 or H2O or N2

By reacting them with each other

18
Q

What metal catalyst do catalytic convertors use?

A

Rhodium catalyst

19
Q

Why are unburnt hydrocarbons/carbon particulates bad?

A

They can cause respiratory problems

20
Q

What can sulphur dioxide cause?

A

Acid rain

21
Q

How is sulfur dioxide removed from flue gases?

A

Flue gas desulfurisation , using CaO and water to produce CaSO4. CaCO3 can also be used, producing CO2 and CaSO4

22
Q

Name the three stages of a free radical substitution.

A

Initiation, Propagation, Termination

23
Q

Under what condition do alkanes react with halogens to form halogenoalkanes?

A

UV light

24
Q

What is propagation?

A

The replacement of a hydrogen with a halogen atom, and the radical halogen reforming.

25
Q

What is a radical?

A

A highly reactive molecule with an unpaired electron.

26
Q

What occurs in termination?

A

Two radicals react to form a stable compound with no unpaired electrons.

27
Q

Why might values of enthalpy for combustion of a liquid alkane be different to that using an expression using bond enthalpies?

A

Enthalpy expression uses gaseous; alkane is not gaseous