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chemistry: organic > topic 6B - alkanes > Flashcards

topic 6B - alkanes Flashcards

1
Q

What is the general formula of alkanes?

A

CnH2n+2.

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2
Q

Why are alkanes considered saturated hydrocarbons?

A

They contain only carbon and hydrogen atoms and all the carbon-carbon bonds are single bonds.

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3
Q

How many bonds does every carbon atom in an alkane form?

A

Four single bonds with other atoms.

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4
Q

How does the formula of cycloalkanes compare to alkanes?

A

They have two fewer hydrogens; general formula is CnH2n.

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5
Q

What are cycloalkanes a common isomer of?

A

Alkenes.

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6
Q

Why are alkanes very unreactive?

A

C-C and C-H bonds are relatively strong and non-polar, so are unreactive with ionic reagents.

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7
Q

What is halogenation?

A

A halogen reacts with alkanes in photochemical reactions started by UV light.

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8
Q

What type of reaction is halogenation?

A

Substitution reaction where a H atom is replaced by a halogen atom.

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9
Q

What is the name of the reaction when a halogen substitutes a hydrogen in an alkane?

A

Free radical substitution reaction.

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10
Q

For every hydrogen replaced in halogenation, what is used and formed?

A

One X2 is used and one HX molecule is formed.

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11
Q

What happens in the initiation stage of the free radical mechanism?

A

UV light provides energy to break the X-X bond in halogen molecules to form two free radicals by homolytic fission.

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12
Q

What happens in the propagation stage of the free radical mechanism?

A

Halogen radicals react with alkanes where the halogen radical acts as a catalyst; for each H replaced, there is a pair of propagation steps.

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13
Q

What does the halogen radical do during propagation?

A

Removes a hydrogen to leave another free radical.

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14
Q

What happens after the hydrogen is removed during propagation?

A

The free radical reacts with a halogen molecule to produce the same free radical as in the initiation step.

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15
Q

What does the regeneration of free radicals allow during propagation?

A

It can react with several more alkane molecules in a chain reaction.

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16
Q

What happens in the termination stage of the free radical mechanism?

A

Two radicals react together to form a stable molecule, preventing the regeneration of free radicals and stopping the chain reaction.

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17
Q

Why are termination reactions less common?

A

The radical is more likely to meet a molecule than a radical.

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18
Q

What type of formulae should be used in radical substitution mechanisms?

A

Structural formulae, not molecular formulae.

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19
Q

What is a problem with radical substitution when trying to produce a specific product?

A

It often leads to a mixture of products rather than the desired one.

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20
Q

What happens if reaction conditions aren’t carefully controlled during radical substitution?

A

Further substitution can occur, replacing more hydrogen atoms in the molecule. This leads to the formation of molecules with progressively more substitutions, each involving the same fundamental reaction mechanism.

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21
Q

How can the formation of by-products be reduced in radical substitution?

A

Ensure an excess of the original molecule containing hydrogen atoms.

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22
Q

Why does an excess of the original molecule help reduce by-products?

A

It increases the probability that a reactive species will collide with the starting material rather than a partially substituted molecule

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23
Q

What other issue occurs with radical substitution in terms of product structure?

A

It can occur at multiple positions along a carbon chain, leading to a mixture of structural isomers.

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24
Q

What is crude oil?

A

A mixture of hydrocarbons mostly made up of alkanes.

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25
How can crude oil be separated into petroleum fractions?
It can be separated into petroleum fractions (useful hydrocarbons with a similar chain length and boiling point range) by fractional distillation, which is a physical process involving the splitting of weak London forces between molecules .
26
What temperature is crude oil heated to for vaporisation?
400ºC.
27
What does the fractionating tower contain?
About 40 horizontal ‘trays’ pierced with small holes.
28
How does the temperature of the fractioning column change?
The fractioning column its hottest at the bottom and coldest at the top
29
Why do different compounds condense at different levels in the column?
They have different boiling points and condense at different temperatures.
30
What happens when rising vapour condenses in the column?
It releases energy (IMFs formed) and heats the liquid on the tray, evaporating more volatile compounds in the mixture on the tray
31
What happens to hydrocarbons with many carbon atoms during distillation?
Hydrocarbons with lots of carbon atoms (larger fractions) have high boiling points which means they don’t vaporise, so run near the bottom of the column and form residue.
32
Where is the residue separated?
In a vacuum distillation column.
33
What happens to hydrocarbons with fewer carbon atoms?
Hydrocarbons with a small number of carbon atoms (smaller fractions) atoms have low boiling points, so they condense near the top of the column and are collected. Those with the lowest boiling points don’t condense and drawn off as gases at the top
34
How can separation of hydrocarbons be enabled?
Lowering the pressure to reduce their boiling temperatures.
35
What is the supply issue created by fractional distillation?
Larger supply of heavier fractions than needed, lower supply of short-chain fractions.
36
Why is cracking used?
To break long-chain alkanes into smaller, useful hydrocarbons by breaking C-C bonds.
37
Why is cracking useful?
* Smaller petroleum fractions are in more demand than larger fractions * To make use of excess larger hydrocarbons * Products of cracking are more useful and valuable than the starting materials
38
What is the general reaction for cracking?
High Mr alkanes → smaller Mr alkanes + alkenes + (hydrogen).
39
What are the conditions for thermal cracking?
High temperatures (~900ºC) and high pressure (up to 70 atm).
40
What does thermal cracking produce?
A higher proportion of alkenes.
41
What catalyst is used in catalytic cracking?
Zeolite catalyst (hydrated alumina silicate).
42
What are the conditions for catalytic cracking?
Slight pressure and high temperature (about 500ºC).
43
What does catalytic cracking produce?
Higher yields of branched and cyclic alkanes.
44
Why is using a catalyst in cracking beneficial?
Cuts costs by allowing lower pressure and temperature and speeds up the reaction.
45
What is reforming?
Processing of straight-chain alkanes into branched-chain alkanes, cyclic alkanes, or arenes for more efficient combustion and used to give fuels a higher octane number
46
What catalysts are used in reforming?
Platinum or rhodium, supported on an inert material such as aluminium oxide.
47
What are the conditions for reforming?
~500ºC under high pressure.
48
What is a valuable by-product of reforming?
Hydrogen, which is used elsewhere in the refinery.
49
What is complete combustion?
Complete combustion is an exothermic reaction as heat is released when alkanes are burnt (oxidised) with oxygen to produce carbon dioxide and water.
50
What is incomplete combustion?
Incomplete combustion occurs when the alkane is burnt with limited oxygen, carbon monoxide, carbon and water (sometimes carbon dioxide as well) is produced.
51
Why does incomplete combustion release less energy per mole?
Not all bonds are fully oxidised.
52
Why do liquid alkanes need vaporisation before combustion?
Combustion occurs between gases.
53
Why do smaller alkanes burn more easily?
They are more volatile and turn to gas more easily.
54
Why do larger alkanes release more energy per mole?
They have more bonds to react, meaning they are good fuels
55
How do carbon dioxide and water vapour affect the environment?
They are greenhouse gases contributing to global warming.
56
How can greenhouse gas emissions be reduced?
Burn fewer fossil fuels.
57
Why is carbon monoxide dangerous?
It binds to haemoglobin better than oxygen, reducing oxygen transport.
58
How can carbon monoxide emissions be reduced?
Ensure a good oxygen supply during combustion.
59
What are the effects of soot?
Blackens buildings, reduces air quality, causes global dimming, worsens respiratory problems.
60
How can soot formation be reduced?
Ensure a good supply of oxygen during combustion.
61
How are nitrogen oxides formed in car engines?
High pressure and temperature cause nitrogen and oxygen in air to react.
62
What happens to nitrogen oxides in the atmosphere?
Form nitric acid in moisture, falling as acid rain.
63
What are the effects of acid rain?
Destroys vegetation, corrodes buildings, kills fish.
64
What else can nitrogen oxides cause?
Photochemical smog when reacting with sunlight and other substances.
65
How can nitrogen oxide emissions be reduced?
Use catalytic converters in cars.
66
How is sulfur dioxide formed?
From sulfur impurities in fossil fuels oxidising during combustion.
67
What happens to sulfur dioxide in the atmosphere?
For sulfuric acid in moisture, leading to acid rain.
68
How can sulfur dioxide emissions be reduced?
Remove sulfur before burning and use flue-gas desulfurisation.
69
What causes unburned hydrocarbons?
Some fuel may not fully combust, wasting fuel.
70
How can unburned hydrocarbons be reduced?
Ensure engines are well tuned and oxygen supply is sufficient.
71
What is the difference between renewable and non-renewable energy sources?
Renewable sources are naturally replenished; non-renewables are finite.
72
What are biofuels?
Fuels derived from living matter over a short time frame.
73
How is bioethanol produced?
By fermenting sugars from crops like maize.
74
How is biodiesel made?
By refining renewable fats and oils, such as vegetable oil.
75
How is biogas generated?
Through the breakdown of organic waste material.
76
What are the advantages of biofuels?
* Releases CO2 that was absorbed by plants during growth, considered carbon neutral * Biodisel and biogas are made form waste that would otherwise go to landfills * Allows fossil fuels to be used as feedstock for organic compounds
77
What are the disadvantages of biofuels?
* Carbon dioxide is also emitted during refining, transport, and use of machinery. * Petrol engines needed to be modified for high ethanol concentrations * Land for fuel crops can't be used for food crops * Drives demand in developed countries; food shortages in developing countries
78
What are catalytic converters made of?
Alloys of platinum, rhodium, and palladium.
79
How are catalytic converters constructed?
As a honeycomb to ensure maximum gas contact/collisons by giving a large surface area.
80
What type of catalysis is used in catalytic converters?
Heterogeneous catalysis is when catalyst is in a different physical phase/state to the reactants, e.g. a solid catalyst in a gaseous reaction
81
What are key equations for pollution removal by catalytic converters?
2NO → N2 + O2 (reduction of NO by CO); CO + O2 → 2CO2 (oxidation of CO); CxHy + O2 → CO2 + H2O (oxidation of unburnt hydrocarbon)
82
How does catalysis work in three stages?
1. Formation of weak bonds with surface weakens bonds in gas molecules makes a subsequent reaction easier; takes place at active sites on the surface of a solid 2. Adsorbed gases may be held on the surface increases chances of favourable collisions 3. The products are then released from the active sites
83
Why is adsorption strength important in catalysis?
Must be strong enough to attract reactants but weak enough so that products an leave to open up active sites
84
What happens if adsorption is too weak?
Molecules won’t bond to the surface.
85
What happens if adsorption is too strong?
Molecules stay on surface, block active sites, and prevent further reaction.

chemistry: organic (6 decks)

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