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A-level chemistry > Alkanes > Flashcards

Alkanes Flashcards

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1
Q

alkane general formula

A

CnH2n+2

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2
Q

what is a hydrocarbon

A

a compound made of only hydrogen and carbon atoms

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3
Q

saturated or unsaturated and why

A

alkanes and cycloalkanes are saturated hydrocarbons because they contain c-c single bonds

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4
Q

general formula of a cycloalkane

A

CnH2n

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5
Q

define heterolytic bond fission

A

when the bond breaks unevenly (one bonded atom gets both electrons)

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6
Q

heterolytic bond fission arrows

A

X—– Y -> X+ + Y-
—>

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7
Q

define homolytic bond fission

A

when the bond breaks evenly (both bonded atoms receive an electron)

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8
Q

homolytic bond fission arrows

A

X—–Y -> X* + Y*
<—–>

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9
Q

what is produced through homolytic bond fission and what are they

A

free radicals (very reactive species that have an unpaired electron)

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10
Q

what type of reactions are reactions with halogens

A

photochemical reactions and radical substitution reactions

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11
Q

define photochemical reaction

A

reaction that requires UV light

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12
Q

why are reactions with halogens substitution

A

because hydrogen is substituted

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13
Q

symbol equation for methane + chlorine

A

CH4 + Cl2 -> CH3Cl + HCl

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14
Q

name and describe the three steps in reactions with halogens

A

1- initiation (radical produced)
2- propagation (radicals used up and created)
3- termination (radicals reacted together)

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15
Q

methane + chlorine initiation

A

Cl2 –(UV) –> 2Cl*

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16
Q

methane + chlorine propagation

A

Cl* + CH4 –> *CH3 + HCl
CH3 + Cl2 –> CH3Cl + Cl

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17
Q

methane + chlorine termination

A

Cl* + Cl* –> Cl2
Cl* + *CH3 –> CH3Cl
*CH3 + *CH3 –> C2H6

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18
Q

Limitations of radical substitution (carbon chain)

A

the reactions can occur anywhere along the carbon chain
structural isomers can form

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19
Q

Limitations of radical substitution (too much chlorine)
- describe and show with symbol equations

A

Cl* + CH3Cl –> *CH2Cl + HCl
CH2Cl + Cl2 –> CH2Cl2 + Cl
with too much chlorine, the reaction continues forming dichloromethane, this continues with more sub-reactions
- forming trichloromethane, tetrachloromethane…

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20
Q

how to solve limits of radical substitution- too much chlorine

A

solve with an excess of methane
- chlorine radical more likely to collide with methane and not chloromethane

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21
Q

what is another name for crude oil

A

petroleum

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22
Q

what is crude oil

A

a mixture of long-chained and short-chained hydrocarbons

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23
Q

how can you separate crude oil

A

through fractional distillation

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24
Q

describe the process of fractional distillation

A
  • crude oil is vaporised at 350degrees celsius
  • vaporised crude oil then placed in a fractionating column, it rises through the trays.
  • the largest hydrocarbons remain at the bottom as ‘residue’ due to their boiling point being too high, they don’t vaporise
  • each fraction condenses at their boiling point, the coolest being at the top
  • separate fractions are collected
  • hydrocarbons with the lowest boiling point don’t condense and are drawn off as gases at the top
25
Q

define cracking

A

breaking long-chained alkanes into smaller hydrocarbons

26
Q

cracking produces which two hydrocarbons

A

alkane + alkene

27
Q

what are the two types of cracking

A

thermal + catalytic

28
Q

conditions required for thermal cracking

A

1000degrees celsius, 70atm

29
Q

what is produced in thermal cracking

A

lots of alkenes which can become polymers

30
Q

conditions required for catalytic cracking

A

zeolite catalyst, 450degrees celsius, slight pressure

31
Q

what is produced in catalytic cracking

A

aromatics and motor fuels

32
Q

benefits of catalytic cracking

A

low costs as a low temperature and pressure required. catalyst speeds up reaction without being used up

33
Q

what is ‘knocking’

A

when alkanes explode on their own accord when the fuel mixture in engines is compressed

34
Q

what type of hydrocarbons are most likely to knock

A

straight chained hydrocarbons

35
Q

how can you make knocking less likely and combustion more efficient and what is it called

A

convert straight chained hydrocarbons into branched/cyclic hydrocarbons (reforming)

36
Q

what is required for reforming and give an example

A

a catalyst (platinum stuck on aluminium oxide)

37
Q

reforming of hexane

A

hexane -> benzene (C6H6) + 4H2

38
Q

reformation of octane

A

2,5- dimethlyhexane

39
Q

name compounds produced by reforming

A

cycloalkanes, branched alkanes, arenes, aromatic hydrocarbons

40
Q

combustion of alkanes word equation

A

alkane + oxygen -> carbon dioxide + water

41
Q

what state must everything be in for combustion to take place

A

gas (vapour)

42
Q

word equation for incomplete combustion

A

alkane + oxygen –> carbon + carbon monoxide + water

43
Q

name pollutants produced from the combustion of alkane fuels

A
  • carbon monoxide
  • oxides of nitrogen
  • sulfur dioxide
  • carbon particulates
  • unburnt hydrocarbons
44
Q

problems with carbon monoxide

A
  • it is toxic
  • binds to haemoglobin in the blood
  • less oxygen is circulated around the body
  • victim dies of oxygen deprivation
45
Q

problems with sulfur dioxide

A
  • burning fossil fuels that contain sulfur
  • sulfur dioxide gas enters atmosphere
  • dissolves in moisture, becomes sulfuric acid
  • creates acid rain
46
Q

problems with oxides of nitrogen

A
  • in car engines, at high temperature and pressure oxygen and nitrogen react together
  • escapes into the atmosphere
  • dissolves in moisture, forming nitric acid
  • creates acid rain
47
Q

describe the harm created by acid rain

A
  • destroys trees and vegetation
  • corrodes buildings and statues
  • kills fish in lakes
48
Q

describe the role of catalytic converters

A

to convert dangerous pollutants into harmless/less harmful gases

49
Q

what type of catalyst is used in a catalytic converter

A

platinum catalyst

50
Q

what can a catalytic converter turn the harmful gases into

A

carbon dioxide, nitrogen gas, water vapour

51
Q

give an example of a symbol equation for a catalytic converter

A
52
Q

define biofuels

A

fuels made from living matter over a short period of time

53
Q

name three types of biofuels

A
  • bioethanol
  • biogas
  • biodiesel
54
Q

describe how bioethanol is made

A

ethanol is made from the fermentation of sugar from crop such as maize

55
Q

describe how biogas is made

A

from the breakdown of organic matter

56
Q

describe how biodiesel is made

A

from refining renewable fats and oils ( for example vegetable oils )

57
Q

why are biofuels seen as carbon neutral

A

they release carbon dioxide when burnt but the carbon dioxide has come originally from the plants during their growth

58
Q

which fuels are made from waste going to landfill

A

biodiesel and biogas

59
Q

name 3 problems with biofuels

A
  • petrol car engines must be modified to handle fuels with high ethanol concentrations
  • land used to grow crops for fuel cant be used for food (problem in lic who rely on agriculture for their income)
  • CO2 is still produced through the farming, refining and transportation of the crops ( can’t be completely classified as carbon neutral )

A-level chemistry (21 decks)

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