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A-Level Chemistry > Alkanes > Flashcards

Alkanes Flashcards

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1
Q

are alkanes saturated or unsaturated? why?

A
  • saturated
  • each carbon is bonded 4 times with the maximum number of hydrogens possible
  • there are no double bonds
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2
Q

why do cycloalkanes differ from alkanes? how are they similar?

A
  • they are a functional group isomer of alkenes
  • general formula CnH2n
  • still saturated
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3
Q

what is crude oil?

A

a mixture of different length hydrocarbons

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4
Q

describe the process of fraction distillation

A
  • crude oil vaporised to 350c using a furnace
  • vapour oil enters the column and rises through the trays, the longest hydrocarbons do not vaporise and run down to the bottom
  • column has a negative temperature gradient - cooler at the top
  • fractions condense at different heights due to the mixture of different length chains and therefore the differing boiling points (fractions separated by bp)
  • when they reach a level at a lower temp than their boiling point, they condense and are removed
  • small molecules condense at the top at lower temperatures and big molecules condense at the bottom at higher temperatures
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5
Q

what is petroleum?

A

mixture consisting mainly of alkane hydrocarbons that can be separated by fractional distillation

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6
Q

what is cracking? why is it useful?

A
  • breaking of longer chain hydrocarbons to shorter chain hydrocarbons through the breaking of a C-C bond
  • heavier fractions can be cracked to lighter fractions which are of a higher demand
  • an alkene and a shorter chain hydrocarbon is produced
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7
Q

what are the 2 types of cracking?

A
  • thermal cracking
  • catalytic cracking
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8
Q

what is thermal cracking?

A
  • high temperature and pressure used (700-1200K and 7000kPa)
  • high percentage of alkenes are produced
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9
Q

what is catalytic cracking?

A
  • higher temperature and slight pressure used (720-820K and 70-140kPa)
  • zeolite catalyst used (lowers cost + speeds up process)
  • produces mainly aromatic hydrocarbons used in fuels for vehicles
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10
Q

what are the products of complete combustion?

A

carbon dioxide and water

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11
Q

what is produced when alkanes burn with a limited oxygen supply? what will be produced if they burn with even less oxygen?

A
  • carbon monoxide and water
  • soot (carbon) and water
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12
Q

what are the problems associated with incomplete combustion?

A
  • carbon monoxide is poisonous, it binds to haemoglobin in the blood and therefore prevents oxygen from binding
  • soot causes breathing problems, makes buildings look dirty and can clog up buildings
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13
Q

what is the greenhouse effect?

A

carbon dioxide absorbs infrared radiation from the sun but emits some of it back into the earth causing a global temperature increase

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14
Q

what is photochemical smog? what are the problems associated with it?

A
  • when solid carbon particulates and ozone mix a photochemical smog is created
  • harms respiratory systems of animals and damages plants
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15
Q

what are oxides of nitrogen? what can be done to minimise the amount going into the atmosphere?

A
  • oxides of nitrogen (NOx) are made when oxygen and nitrogen found in the air combine under high pressure and temperature
  • car engines provide these conditions
  • catalytic converters help to reduce the amount of unburnt hydrocarbons and NOx going into the atmosphere
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16
Q

What is acid rain? How does it form and why is it dangerous?

A

-burning fossil fuels can release sulfur dioxide
- some fossil fuels contain Sulfur based impurities
- when burned, the sulfur reacts with Oxygen producing sulfur dioxide (SO2)
-sulfur dioxide = acidic gas - reacts with Water in the atmosphere to form sulfuric acid which falls as acid rain (NOx do the same)
-acid rain causes damage to plants, kills fish + erodes buildings

17
Q

How can SO2 be removed from flue gases?

A
  • wet scrubbing
  • an alkali is used to neutralise sulfur dioxide in flue gases
  • involves dissolving calcium carbonate, oxide in water and spraying acidic SO2 gas
18
Q

state the 3 steps of free radical chain reactions

A
  • initiation
  • propagation
  • termination
19
Q

what happens in the initiation stage of a free radical chain reaction ?

A

a radical is created using UV light (photochemical reaction)

20
Q

what happens during the propagation stage of a free radical chain reaction?

A

when a radical reacts with a non-radical molecule, new radicals are created + then react further with more non-radicals, resulting in a chain reaction

21
Q

what happens in the termination stage of a free radical chain reaction?

A

2 radicals react, forming a non-radical therefore ending the chain reaction

22
Q

what happens in the initiation stage of the free radical substitution reaction with methane and chlorine? give any appropriate equations.

A
  • the Cl-Cl bond is broken by UV light (photodissociation)
  • 2 highly reactive radicals are produced
    Cl2 -UV-> 2Cl○
23
Q

what happens in the propagation stage of the free radical substitution reaction with methane and chlorine? give any appropriate equations.

A
  • Cl radical reacts with a methane molecule to make a methyl radical (○CH3) and HCl
  • the methyl radical reacts with a Cl2 molecule, forming a chloromethane and a Cl○ radical
  • this Cl○ radical can react with more methane
    Cl○ + CH4 –> HCl + ○CH3
    ○CH3 + Cl2 –> CH3Cl + Cl○
24
Q

what happens in the termination stage of the free radical substitution reaction with methane and chlorine? give any appropriate equations.

A
  • 2 radicals react to make a stable non-radical molecule
    Cl○ + Cl○ –> Cl
24
Q

what happens in the termination stage of the free radical substitution reaction with methane and chlorine? give any appropriate equations.

A
  • 2 radicals react to make a stable non-radical molecule
    Cl○ + Cl○ –> Cl2
    ○CH3 + ○CH3 –> CH3CH3 (C2H6)
    Cl○ + ○CH3 –> CH3Cl
  • if many Cl○ present, will get a di/tri/tetra chloromethane (CH2Cl2/CHCl3/CCl4)
  • if not, mainly just get single substitution (CH3Cl)

A-Level Chemistry (23 decks)

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